U 1. Uracil. 2. Uridine. 3. Enzyme Unit. 4. Uranium. 5. Uniformly Labeled. Ul

U

U 1. Uracil. 2. Uridine. 3. Enzyme unit. 4. UEP Unit evolutionary period. Uranium. 5. Uniformly labeled. UFA Unesterified fatty acids. Ul One of a group of small nuclear UIBC Unsaturated iron-binding capacity. ribonucleoproteins (snRNP) that function in ultimate carcinogen The form of a carcinogen the splicing of nuclear pre-mRNA. Ul con- in which it ultimately reacts with cellular mac- sists of several polypeptides and an RNA romolecules; the reactive intermediate formed molecule (165 nucleotides) that contains a from a carcinogen that is inacitve per se. The segment that is complementary to the accep- ultimate carcinogen is believed to be a form tor and donor junctions of introns. Other that contains an electrophilic center (such as a members of the group that have been iden- carbonium ion, free radical, epoxide, or metal tified include U2, U4, U5, and U6. cation) that attacks electron-rich centers in Ubbelohde viscometer A capillary-type visco- proteins and nucleic acids. meter that permits the measurements of vis- ultimate precision See method of ultimate pre- cosity as a function of concentration, since the cision. solution can be diluted directly in the visco- ultra- 1. Prefix meaning excessive. 2. Prefix meter. meaning beyond the range of. ubiquinol Reduced coenzyme Q; CoQH2. ultracentrifugation Centrifugation, performed ubiquinol: cytochrome C oxidoreductase Com- in an ultracentrifuge, in which high centrifugal plex III. forces are used. ubiquinone Coenzyme Q. ultracentrifuge A high-speed centrifuge cap- ubiquitin A small, heat-stable, nonenzymatic able of generating speeds of approximately polypeptide of 76 amino acids that is widely 60,000 rpm and centrifugal forces of approx- distributed (as its name implies) in eukaryotic imately 500,000 x g. cells. ATP hydrolysis drives a reaction where- ultracryotomy The preparation of thin sections by ubiquitin becomes covalently linked, via its from frozen tissue specimens. carboxyl group, to the amino group of lysine ultrafiltration The filtration of a solution in proteins that are substrates for proteolysis. through a filter or a membrane that will retain It is believed that the attachment of ubiquitin macromolecules. marks a given protein for rapid intracellular ultramicrotome A microtome for cutting very degradation; it may serve to tag damaged or thin sections of tissues, about 0.05 to 0.10 jim abnormal proteins. Ubiquitin is also involved in thickness, for electron microscopic ex- in histone modification and appears to be part amination. of certain receptor sites on lymphocyte sur- ultrasonic Of, or pertaining to, ultrasound. faces. ultrasonication The exposure of material to ubiquitination The covalent linking of ubi- ultrasound; used for the rupture of cells and quitin to a protein. the denaturation of proteins. ubiquitous RNA A class of RNA molecules ultrasound Sound waves that have frequencies that form part of the small nuclear RNA greater than those of audible sound; refers complement and that have wide occurrence. particularly to sound waves that have frequen- Ribosomal 5S RNA and transfer RNAs are cies of 500,000 cps or higher. examples. ultrastructure The fine structure of tissues, UDP 1. Uridine diphosphate. 2. Uridine-5'- cells, and subcellular particles beyond that re- diphosphate. vealed by the light microscope. UDPG Uridine diphosphate glucose; see UDP- ultraviolet dichroism The dichroism produced glucose. when ultraviolet light is absorbed by oriented UDP-GaI Uridine diphosphate galactose, samples such as nucleic acid preparations. UDP-GIc Uridine diphosphate glucose; see ultraviolet microscope A microscope that util- UDP-glucose. izes ultraviolet light for illumination of the UDP-glucose Uridine diphosphate glucose; a specimen and for image formation. It has nucleoside diphosphate sugar that serves as twice the resolving power of an ordinary light the donor of a glucose residue in the biosyn- microscope and may be used for estimating thesis of glycogen. Abbr UDPG; UDP-GIc. the amount of nucleic acid in a sample. ultraviolet optics ABSORPTION OPTICAL SYSTEM. uncompensated acidosis An acidosis in which ultraviolet spectrum That part of the electro- the blood pH falls due to insufficient com- magnetic spectrum that covers the wavelength pensatory mechanisms. range of about 1.3 x 10~6 to 4 x 10~5 cm and uncompensated alkalosis An alkalosis in which that includes photons which are emitted or the blood pH rises due to insufficient com- absorbed during electronic transitions; radia- pensatory mechanisms. tion of the shorter wavelengths is known as uncompetitive inhibition The inhibition of the far ultraviolet, and that of the longer wave- activity of an enzyme that is characterized by lengths is known as near ultraviolet. Abbr a decrease in the maximum velocity compared UV spectrum. to that of the uninhibited reaction and by a umber codon The codon UGA; one of the reciprocal plot (I/velocity versus I/substrate three termination codons. Aka opal codon. concentration) which is parallel to that of the umber mutation A mutation in which a codon uninhibited reaction. See also degree of in- is mutated to the umber codon, thereby caus- hibition. ing the premature termination of the synthesis uncompetitive inhibitor An inhibitor that binds of a polypeptide chain. to the enzyme-substrate complex but does not UMP 1. Uridine monophosphate (uridylic bind to the free enzyme. acid). 2. Uridine-5'-monophosphate (5'- uncoupler Uncoupling agent. uridylic acid). uncoupling 1. The separation of the two pro- unbalanced growth The growth of cells cesses that constitute oxidative phosphoryla- such that not all of the cellular components tion such that ATP synthesis is dissociated increase by the same factor and, hence, the fromt he electron transport system at one or overall composition of the cells does not re- more of the phosphorylation sites. As a result main constant. A given increase in the con- of uncoupling, ATP synthesis is inhibited but centration of protein, DNA, RNA, lipid, or electron transport and respiration are allowed some other macromolecule is not coincident to proceed and may even be stimulated. 2. with the same increase in the mass and num- The decreased sensitivity of target cells to sti- ber of cells. mulation by a ligand that is due to the fact uncertainty principle The principle, enunciated that specific receptors, while still present, are by Heisenberg, that it is impossible to know unable to activate a particular enzyme. The simultaneously, with absolute precision, both two processes of ligand binding to a receptor the position and the speed of a small particle, and enzyme activation by the ligand-receptor such as an electron. Specifically, the product complex have been separated. See also desen- of the uncertainty in position Ax and the un- sitization (3). certainty in momentum Ap can be no smaller uncoupling agent An inhibitor that brings than Planck's constant h. Thus, AJC-Ap ^ h. about the uncoupling of ATP synthesis from Depending on the precise definition of Ax and the electron transport system at one or more Ap, the right side of this expression can be /t, of the phosphorylation sites. /i/2ir, or /i/4ir. Aka Heisenberg uncertainty undecaprenol A long-chain lipid, containing 11 principle. isoprene units, that serves as a carrier (in the uncharged polar amino acid A polar amino form of undecaprenol phosphate) of carbohy- acid that carries no charge in the intracellular drates in the biosynthesis of peptidoglycan in pH range of about 6 to 7. bacteria. See also glycosylation. uncharged tRNA A transfer RNA molecule undecaprenol phosphate See undecaprenol. that does not have an amino acid covalently undernutrition Malnutrition resulting from the linked to it. consumption of inadequate amounts of food uncoating The removal of the protein coat so that one or more of the essential nutrients from a virus that occurs extracellularly in the are lacking in the diet. Aka undernourish- case of bacterial viruses and, apparently, in- ment. tracellularly in the case of animal viruses. underwinding Negative supercoiling; see super- uncoating enzyme An enzyme that functions in helix. the removal of the protein coat from a virus. uneconomic species An undesirable species; the uncoded amino acid An amino acid, such as species that is eliminated by another, desir- hydroxyproline or hydroxylysine, for which no able, species through the use of chemicals. codon exists. Such an amino acid is derived by uni- 1. Referring to one kinetically important enzymatic modification of the parent amino substrate and/or product of a reaction; thus a acid (e.g., proline or lysine) after the parent uni bi reaction is a reaction with one substrate amino acid has become incorporated into a and two products. 2. Prefix meaning one. polypeptide chain in response to its codon. unidirectional replication DNA replication that proceeds in one direction; one replicating fork unit membrane hypothesis The hypothesis that moving in a given direction, as in the rolling the structure of biological membranes can circle model. be described in terms of a unit membrane, uniformitarianism See doctrine of uniformitar- about 90 A thick. The unit membrane is con- ianism. sidered to be a bilayer of polar lipids, such uniformly labeled Designating a compound as phospholipids, surrounded on both sides that is labeled in all of the positions in the by protein. The lipids are arranged with their molecule in a statistically uniform or nearly nonpolar portions inward and their polar por- uniform manner; generally refers to 14C- tions outward where they are coated with labeled compounds. Sym U. protein molecules that are in their beta con- unifunctional feedback A feedback mechanism figuration. Aka Danielli-Davson-Robertson that affords control in only one direction so model; Robertson model. See also Danielli- that the input of a system is affected by either Davson model; fluid mosaic model. an increase or a decrease of the output; a unit mitochondrion hypothesis A controversial system in which the pH will be adjusted if it idea, proposed by some biologists, according rises above the normal value is an example. to which there exists only one mitochondrion unilamellar vesicle See vesicle. per cell in vivo. The hundreds and thousands unimolecular reaction A chemical reaction in of mitochondria seen in electron microscope which one molecule (or other entity) of a preparations are considered to be artifacts, single reactant is transformed into products. arising from disruption of massive in vivo unineme hypothesis The hypothesis that a new- mitochondria as the sample is prepared for ly formed chromatid contains a single double- electron microscopy. stranded DNA molecule. unit of complement The amount of comple- unionized calcium The physiologically inactive ment that lyses 50% of the sensitized erythro- form of calcium that cannot diffuse through cytes. Abbr C H50 unit. a semipermeable membrane through which unit of enzyme See enzyme unit. calcium ions (Ca2"1") can pass; represents unit of information See bit. largely that fraction of calcium that is bound unit of transcription See transcription unit. to plasma proteins, particularly albumin. unity of biochemistry The phenomenon that uniport The transport of a substance across a widely different microbial, plant, and animal membrane that is not linked to the transport cells show a high degree of similarity with of another substance across the same mem- respect to both the types of molecules that brane. See also antiport; symport. they contain and the metabolic reactions that unique DNA DNA sequences that occur only these molecules undergo. once in the haploid genome. Most structural universal buffer A mixture of several buffers genes and their introns are unique DNA. Aka that can be used over a relatively wide pH nonrepetitive DNA. See also repetitive DNA. range. Unitarian hypothesis The hypothesis that the universal code A code in which the same injection of an antigen results in the formation codons code for the same amino acids in all of a single type of antibody that has multiple types of organisms. functions and that can react with antigens universal donor An individual of the O type in under all conditions, such as those of agglu- the ABO blood group system, who can tination, precipitation, and complement fixa- donate blood to any recipient. tion. universal recipient An individual of the AB unitary rate The rate constant of an elemen- type in the ABO blood group system, who tary step. can receive blood from any donor. unit cell The smallest portion of a crystal that universal red shift The phenomenon that the embodies the structural characteristics of the wavelength of maximum absorption for a crystal and that, when repeated indefinitely, compound in solution generally occurs at a will generate the entire structure. longer wavelength than that for the compound unit evolutionary period The time required for in the gas phase. the amino acid sequence of a protein from universe The thermodynamic system plus its two species to change by 1% (that is, develop surroundings. a difference of 1 amino acid residue per 100 unmasking The conversion of an unreactive residues) after the two species have diverged amino acid residue or an unreactive site on a in the course of evolution. Abbr UEP. protein molecule to a reactive one that is ac- unit mass The quantity m/z, used in mass spec- cessible to specific reagents and that can par- trometry, where m and z are the mass and ticipate in specific binding or other reactions. charge of the fragment, respectively. The process may involve conformational changes in the molecule and/or proteolytic re- moval of blocking groups. maps of the fragment and the vector have unmixing 1. The sorting out of solute mole- different orientations. See also n orientation. cules from solvent molecules in a solution, UP Uroporphyrin. a process that is accompanied by a decrease upfield Describing a peak in nuclear magnetic in entropy. 2. The formation of a single com- resonance that is on the high magnetic field plex from separate particles, a process that is side of the spectrum. See also shielded nuc- accompanied by a decrease in entropy. leus. unpaired electron A single electron, instead of UPG Uroporphyrinogen. the usual two, in an orbital. uphill reaction ENDERGONIC REACTION. unresponsiveness IMMUNOLOGICAL UNRESPON- up promoter mutation See promoter up muta- SIVENESS. tion. unrestricted Descriptive of a restricted mutant U protein Unstable protein; see trigger protein. that has undergone the opposite modification. upstream Describing a location or a sequence See also restricted virus. of units in the opposite direction to that in unsaponifiable lipid The fraction of lipid in a which a process occurs: (a) during transcrip- sample that remains insoluble after saponifica- tion, the location or sequence of deoxyribo- tion of the sample with alkali; it consists prin- nucleotides on the transcribed strand of DNA cipally of steroids and terpenes. from the 5'- to the 3'-end; (b) during transla- unsaturated fatty acid A fatty acid that con- tion, the location or the sequence of ribonuc- tains one or more double bonds in the alkyl leotides on the mRNA from the 3'- to the chain. Unsaturated fatty acids have lower 5'-end; (c) during replication, the location in a melting points than corresponding saturated direction that is opposite to that of the repli- fatty acids; all of the common unsaturated cating fork movement; (d) in a polypeptide fatty acids are liquid at room temperature. chain, the sequence of amino acids in which Some unsaturated fatty acids are essential in they are linked together, from the C- to the animals but they are probably obtained in suf- N-terminal; (e) in the electron transport sys- ficient amounts in the typical human diet. tem, the sequence of electron carriers in a unsaturated iron-binding capacity The differ- direction opposite to the flow of electrons, ence in the concentration of iron between the from oxygen to metabolite. See also down- maximum potential concentration (transferrin stream. fully saturated with iron) and the actual con- upstream activation sites Sites in eukaryotic centration (transferrin only about 25 to 30% DNA that are usually 50-300 bp upstream saturated with iron); generally expressed in from the promoter and that have a regulatory terms of micrograms per 100 mL of plasma. role in transcription; they are believed to be Abbr UIBC. binding sites for transcription factors. unscheduled DNA synthesis DNA synthesis that upward flow A column chromatographic tech- occurs during some stage of the cell cycle nique, frequently used in gel filtration, in other than the S phase. See also cell cycle. which the eluent is passed upward through the unselective marker A marker that does not column. This minimizes compression of the affect the growth of the organism on a selec- gel bed and allows for finer control of the flow tive medium. rate. unstable mutation A mutation that has a high UQ Ubiquinone. frequency of back mutation to the original, UQH2 Dihydroubiquinone; reduced coenzyme nonmutated state. Q. unstable protein TRIGGER PROTEIN. Ura Uracil. unstructured smectic See liquid crystal. uracil The pyrimidine 2,4-dioxypyrimidine that untargeted mutagenesis The production of occurs in RNA. Abbr U; Ura. mutations that occur at locations other than uracil-DNA glycosidase See uracil-N-glycosy- the actual sites of damage in DNA. Mutations lase. arising in ultraviolet irradiated DNA that do uracil fragments Fragments that are excised not occur at thymine dimer sites, are an ex- during DNA replication as a result of the mis- ample. incorporation of uracil in place of thymine. untwistase See topoisomerase. The fragments consists of uracil and nuc- untwisting enzyme See topoisomerase. leotides from which uracil has been removed. unwindase SINGLE-STRAND BINDING PROTEIN. The fragments are formed by the combined unwinding protein SINGLE-STRAND BINDING PRO- action of several enzymes, including uracil-JV- TEIN. glycosylase and AP endonuclease. unzippering See zippering. uracil-N-glycosylase An enzyme that functions u orientation The sense of insertion of a DNA in DNA repair by removing a uracil that was fragment into a vector such that the genetic either erroneously incorporated or formed by mutation. The action of this enzyme is then peroxide. followed by that of AP endonuclease and uricolysis The reaction catalyzed by the en- DNA polymerase. Aka uracil-DNA glycosi- zyme uricase. dase. uricosome An early term for peroxisome; so uracil-rich code An early version of the genetic coined, because of the presence of urate code according to which the codons of all the oxidase in these organelles. amino acids contain at least one uracil nuc- uricosuria The presence of excessive amounts leotide. of uric acid in the urine. urate A salt of uric acid. uricosuric drug A drug that tends to promote urate oxidase URICASE. the excretion of uric acid. Urd Undine. uricotelic organism An organism, such as a urea A compound that is formed in the urea bird, that excretes the nitrogen from amino acid cycle during amino acid catabolism and that is and purine catabolism primarily in the form one of the major forms in which nitrogen is of uric acid. excreted in the urine; urea is also the end uridine The ribonucleoside of uracil. Uridine product of purine catabolism in most fishes mono-, di-, and triphosphate are abbreviated, and amphibia. Aka carbamide. respectively, as UMP, UDP, and UTP. The urea cycle The cyclic set of reactions whereby abbreviations refer to the 5'-nucleoside phos- two amino groups, derived from the catabol- phates unless otherwise indicated. Abbr Urd; ism of two amino acids, and a molecule of U. carbon dioxide are converted to urea which is uridine diphosphate glucose See UDP-glucose. then excreted. uridine nucleotide coenzyme See nucleotide urease An enzyme that appears to have an coenzyme. absolute specificity and that catalyzes only the uridylic acid The ribonucleotide of uracil. single reaction whereby urea is hydrolyzed to uridylylation The transfer of a 5'-UMP group carbon dioxide and ammonia. (5'-uridylyl group) from UTP; specifically, the urenzyme A rudimentary form of an enzyme in transfer of a uridylyl group from UTP, cataly- a primitive cell. zed by the enzyme uridylyl-transferase. This ureotelic organism An organism, such as a reaction is part of the complex regulation of mammal, the excretes the nitrogen from ami- the activity of the enzyme glutamine syn- no acid and purine catabolism primarily in the thetase in E. coli. form of urea. urinalysis The chemical and physical analysis urethane Ethyl carbamate; a carcinogenic of urine. agent. urinometer A hydrometer designed for mea- Urey equilibrium The equilibrium established suring the specific gravity of urine. in the reaction in which the reactants are car- urkaryote The original eukaryotic cell from bon dioxide and calcium silicate, and the pro- which present day eukaryotes evolved. ducts are calcium carbonate and silica. The Ul RNA See Ul. reaction has been proposed by Urey to be the urochrome The principal pigment of urine that one by means of which the carbon dioxide is composed of a peptide of unknown struc- pressure was maintained at low levels in the ture and either urobilin or urobilinogen. primitive atmosphere of the earth. urogastrone A gastrointestinal hormone that ur genes The genes coding for a "minimal" inhibits the secretion of HCl by the stomach; biological system that is essential for all appears to be identical to human epidermal organisms; the genes coding for rRNA, ribo- growth factor. somal proteins, and the enzymes and other urokinase A serine protease, synthesized in the proteins involved in DNA replication, DNA kidney and found, as its name implies, in the transcription, and RNA translation. urine; a plasminogen activator that converts -uria 1. Combining form meaning the presence plasminogen to plasmin. of a substance in the urine. 2. Combining uromodulin A glycoprotein of 616 amino acids form meaning the presence of excessive that has immunosuppressive properties in amounts of a substance in the urine. vitro. It contains about 30% carbohydrate uric acid A purine that does not occur in nuc- and is synthesized by the kidney; it appears leic acids but that is an intermediate of purine to be identical to the Tamm-Horsfall glyco- catabolism in some organisms, and an end protein which is the most abundant protein product of purine catabolism in humans and of renal origin in normal human urine. other organisms. uronic acid A monocarboxylic sugar acid of an uricase A copper-containing enzyme that aldose in which the primary alcohol group has catalyzes the oxidation of uric acid by mole- been oxidized to a carboxyl group. cular oxygen to allantoin and hydrogen uroporphyrin The urinary pigment derived from uroporphyrinogen. Abbr UP. See also are plant parasites). porphyrin. utile cycle A cyclic set of reactions that does uroporphyrinogen An intermediate in the bio- accomplish something in metabolism and that, synthesis of the porphyrins that is formed by therefore, does not constitute a futile cycle. the linking of four molecules of porphobili- UTP 1. Uridine triphosphate. 2. Uridine-5'- nogen. Abbr UPG. triphosphate. ursolic acid An unsaturated, pentacyclic, tri- UV Ultraviolet. terpene carboxylic acid that occurs widely in UV endonuclease An E. coli enzyme (com- plants as the free acid or as the aglycone of posed of three subunits) that functions in the triterpene saponins. repair of DNA which contains thymine dimers usnic acid A antibiotic produced by some that were produced by ultraviolet irradiation. lichens that is active against many gram- The enzyme makes a cut at or near the 5'-end positive bacteria (including Mycobacteriwn of the thymine dimer. Aka UVR"1" endonuc- tuberculosis) and some fungi. lease. USP Unites States Pharmacopeia; denotes a UV-induced dimer A pyrimidine dimer that is chemical that meets specifications set out in formed in DNA by exposure of the DNA to the U.S. Pharmacopeia. ultraviolet light. See also pyrimidine dimer; USPHS United States Public Health Service. thymine dimer. Ussing chamber technique A technique for UV monitor A monitor, the operation of which evaluating active transport by mounting a is based on the measurement of the absorp- piece of tissue in a Lucite chamber in such a tion of ultraviolet light by a solution. way that both electrical and chemical gra- UV reactivation The phenomenon that the sur- dients are eliminated. vival of UV-irradiated lambda phage particles Ussing's short circuit An experimental setup is higher on an irradiated host than on an for the measurement of membrane potentials unirradiated host; represents a DNA repair and of concentration gradients across a mem- mechanism that involves the SOS repair sys- brane. tem. Aka Weigle reactivation; W reactivation. ustilagic acids A group of extracellular glycoli- UVR* endonuclease uv ENDONUCLEASE. pids, produced by Ustilago species (fungi that V

v Reaction rate. V0 1. Initial velocity. 2. Control velocity; initial bonding that is developed by considering the velocity in the absence of an inhibitor. atoms to have intact atomic orbitals, and then v Partial specific volume. moving the atoms closer to each other with V 1. Maximum velocity. 2. Volume. 3. Volt. 4. a resultant overlap of the atomic orbitals. Valine. 5. Vanadium. According to this theory, a covalent bond is vacant lattice point model A model for the formed by the overlap of two orbitals, one structure of water according to which the from each bonding atom, and with each orbit- water structure is an essentially crystalline al holding one electron. A coordinate cova- system that is closely related to an open, lent bond is formed by the overlap of an orbit- expanded, ice-like structure into which free al of one atom, holding two electrons, with an and nonassociated water molecules can easily unoccupied orbital of a second atom. fit. valence electron An electron that is located in vaccenic acid An unsaturated fatty acid that the outermost energy shell of an atom and contains 18 carbon atoms and one double that participates in the formation of chemical bond; it is the major unsaturated fatty acid in bonds. E. coli. valency Variant spelling of valence. vaccination An immunization in which a vac- -valent Combining form meaning valence; used cine is administered to humans or to animals either with mono, di, ..., poly to indicate the for the purpose of establishing resistance to an chemical valence of atoms and ions, or with infectious disease. uni, bi, ..., multi to indicate the immunolo- vaccine A suspension of antigens that are de- gical valence of antigens or antibodies. rived from infectious bacteria or viruses and valine An aliphatic, branched, nonpolar alpha that, upon administration to humans or to amino acid that contains five carbon atoms. animals, will produce active immunity and will Abbr VaI; V. provide protection against infection by these, valinemia A genetically inherited metabolic or by related, bacteria or viruses. defect in humans due to a deficiency of the vaccinia virus A virus of the poxvirus group enzyme valine aminotransferase. that infects humans and many animals. valinomycin An ionophorous antibiotic, pro- vacuole An intracellular structure, surrounded duced by Streptomyces fulvissimus, that acts by a single membrane (tonoplast) and filled as an uncoupler of oxidative phosphorylation; with fluid. Plant vacuoles are usually large and a depsipeptide antibiotic. occupy a major portion of the cell volume. vanadium An element that is essential to hu- Animal vacuoles are much smaller and are mans and animals. Symbol, V; atomic num- also known as secretory vesicles or secretory ber, 23; atomic weight, 50.942; oxidation granules. states, +2, +3, -1-4, +5; most abundant iso- vacuolysosome A lysosome that has fused with tope, 51V; a radioactive isotope, 48V, half-life, a vacuole. 16 days, radiation emitted, positrons and vacutainer An evacuated tube used in the gamma rays. drawing of blood. vancomycin A glycopeptide antibiotic, pro- vacuum evaporator A vacuum chamber in duced by some species of Actinomycetes, that which metal atoms are evaporated in the pro- inhibits peptidoglycan biosynthesis. cess of shadowcasting specimens for electron van Deemter equation An equation that ex- microscopy. presses the height equivalent of a theoretical vacuum ultraviolet The range of the ultraviolet plate in chromatography as the sum of three spectrum that encompasses wavelengths less terms: an eddy diffusion term, a molecular than 1.8 x 10~5 cm. diffusion term, and a mass transfer term. VaI 1. Valine. 2. Valyl. van Deemter plot A plot of height equivalent valence 1. The number of electrons of an atom of a theoretical plate as a function of average or a group of atoms that participate in the gas flow rate in a gas chromatographic col- formation of chemical bonds. 2. ANTIGEN umn. VALENCE. 3. ANTIBODY VALENCE. van den Bergh reaction A colorimetric reaction valence bond theory The theory of chemical for bilirubin that is based on the formation of a diazo dye from bilirubin and dia- mical reaction is measured by either the volume zotized sulfanilic acid. The direct van den or the pressure of a gas released during Bergh reaction measures direct-acting bilir- the reaction. ubin, and the indirect van den Bergh reaction van't Hoff complex A catalyst-reactant com- measures indirect-acting bilirubin. plex for which the conversion to free catalyst van der Waals compound A compound in plus product proceeds at a much faster rate which neutral atoms or molecules are held than the reversion to free catalyst plus reac- together by van der Waals interactions; the tant. See also Arrhenius complex. dimers formed from noble gas atoms are an van't Hoff equation An equation that describes example. the variation of the equilibrium constant K van der Waals distance The distance between with the absolute temperature T at constant two atoms at which the attractive van der pressure; specifically, dlnKldt = &H°/RT2, Waals force is just balanced by the repulsive where A/f ° is the standard enthalpy change force due to the orbital electrons of the two and R is the gas constant. atoms; the distance is equal to the sum of the van't Hoff factor A measure of the deviation van der Waals radii of the two atoms. Aka of a solution from ideality; equal to the ratio van der Waals contact distance; van der Waals of the measured value of a colligative pro- separation. perty of the solution to the value calculated van der Waals forces VAN DER WAALS INTERAC- on a molar basis. TIONS. van't Hoff isobar VAN'T HOFF EQUATION. van der Waals interactions The attractive and van't Hoff isochore VAN'T HOFF EQUATION. repulsive forces between atoms and molecules van't Hoff limiting law An expression for the that involve various dipole-dipole interactions limiting value of the osmotic pressure of and that consist of three components: a dis- an ideal solution; specifically, limc_0 H = persion effect, an induction effect, and an RTIM9 where Il is the osmotic pressure, c orientation effect. The three components are is the concentration of the solute, M is the classified as weak interactions, since the ener- molecular weight of the solute, R is the gas gy of these interactions is proportional to r~6 constant, and T is the absolute temperature. where r is the distance between the interacting van't Hoff plot A plot of the logarithm of species. See also dispersion effect; induction the equilibrium constant as a function of the effect; orientation effect; weak interactions. reciprocal of the absolute temperature. See van der Waals radius One-half of the van der also van't Hoff equation. Waals distance between two like atoms; one- V antigen A surface antigen of some viruses. half of the equilibrium internuclear distance vapor diffusion method A method for the slow between two non-bonded atoms when the crystallization of a substance; used in the pre- attractive and repulsive forces between the paration of transfer RNA crystals. two atoms are exactly balanced. Aka van der vapor-liquid partition chromatography GAS Waals contact radius. CHROMATOGRAPHY (2). van der Waals repulsive force A repulsive vapor phase chromatography GAS CHROMA- force that develops when noncovalently TOGRAPHY. bonded atoms or molecules approach each vapor pressure osmometer An osmometer, the other closer than their van der Waals dis- operation of which is based on the lowering of tance; due to the repulsion of the overlapping the vapor pressure of a solvent by the addition electron clouds. of a solute. van der Waals shell The space surrounding the variable Generally, any quantity that varies. nucleus of an atom that is described by the More precisely, a variable in the mathematical van der Waals radius. sense; that is, a quantity that may take on any van der Waals surface The complex surface of one of a specified set of values. See also de- a folded protein that results when each atom pendent variable; independent variable; vari- is depicted as a sphere of its van der Waals ate. radius and overlapping spheres, where the variable arm See arm. atoms are covalently bonded, are truncated. variable genes See V genes. This surface has a strictly defined area and variable of state STATE FUNCTION. encloses a definite volume. variable region That part of the immunoglobu- vanilmandelic acid 3-Methoxy-4-hydroxy-mande- Hn molecule in which differences in amino lic acid; a compound that is quantitatively the acid sequences are found when immunoglobu- most important metabolite of the catechola- lins from different sources are compared. The mines and that is used to assess the endogenous region comprises portions of both the light production of catecholamines. Abbr VMA. and the heavy chains and includes the two van Slyke method A method in which a che- antigen binding sites. Variable regions con- sists of hypervariable regions, where the bulk vasoactive intestinal peptide A peptide hor- of amino acid sequence variability occurs, and mone of 28 amino acids that is found in the of framework regions, in which the amino gastrointestinal tract and that possesses a wide acid sequence is relatively constant. Aka vari- variety of biological functions when infused able domain. See also constant region. into an animal. These functions include vaso- variable substrate The substrate the concentra- dilation, increase of cardiac output, increase tion of which is varied while the concentra- of glycogenolysis, inhibition of gastric secre- tions of other substrates are maintained at fixed tion, stimulation of intestinal secretion, and values; used in kinetic studies of multisub- stimulation of lipolysis. Abbr VIP. strate enzyme systems. vasoconstriction A decrease in the diameter of variance 1. The square of the standard devia- the blood vessels. tion. 2. DEGREES OF FREEDOM (1). vasoconstrictor An agent that causes vasocon- variance analysis See analysis of variance. striction. variance-ratio distribution An F distribution; vasodepressor An agent that brings about a See F test. lowering of the blood pressure. variance-ratio test F TEST. vasodilation An increase in the diameter of the variant 1. n One of several forms of a protein, blood vessels. occurring within one species or distributed vasodilator An agent that causes vasodilation. among several species, that differ from each vasopressin A cyclic peptide hormone, consist- other either in their state of aggregation, as in ing of nine amino acids, that increases the the case of isozymes, or in their amino acid blood pressure and increases the absorption sequence, as in the case of abnormal hemog- of water by the kidneys; it is secreted by lobins. See also multiple forms of an enzyme. the posterior lobe of the pituitary gland and 2. A strain that differs in some aspects from occurs in mammals. Abbr VP. the given microorganism. 3. adj Dissimilar; vasopressor principle VASOPRESSIN. showing variation. Said of nonidentical amino vasotocin A cyclic peptide hormone, consisting acid residues that occupy similar positions in of nine amino acids, that is related to vasop- the same polypeptide chain which is isolated ressin in its structure and in its function; it is from different sources. secreted by the posterior lobe of the pituitary variant repetition The occurrence of non- gland and occurs in birds, reptiles, and some identical, but related, genes per nucleus; gives amphibians. rise to a family of related proteins (variants). VCD Vibrational circular dichroism. variate A random variable; a variable that can vector 1. A directional quantity. 2. An organ- take on any of the values of a specified set ism that serves to transfer a parasite from one with specified relative frequency or probabil- host to another. 3. A self-replicating DNA ity. Aka random variable; chance variable; molecule that serves to transfer a DNA seg- stochastic variable. ment (passenger) into a host cell in recom- varicella The primary form of the disease pro- binant DNA technology. The three most com- duced by varicella virus in a host that was not mon types of vectors used are bacterial pias- previously infected by the virus. mids, phages, and other viruses. varicella virus A virus of the herpesvirus group vectorial Of, or pertaining to, a vector. that leads to varicella in hosts without im- vectorial discharge Descriptive of the trans- munity to the disease and to herpes zoster in membrane insertion of proteins during protein hosts with immunity. It is the causal agent of synthesis according to the signal hypothesis; chicken pox and shingles. Aka varicella- the term vectorial indicates directionality of zoster virus. transport from the cytosol to the lumen of the variola virus The virus of the poxvirus group rough endoplasmic reticulum and directional- that causes smallpox. ity with respect to the protein (N-terminal end varion A variable codon; the number of inserted into the membrane first); the term codons for a pair of homologous proteins that discharge emphasizes that this is an active, have been free to fix mutations over some energy-requiring process. See also signal hypo- part of the period during which the two pro- thesis. teins have diverged from their common an- vectorial enzyme An enzyme that is directional cestor; it is a joint property of the pair of in its action, such as one that is fixed in a nucleotide sequences. See also covarion. membrane. vascular Of, or pertaining to, vessels that con- vectorial phosphorylation The coupled pro- duct a biological fluid; such vessels include cesses of transport and phosphorylation of those that carry the blood and those that con- monosaccharides in the bacterial phospho- duct the sap of plants. transferase system. vasocative Having an effect on blood vessels. vectorial reaction A reaction that is directional and in which the components either are not Verner-Morrison syndrome PANCREATIC CHOLERA. free to move at random or are fixed in a vernier A small, movable, auxiliary scale that nonrandom order; a reaction across a mem- is attached to a larger scale and that has brane is generally a vectorial reaction. divisions of slightly different width than those vectorial thioesterification The coupled pro- of the larger scale; used for increasing the cesses of transport and thioesterification accuracy of a measurement. whereby a fatty acid is converted to fatty acyl Veronal Trademark for barbital. coenzyme A in E. coli. Versene Trademark for ethylene dinitrolotetra- vectorial transfer CHANNELING. acetic acid. vegan A strict vegetarian; an individual who vertebrate 1. n An animal that has a backbone; eats only plant food products. See also lacto includes fishes, amphibia, reptiles, birds, and vegetarian; lacto-ovo vegetarian. mammals. 2, Adj Of, or pertaining to, an vegetative Pertaining to growth, particularly of animal that has a backbone. plants, as opposed to reproduction. vertical evolution The process wereby a species vegetative cell An actively growing cell, as dis- changes over time, without splitting, and tinct from one forming spores. evolves into a new and distinct species. vegetative DNA The pool of genetically compe- vertical transmission The transmission of tent DNA of a phage that is produced during viruses from one generation of hosts to the the vegetative state of the phage but that has next. Aka vertical infection. not yet been assembled into complete phage very early RNA PREEARLY RNA. particles. very high-density lipoprotein A plasma lipo- vegetative map The genetic map of a phage protein that has a density above 1.21 g/mL. that is deduced from its vegetative replication. An increase in the concentration of very high- vegetative mycelium That portion of the myce- density lipoproteins (VHDL) is believed to be lium of a fungus that penetrates into the linked to a decrease in the incidence of ather- medium and absorbs nutrients. osclerosis. VHDL contain about 57% protein, vegetative nucleus 1. The macronucleus of a 21% phospholipid, 17% cholesterol and ciliate. 2. The tube nucleus of a pollen grain. cholesterol esters, and 5% triacylglycerols 3. The nucleus of a cell during interphase. (triglycerides). VHDL have molecular weights vegetative phage A phage in its vegetative of about 1.5-2.8 x 105, a sedimentation coef- state. ficient of 2-1OS, and are classified as the a- vegetative replication The replication of vegeta- fraction on the basis of electrophoresis. Abbr tive DNA. VHDL. See also lipoprotein. vegetative reproduction 1. Asexual reproduc- very high-lipid lipoprotein VERY LOW-DENSITY tion. 2. The reproduction in plants without LIPOPROTEIN. true seeds. very long-chain fatty acids See adrenoleukody- vegetative state The state of a phage during strophy. which it replicates actively and autonomously very low-density lipoprotein A plasma lipopro- within the host cell; a noninfective state dur- tein that has a density of 0.95-1.006 g/mL. ing which infective phage particles have not An increase in the concentration of very low- yet been assembled. During this state the density lipoproteins (VLDL) is believed to phage controls the synthesis by the host of be linked to an increase in the incidence of components that are necessary for the produc- atherosclerosis. VLDL contain about 10% tion of infective phage particles. protein, 18% phospholipid, 22% cholesterol vehicle VECTOR (3). and cholesterol esters, and 50% triacylgly- vein A blood vessel that transports blood from cerols (triglycerides). VLDL have molecular the tissues to the heart. weights of about 5.0-20 x 106, a flotation velocity REACTION RATE. coefficient of 12-400S, and are classified as velocity constant RATE CONSTANT. the pre-beta fraction on the basis of electro- venom substrate STUART FACTOR. phoresis. Abbr VLDL. See also lipoprotein. venous Of, or pertaining to, veins. very low-Iipid lipoprotein VERY HIGH-DENSITY ventilation The process whereby oxygen is sup- LIPOPROTEIN. plied to the lungs and to the blood in the vesicle 1. A small sac; a membranous cavity. 2. capillaries of the lungs; aeration of the blood A closed bilayer structure. The structure may in the lungs. consist of a single bilayer (unilamellar vesicle) venule A small vein or veinlet. or of several bilayers (multilamellar vesicle). verification The deacylation of a misacylated Unilamellar vesicles may be small (SUV; transfer RNA molecule by its aminoacyl-tRNA 0.02-0.05 fjim), or large (LUV; > 0.06 jjim); synthetase, thereby regenerating the uncharged multilamellar vesicles (MLV) range in size tRNA molecule. from 0.1 to 5.0 jxm. Vesicles formed from synthetic surfactants are called surfactant filaments are found in fibroblasts. vesicles; those composed of phospholipids vinblastine An indole alkaloid used as an anti- are called liposomes. tumor agent in the treatment of Hodgkin's vesicular stomatitis virus A virus of the genus disease and other cancers. Vinblastine inhibits rhabdovirus that infects cattle, pigs, and microtubule formation; it disrupts microtu- horses, producing symptoms as in foot and bules of the mitotic spindle and preferentially mouth disease. Abbr VSV. kills rapidly dividing cells. vesiculin A highly acidic, low molecular weight vinca alkaloids A group of indole alkaloids (10,000) protein in synaptosome vesicles. that includes mitotic poisons, hypertensive V genes Genes that code for segments of the agents, and anticancer compounds. variable region of immunoglobulin molecules. vincaleukoblastine VINBLASTINE. VH The variable region of the heavy chains of vincristine An indole alkaloid, related to vinb- the immunoglobulins. lastine and used in the treatment of leukemia VHDL Very high-density lipoprotein. and various other types of cancer. VI Initial velocity in the presence of an inhibi- vinculin A fibrous protein, believed to be in- tor. volved in anchoring actin filaments to the in- viable Describing a cell or an organism that is ner side of the cell membrane. Vinculin is also alive and capable of reproduction. present at patches on the cell surface, called viable count The number of viable cells in a adhesion plaques, believed to be responsible bacterial culture. for intercellular adhesion. Cells infected with Vi antigen A surface antigen of bacteria that is Rous sarcoma virus produce a kinase (src pro- different from the O antigen. tein) that modifies vinculin, a reaction that vibrating reed electrometer A sensitive electro- may play a role in cytoskeletal changes follow- meter that is used for measuring the small ing cell transformation by the virus. currents produced in an ionization chamber. vinegar souring ACETIFICATION. vibrational circular dichroism A spectroscopic vinyl The radical -CH=CH2 that is derived method in which one measures the difference from ethylene. in absorption of right- and left-circularly pola- viologen A 4,4'-bipyridinium salt. Compounds rized light by chiral compounds. Abbr VCD. of this type can be polymerized to yield redox vibrational optical activity A group of related polymers (polyviologens or polymeric violo- spectroscopic methods which includes vibra- gens) that can serve as electron donors for tional circular dichroism and Raman optical some dehydrogenases. activity. Abbr VOA. viosterol CALCIFEROL. vibrational transition The transition of a mole- VIP Vasoactive intestinal peptide. cule in which, as a result of the stretching or viral Of, or pertaining to, viruses. the bending of a bond, the molecule under- viral coat CAPSID. goes a vibration so that the atomic nuclei are viral hepatitis See hepatitis. temporarily displaced, but their equilibrium viral interference The inhibition of viral multi- positions are not changed. Vibrational transi- plication that may occur upon multiple infec- tions require energies that are intermediate tion of cells with the same virus or upon between those of electronic and those of rota- mixed infection with two or more different tional transitions, and they are induced by viruses; due to a large extent to the action of short wavelength infrared radiation. interferon that is produced in response to the vibronic transition A molecular transition in infecting viruses. which the energy of vibration is added to that viral multiplication cycle See multiplication of the electronic transition; a transition in cycle. which both the electronic and the vibrational viral particle VIRUS. quantum numbers change. viral-specific viRus-spECific. vie- Combining form meaning vicinal. viral yield The average number of infectious vicinal Referring to two substituents on adja- viral particles obtained per productive cell. cent carbon atoms. Abbr vie. viremia The presence of virus particles in the villi Large, fringe-like projections of an epithe- blood. Viremia is said to be primary or secon- lium. See also micro villi. dary depending on whether it is due to pro- villikinin A gastrointestinal hormone that con- geny virions produced principally at the site trols the movement of the villi. of the initial infection, or whether it is due villin A protein, isolated from intestinal micro- to those produced in other organs. villi, that binds to and fragments actin fila- virgin cell A lymphocyte in peripheral lym- ments. phoid tissue that has not yet encountered vimentin A protein (MW 55,000) that is a com- an antigen. When such a cell encounters an ponent of intermediate filaments; vimentin antigen, it may be stimulated to multiply and to become an effector cell or a memory cell. that they contain and the mechanism involved virial coefficient See virial expansion. in the production of viral mRNA (± repre- virial expansion A mathematical expansion of sents double-stranded nucleic acid; +DNA the general form Y = A + BX + CX2 + • • •, has the same polarity as the mRNA; +RNA where X is a variable and A9 B9 C, etc. are is identical to the mRNA; and -RNA is com- the first, second, third, etc., virial coefficients. plementary to the mRNA): Aka virial equation. Class I (±DNA) -» +mRNA -» Protein viricide A chemical or a physical agent that Class II (+DNA) -» ±DNA -> +mRNA inactivates viruses. —» Protein virion A complete viral particle consisting of a Class III (±RNA) -» +mRNA -» Protein nucleocapsid and any additional structural Class IV (+RNA) -» -RNA -> +mRNA proteins and/or envelopes; a virus. -» Protein virogene theory ONCOGENE THEORY. Class V (-RNA) -> + mRNA -» Protein viroid A virus-like infectious particle that Class VI (+RNA) -» -DNA -* ±DNA -» consists of a single-stranded, covalently closed, +mRNA -* Protein circular RNA molecule, does not have a pro- See also plus strand. tein coat, and has a molecular weight of about virus antigen A surface antigen of some 100,000 (about 300 nucleotides). Viroids are viruses. the causal agents of some plant diseases, such virusoid Plant satellite RNA. as potato spindle tuber disease, and are resis- virus receptor A cell membrane receptor to tant to a wide range of treatments to which which a virus attaches. viruses are usually sensitive. Viroids appear to virus-specific Designating a protein or a nuc- be related structurally to some introns and are leic acid molecule that is coded for by a viral believed by some to represent escaped in- gene rather than by a host cell gene, and that trons. Aka infectious RNA; pathogene; is produced in the host cell after its infection pathogenic RNA. by the virus. virology The science that deals with viruses virus theory of cancer See oncogene theory. and viral diseases. viscid viscous. viroplasms Electron-dense aggregates of viral viscoelastic Descriptive of a substance, such as particles or viral components that occur in the a concentrated polymer solution, that exhibits cytoplasm of some virus-infected cells. properties of both liquids and solids. viroplast One of a group of spherical or elon- viscometer An instrument for measuring the gated bodies that occur in the cytoplasm of viscosity of a liquid. some virus-infected plant cells; consists of an Visconti-Delbrueck hypothesis The hypothesis accumulation of viral particles with or without according to which the types and frequencies cytoplasmic components. of phage recombinants that are obtained upon virosome An artificial macromolecular com- phage infection of bacterial cells are due to plex that is formed when animal viral mem- the multiplication and the repeated mating of brane proteins and lipid films (prepared from the phages in the host cells. Mating occurs in lecithin and diacetyl phosphate) are mixed pairs and at random, and each mating pro- and dialyzed free of nonionic detergents. duces phage recombinants as a result of the virulence 1. PATHOGENICITY. 2. The multiplica- exchange of genetic material by one or more tion of virulent viruses. crossovers between the parental phages. The virulent Of, or pertaining to, virulence. total intracellular collection of vegetative virulent virus A virus that causes lysis of the phage genomes is called a mating pool. host cell that it infects; such a virus multiples viscose rayon Fibers of regenerated cellulose. in the host cell and forms progeny viral parti- viscosimeter VISCOMETER. cles which are released into the medium when viscosity The resistance of a fluid to flow; the the cell bursts. In the case of bacteria, a viru- internal friction of a fluid. For Newtonian lent phage cannot become a prophage. fluids, the viscosity is the force required to virus An infectious agent that consists of pro- maintain unit velocity for a fluid flowing be- tein and either DNA or RNA, both of which tween two parallel plates of unit area each and are arranged in an ordered array and are a unit distance apart. Sym T). Aka coefficient sometimes surrounded by a membrane. A of viscosity. virus is generally smaller than a bacterium and viscosity-average molecular weight An average is an obligate intracellular parasite at the molecular weight that is obtained from viscos- genetic level; it uses the cell machinery to ity measurements. It is given by Mv = 17 produce viral products specified by the viral (En1-M*+VEn/M/) " where nt is the number nucleic acid. Viruses can be classified into 6 of moles of component /, MI is the molecular classes on the basis of the type of nucleic acid weight of component i, and a is a quantity that varies with _the system being studied. also mechanistic philosophy. When a = 1, MV becomes equal to the vital stain A stain that can penetrate the cell weight-average molecular weight (Afw)» when membrane of a living cell and that can stain OL _< 1, MV falls between the number-average the contents without injury to the cell. (Mn) and the weight-average molecular vitamer One of two or more forms of a vita- weight; and when a > 1, Mv falls between the min; vitamins AI and A2 are examples of weight-average molecular weight and_the z- vitamers. average molecular weight (M2). Sym Mv. vitamin An organic compound that (a) occurs viscosity increment A measure of the asymmet- in natural food in extremely small concentra- ry of a solute molecule that is equal to the tions and is distinct from carbohydrates, ratio of the intrinsic viscosity of the solution lipids, proteins, and nucleic acids; (b) is re- to the partial specific volume of the solute. quired by the organism (generally restricted to viscosity number REDUCED VISCOSITY. animals) in minute amounts for normal health viscosity ratio RELATIVE VISCOSITY. and growth, and generally functions as a com- viscotoxins A group of homologous proteins, ponent of a coenzyme; (c) when absent from containing 46 amino acid residues and 3 dis- the diet, or improperly absorbed from the ulfide bonds; the viscotoxins are plant toxins food, leads to the development of a specific (phytotoxins) that act as hypotensive agents, deficiency disease; (d) cannot be synthesized slowing the rate of heart beat. by the organism and must, therefore, be viscous 1. Possessing viscosity. 2. Thick; sticky; obtained exclusively through the diet. glutinous. vitamin A A generic descriptor of all p-ionone viscous drag The factional force that counter- derivatives, other than provitamin A caro- acts and balances either the electrial driving tenoids, that exhibit qualitatively the biologi- force in electrophoresis or the centrifugal cal activity of all-trans-retinol. Vitamin A force in sedimentation. is a fat-soluble vitamin that is structurally visible dichroism The dichroism that is pro- related to the carotenes and that is required duced when visible polarized light is absorbed for certain aspects of metabolism, particu- by oriented samples. larly the biochemistry of vision. Vitamin A1 visible mutation A mutation that results in (retinolj) predominates in higher animals and some alteration of the morphology of an marine fish, and vitamin A2 (retino!2) pre- organism. dominates in freshwater fish; the two forms visible spectrum That part of the electro- differ by one double bond in the molecule. A magnetic spectrum that covers the wavelength deficiency of vitamin A causes night blindness range of about 4 x 10~5 to 7.5 x 10~5 cm and and xerophthalmia. The recommended names that includes photons that are emitted or for vitamin A derivatives are as follows: re- absorbed during electronic transitions. tinol (vitamin Ax alcohol; axerophthol); retinal visual cycle A cyclic set of reactions that occur or retinaldehyde (vitamin A1 aldehyde; re- in both the rods and the cones of the retina tinene); retinoic acid (vitamin A1 acid); and whereby (a) light leads to the isomerization of 3-dehydroretinol (vitamin A2). See also anti- 11-cw-retinal to the all-frans-retinal and to its promoter. dissociation from the appropriate opsin, and vitamin AI See vitamin A. (b) the all-trans isomer is reconverted en- vitamin A2 See vitamin A. zymatically to the 11-cis isomer which recom- vitamin A1 acid See vitamin A. bines with the opsin. vitamin AI alcohol See vitamin A. visual pigment One of several conjugated pro- vitamin AI aldehyde See vitamin A. teins that consist of an opsin and a form of vitamin B 1. VITAMIN B COMPLEX. 2. The origin- vitamin A aldehyde and that function in the al antiberiberi activity. biochemical reactions that pertain to vision. vitamin BI THIAMINE. visual purple RHODOPSIN. vitamin B2 RIBOFLAVIN. visual threshold The minimum light intensity vitamin B3 PANTOTHENIC ACID. required to produce a visual sensation. vitamin B4 An activity, isolated from yeast or vital capacity The greatest volume of air that liver, that could alleviate muscular weakness can be expired after a forced inspiration; in- in rats and chicks. The existence of vitamin B4 cludes the tidal, supplemental, and comple- has not been confirmed since all purported mental airs. vitamin B4 deficiency symptoms could be vitalism The doctrine that life and its phe- alleviated by known nutritional factors such as nomena are not fully explicable in terms of thiamine, glycine, arginine, and cystine. the laws and processes of chemistry and phy- vitamin B5 A growth-stimulating activity in sics, and that they require special vital forces pigeons that is probably identical with nico- that are found only in living organisms. See tinic acid. vitamin B6 A generic descriptor for all 3- anemia in the chick and that is known to be hydroxyl-2-methyl pyridine derivatives that folic acid. exhibit qualitatively the biological activity of vitamin B complex A group of diverse water- pyridoxine. Major forms of the vitamin are soluble vitamins that are classified as a group pyridoxine, pyridoxamine, and pyridoxal. primarily for historical reasons though, to Vitamin B$ is widely distributed in nature, some extent, they are found together in and its phosphorylated forms function as nature. The complex includes niacin, coenzymes in amino acid metabolism, as in riboflavin, folic acid, thiamine, pyridoxine, the transamination reaction. pantothenic acid, biotin, and cobalamine. The vitamin B7 CARNITINE. compounds choline, lipoic acid, inositol, and vitamin B8 The nucleotide adenylic acid that is p-aminobenzoic acid are usually also classified no longer classified as a vitamin. as B vitamins. Most, if not all, of the B vita- vitamin B9 A designation that has not been mins function as components of coenzymes. used for a B vitamin. vitamin B group VITAMIN B COMPLEX. vitamin Bi0 An activity that promotes growth vitamin Bp An activity that prevents perosis in and feathering in the chick and that is a mix- the chick and that is replaceable by a mixture ture of folic acid and vitamin Bi2. of choline and manganese. vitamin B11 An activity that promotes growth vitamin Bt An activity that promotes insect and feathering in the chick and that is a mix- growth and that contains carnitine as the ac- ture of folic acid and vitamin B12. tive component. vitamin B12 A generic descriptor for all corri- vitamin Bw BIOTIN. noids that exhibit qualitatively the biological vitamin Bx p-AMINOBENZOIC ACID. activity of cyanocobalamin. Vitamin B12 is a vitamin C A generic descriptor for all com- cobalt-containing vitamin and its inadequate pounds that exhibit qualitatively the biological absorption from the intestine causes perni- activity of ascorbic acid. Vitamin C is a water- cious anemia. Coenzyme forms of vitamin B12 soluble vitamin that occurs in fruits and veget- are termed cobamides. Different forms of ables. Vitamin C functions in the regulation of vitamin B12 are named by reference to cobala- oxidation-reduction reactions in metabolism; min, a derivative of cobamide. See also coba- a deficiency of vitamin C causes scurvy. See mide; cobalamin. also antipromoter. vitamin B13 A compound, isolated from distil- vitamin C2 See bioflavonoids. lers dried solubles, that was provisionally cal- vitamin D A generic descriptor for all steroids led vitamin B13 and later shown to be orotic that exhibit qualitatively the biological activity acid, an intermediate in pyrimidine metabol- of cholecalciferol; a group of fat-soluble vita- ism. Orotic acid is not recognized as a vitamin mins, structurally related to the sterols, that by United States and Canadian drug author- are active in the prevention and cure of rick- ities. ets. Since they can be derived from sterols by vitamin B14 An unconfirmed vitamin. ultraviolet irradiation, vitamin D is not re- vitamin B15 Preparations marketed in the Un- quired in the diet if the organism has adequ- ited States as vitamin B15 or pangamate (pan- ate access to ultraviolet light (present in sun- gamic acid) have been shown to contain one light). Vitamin D affects the absorption and or more of the following: calcium glucon- deposition of calcium phosphate. See also ate, glycine, W,W-dimethylglycine, and N, N- 1,25-dihydroxycholecalciferol. diisopropylamine dichloroacetate. Only the vitamin D1 Originally considered to be a pure latter compound has pharmacological activity vitamin but later shown to be a mixture of leading to decreased blood pressure and body vitamin D2 (ergocalciferol) and lumisterol. temperature in rats. Since the compositions of vitamin D2 ERGOCALCIFEROL. these preparations are undefined, adequate vitamin D3 CHOLECALCIFEROL. clinical studies to test the claims for panga- vitamin D-resistant rickets A form of rickets mate have not been made. At present, there that is indistinguishable in its symptoms from is no scientific evidence that pangamate pre- ordinary rickets but differs from the latter in parations have vitamin activity. being resistant to treatment with vitamin D. It vitamin B17 A term used to describe laetrile can be due to a number of unrelated causes. and/or amygdalin. Neither laetrile nor amyg- The most common form is a genetically inher- dalin (nor any similar cyanogenic glucosides) ited metabolic defect involving the renal are recognized by United States or Canadian transport mechanism for phosphorus. drug authorities as vitamins; nor is the term vitamin E A generic descriptor for all tocol vitamin B17 recognized by these authorities. and tocotrienol derivatives that exhibit qual- See also laetrile; amygdalin. itatively the biological activity of a-tocop- vitamin Bc An activity that prevents nutritional herol; a group of fat-soluble vitamins that are required for normal growth and fertility vitamin B12, deoxyribesides, and amino acids. of animals. Tocopherols are widely distributed vitamin U The methylsulfonium salts of in nature and function primarily as antiox- methionine, which occur naturally in cabbage idants. See also antipromoter. and other green vegetables, and which have vitamin F 1. Obsolete designation for the been claimed to alleviate peptic ulcers in activity of the essential fatty acids as reflected guinea pigs. These claims were not supported in the prevention of atherosclerosis in anim- by later clinical trials and the compounds are als; confirmed to be the compound cis,cis- not recognized as vitamins by United States linoleic acid. 2. Obsolete designation for and Canadian drug authorities. thiamine. vitamin V An activity from tissue that prom- vitamin G Obsolete designation for riboflavin. otes bacterial growth and that has been shown + vitamin GH3 See gerovital. to be a mixture of NAD and NADH. vitamin H Obsolete designation for biotin. vitellenin The lipid-free protein of lipovitelle- vitamin H3 See gerovital. nin. vitamin hypothesis The hypothesis that certain viteUin The principal protein in hens' egg yolk; compounds having the properties of vitamins the lipid-free protein of lipovitellin. A gly- constitute an essential dietary requirement of coprotein that contains about 1% phosphate an organism. and that is derived from vitellogenin. vitamin I Obsolete designation for vitamin B7. viteUine membrane A membrane that surrounds vitamin J Obsolete designation for vitamin €2. the ovum. vitamin K A generic descriptor for 2-methyl- viteUogenesis The formation of egg yolk. 1,4-naphthoquinone and all of its deriva- viteUogenic hormone ALLATUM HORMONE. tives that exhibit an antihemorrhagic activity vitellogenin An egg yolk glycoprotein of amp- in animals fed a vitamin K-deficient diet. hibia and birds. It is the precursor of the The compound 2-methyl-3-phytyl-l,4-nap- proteins phosvitin and lipovitellin. The synth- hthoquinone is generally called vitamin K esis of vitellogenin is stimulated by female sex or phylloquinone (phytylmenaquinone; phyto- hormones. nadione). The vitamin K derivatives represent vitreous Glass-like. a group of widely distributed fat-soluble vita- vitreous humor The gel-like material that fills mins that have quinone-type structures and the posterior cavity of the eye. that are required for the formation of vivisection The cutting of, or operating on, a prothrombin. A deficiency of vitamin K living animal for purposes of experimentation. causes prolonged clotting times and hemor- vL The variable region of the light chains of rhagic disease. the immunoglobulins. vitamin L1 A liver filtrate activity believed to VLCFA Very long-chain fatty acids. be necessary for lactation and shown to be VLDL Very low-density lipoprotein. anthranilic acid. VMA Vanilmandelic acid. vitamin L.2 A yeast filtrate activity believed to Vmax Maximum velocity. be necessary for lactation and shown to be VOA Vibrational optical activity. adenylthiomethylpentose. Voges-Proskauer test A test for organisms that vitamin M An activity that prevents nutritional carry out butylene glycol fermentation; based anemia and leucopenia in the monkey; the on the production of a pink color by the reac- compound is an active form of folic acid. tion of acetoin, formed during the fermenta- vitamin N Obsolete designation for a mixture tion, with creatine in an alkaline solution. obtained from either brain or stomach and void volume 1. The volume of solvent in col- believed to inhibit cancer. umn chromatography that is equal to the total vitamin P See bioflavonoids. bed volume of the column minus the volume vitamin PP Obsolete designation for nicotinic occupied by the particles of the support; the acid. volume of solvent that is external to the sup- vitamin R An activity that promotes bacterial port particles. 2. OUTER VOLUME. growth and that is apparently related to folic vol Volume. acid. volatile buffer A buffer that can be evaporated vitamin S An activity that promotes the growth without leaving a residue, as one consisting of of chicks and that is related to the peptide ammonium formate and ammonium hydrox- streptogenin. ide. vitamin T A group of activities isolated from volt A unit of electrical potential which is termites, yeasts, or molds and reported to in- equal to the potential required to make a cur- crease growth rates, improve wound healing, rent of 1 A flow through a resistance of 1 H. alleviate skin disorders, and accelerate insect SymV. development; a varied mixture of folacin, voltage clamp technique A technique for study- ing the effects of membrane potentials that in which gas volumes are measured. are intermediate between the resting potential volutin granule A microbial storage granule of and the peak of the action potential; based on polymetaphosphate that stains metachromati- the production of a sudden displacement of cally; found in cyanobacteria, yeast, and other the membrane potential from its resting value microorganisms. by means of a pair of electrodes, and on hold- von Gierke's disease GLYCOGEN STORAGE DIS- ing the potential across the membrane at this EASE TYPE I. new level by means of a feedback amplifier. von Willebrand factor FACTOR VIH. The current that flows through a definite area von Willebrand's disease A genetically inher- of the membrane, maintained by a space ited metabolic defect in humans, characte- clamp, under the influence of this applied vol- rized by blood coagulation disorder and slow tage (called command voltage) is then mea- aggregation of platelets, and due to a deficien- sured with a separate pair of electrodes and a cy of Factor VIII. separate amplifier. vortex stirrer A device for mixing a solution in voltage-gated channel See gated channel. a test tube by forming a vortex in the solution. voltammetry The electroanalytical study of VP Vasopressin. chemical rections by the measurement of the VPC Vapor phase chromatography. currents produced by the electrolysis of ox- VPg Genome-linked viral protein; a small pro- idizable or reducible substances as a function tein that is linked covalently to the 5'-end of of the applied voltage. the RNA of polio virus. Its function is to prime voltanunogram See cyclic voltammetry. viral replication. volume fraction The fraction of the solution V region Variable region. volume that is occupied by the solute. VSV Vesicular stomatitis virus. volume receptor A receptor in the central ner- V system A system composed of a regulatory vous system that responds to changes in the enzyme and an effector, such that the effector volume of the blood. alters the maximum velocity of the reaction volumetric analysis A method of chemical but does not alter the substrate concentration analysis that is based on the measurement of at which one half of the maximum velocity is the volume of a standard solution which is obtained. required to react completely with a sample of vulcanization The cross-linking of natural rub- the substance that is being determined. ber chains by means of sulfur. volumetric pipet A pipet that is enlarged to a v/v The concentration of a solution that is ex- bulb in its center and that is used for the pressed in terms of volume per unit volume. transfer of a fixed volume of liquid. vWF von Willebrand factor. volumetric technique A gasometric technique W

W 1. Tryptophan. 2. Watt. 3. Wyosine. 4. method for studying the cellular respiration of Writhing number. tissue slices or cells in which the only gases Walden inversion The alteration of the con- exchanged are oxygen and carbon dioxide, figuration of an asymmetric center in a mole- and in which respiration is measured by the cule as a result of a bimolecular displacement absorption of carbon dioxide in alkali. reaction. Warburg's respiratory enzyme CYTOCHROME ox- walking See chromosome walking. ^DASE. walking down the helix The movement of the Warburg's yellow enzyme OLD YELLOW EN- replisome along the DNA during replication. ZYME. wall effect The spreading and curving of a zone Warburg theory A theory of cancer that is as it migrates downward in a chromatographic based on the universal occurrence and import- column as a result of the increased flow rate at ance of glycolysis in the metabolism of cells. the wall of the tube compared to that at the According to this theory, cancer results from center. an irreversible injury to respiration, specifical- walling-off effect AFFERENT INHIBITION. ly to the electron transport system, which is wandering spot procedure A method for se- followed by a changeover in the cell to an quencing synthetic oligonucleotides. Involves anaerobic, glycolytic, and fermentative-type labeling the oligonucleotide at the 5'-end with metabolism for energy production; the main 32P, using polynucleotide kinase, and subse- difference between a normal and a tumor cell quent partial digestion of the oligonucleotide, is, therefore, a shift in metabolism toward using snake venom phosphodiesterase. The that of an anaerobic state, in which glycolysis resulting labeled fragments are fractionated, is emphasized. first by electrophoresis on cellulose acetate Warburg vessel A receptacle for the tissue paper at pH 3.5, and then by homochroma- slices or the cells, the cellular respiration of tography on DEAE-cellulose. The sequence which is being measured in a Warburg appar- is determined by the characteristic mobility atus. shifts of the labeled fragments. warfarin A synthetic compound, closely re- Warburg apparatus A manometer that is used lated to dicumarol, that is used as a rat for studying the cellular respiration of tissue poison; it blocks prothrombin activation in slices or cells by measuring oxygen uptake and blood clotting. /or carbon dioxide evolution. Aka Warburg Waring Blendor Trademark for a blender used manometer; Warburg-Barcroft apparatus. in the preparation of tissue homogenates. Warburg coefficient METABOLIC QUOTIENT. warm antibody An antibody that has a higher Warburg-Dickens-Horecker cycle HEXOSE MONO- titer at elevated temperatures. PHOSPHATE SHUNT. warm-blooded HOMOIOTHERMIC. Warburg-Dickens pathway HEXOSE MONOPHOS- Wasserman test A complement fixation test PHATE SHUNT. that is used in the diagnosis of syphilis and Warburg effect 1. The inhibition of photo- that is based on the reaction of cardiolipin synthesis by high concentrations of oxygen. 2. with the Wasserman antibody; cardiolipin The overproduction of lactic acid that occurs serves as an antigen and is mixed with lecithin in many tumors. See also Warburg theory. and cholesterol to form micelle-like structures Warburg method A manometric method for that enhance its reactivity. studying the cellular respiration of tissue slices wasting disease A disease that is characterized or cells by means of a Warburg apparatus; by a decrease in the amount of certain tissues the tissue slices or cells are maintained at a or organs; a pathological condition in which constant temperature and the changes in there is atrophy. gas pressure are measured with a constant vol- water balance The reactions and factors in- ume manometer. See also Warburg's direct volved in maintaining a constant internal en- method. vironment in the body with respect to the Warburg's atmungsferment CYTOCHROME ox- distribution of water between the various fluid IDASE. compartments and with respect to the estab- Warburg's direct method A manometric lishment of an equilibrium between the intake of water and its output. the sense strand. Abbr W strand. water bath A water-filled container for main- watt A unit of power equal to 1 J/s. Sym W. taining immersed tubes, flasks, etc., at a con- wavelength The distance, along the axis of stant temperature. propagation, between two identical points of a water compartment FLUID COMPARTMENT. wave. waterfall sequence CASCADE MECHANISM. wave mechanics See quantum mechanics. water hydrate model A model for the structure wave number The number of waves per unit of water according to which the water struc- length; the reciprocal of the wavelength. ture results from the formation of clathrate- wave shifter SECONDARY FLUOR. type structures which consist of ordered, but wax A neutral lipid consisting of esters formed highly random, labile frameworks in which from fatty acids and long-chain alcohols other the cages are occupied by unbonded water than glycerol. molecules. wax acids Fatty acids that occur in waxes. water intoxication An extreme case of hypoto- wax alcohols Long-chain, monohydroxy, ali- nic expansion that may lead to convulsions phatic alcohols that form waxes by esterifica- and death. tion to fatty acids. water of hydration Water that surrounds and WBC White blood cell. binds to solute molecules and/or forms hy- weak electrolyte An electrolyte that is only drates. partially dissociated into ions in water. water regain The uptake of water by a dry gel weak interactions The attractive and repulsive expressed as grams of water taken up per forces between atoms and molecules that are gram of gel when the latter is suspended in less strong than those of covalent bonds, ionic water and allowed to swell. bonds, and ion-ion interactions. Weak in- water softening The conversion of hard to soft teractions include the forces of hydrogen water by ion exchange chromatography or by bonds, hydrophobic bonds, van der Waals in- some other method. teractions, and charge fluctuation interactions. water-soluble B An early designation for a weak promoter LOW-LEVEL PROMOTER. fraction of water-soluble vitamins prepared weight-average molecular weight An average from egg yolk. molecular weight that is weighted toward the water-soluble vitamin One of a group of vita- heavier molecules in a mixture of molecules; 2 mins that are soluble in water and that include specifically, Mw = 2>i,-Af/ /2>i/M/, where w, is those of the vitamin B complex as well as the number of moles of component i, and M1 some other vitamins such as vitamin C and is^ the molecular weight of component i. Sym vitamin P; most, if not all, of the water- Mw. See also average molecular weight. soluble vitamins function by being compon- weighted average The average of a set of num- ents of coenzymes. bers obtained by multiplying each number by water structure See distorted bond model; a weight (factor) expressing its relative im- flickering cluster model; vacant lattice point portance and then dividing the sum of these model; water hydrate model. products by the sum of the weights. Watson-Crick model The model proposed by weighted mean WEIGHTED AVERAGE. Watson and Crick in 1953 for the structure of Weigle reactivation uv REACTIVATION. DNA. According to this model, DNA consists Weinberg-Salam theory A unified field theory of two right-handed helical polynucleotide of the weak and electromagnetic forces of chains coiled around the same axis to form a particle physics. See also elementary particles. double helix. The chains are antiparallel, with Welan gum A water-soluble, heteropolysac- the deoxyribose-phosphate backbone on the charide, produced by fermentation of some outside and the purines and pyrimidines stack- species of Alcaligenes, that yields solutions of ed perpendicularly to the fiber axis on the high viscosity at low concentrations. inside. The bases are held together by hyd- western blotting A variation of the Southern rogen bonds and they are specifically paired: blotting technique in which proteins are sepa- adenine and thymine by means of two hyd- rated electrophoretically, transferred to a rogen bonds, and guanine and cytosine by special paper which binds them covalently, means of three hydrogen bonds; the two and are then located by reaction with a radio- chains are, therefore, complementary. active probe (for example, radioactive double- Watson-Crick-type DNA A DNA molecule stranded DNA for DNA binding proteins or that can be described by the Watson-Crick radioactive antibodies for protein antigens). model of DNA. Aka western transfer. See also blotting. Watson strand The DNA strand of Watson- wet application A method of sample applica- Crick-type DNA that is transcribed in vivo; tion in electrophoresis in which the sample is applied to a previously wetted support (such wild-type 1. The typical, most frequently en- as cellulose acetate, for example). countered phenotype of an organism in na- wetting agent A surface-active agent that en- ture. 2. The phenotype of an organism that is hances the spreading of a liquid on a solid used as a standard of comparison for mutants surface. of the same organism. wet weight The weight of a tissue, or of col- wild-type allele See wild-type gene. lected cells, from which water has not been wild-type gene 1. The normal, most frequently removed, encountered allele of a given gene of an Wharton's jelly A viscous solution from the organism. 2. An allele of a given gene that is umbilical cord that is frequently used as a arbitrarily selected as a standard of compari- starting material for the preparation of hyalur- son for mutant alleles of the same gene. onic acid. Willebrand factor See von Willebrand factor. Whatman Trademark for a group of filter pap- Wilson chamber CLOUD CHAMBER. ers. Wilson's disease A genetically inherited meta- wheal and erythema response A local, bolic defect in humans that is due to a de- cutaneous anaphylactic reaction in humans ficiency of ceruloplasmin, resulting in an in- that is produced by the intracutaneous in- crease in the level of copper in the brain and jection of an allergen. Aka wheal and flare in the liver. response. Wilzbach method A method for the random wheat germ agglutinin A lectin, isolated from labeling of a compound with tritium by expos- wheat germ, that binds to Af-acetyl- ing the compound to tritium gas in a sealed glucosamine on cell surfaces. container for several weeks. A modification whey proteins The milk proteins that are of this method entails the exposure of the obtained by acidifying skim milk to pH 4.7 compound to tritium gas in the presence of and removing the precipitated casein. Whey a silent electric discharge. Aka Wilzbach gas proteins account for about 20% of the total exposure. milk proteins. winding number LINKING NUMBER. white adipose tissue WHITE FAT. window CHANNEL. white fat 1. Ordinary adipose tissue as opposed windowless counter A radiation counter in to brown fat. 2. Adipose tissue in which the which the sample is not separated from the fat is present in a single droplet within the fat ionization detector by either a window or a cell. Aka white adipose tissue. See also brown membrane. fat. windowless gas flow counter A radiation coun- white muscle A pale skeletal muscle that has a ter that incorporates the characteristics of relatively low content of myoglobin and both a gas flow and a windowless counter. cytochromes; it is nearly devoid of mitochon- wobble base The third base (the 5'-end) in the dria and obtains almost all of its ATP from anticodon of transfer RNA; the wobble base glycolysis. It is capable of short bursts of can bind with one of several possible bases at activity. Aka fast-twitch muscle. the 3'-end of the codon. white plaque A plaque that does not stain with wobble hypothesis A hypothesis proposed by neutral red in the plaque assay; the lack of Crick to explain how one tRNA molecule can staining is due to a destruction of lysosomes in "recognize" more than one codon. According the virus-infected cells. to this hypothesis, the first two bases (the White's solution A synthetic medium for the 3'-end) of the anticodon in tRNA bind to the growth of plant cells. first two bases (the 5'-end) of a codon in regu- whole blood Blood that has not been fraction- lar base-pairing fashion. The third base, ated in any way. however, of the tRNA anticodon (the 5'-end), whole body counter A large, external-sample- while hydrogen bonding, has a certain amount type liquid scintillation counter that is de- of play or wobble that permits it to bind to signed for the determination of total body one of several possible bases at the 3'-end of radioactivity in humans or in animals. the codon. whole plasma Plasma that has not been frac- wobble pairing The base pairing that is allowed tionated in any way. according to the wobble hypothesis. whole serum Serum that has not been fraction- Wohl-Zemplen degradation A degradative ated in any way. technique for aldoses whereby the carbon of wiggle SQUIGGLE. the reducing group is removed by treatment Wys iodine number An iodine number deter- with hydroxylamine, and the sugar is con- mined by the use of a solution of iodine in verted to the next lower aldose. glacial acetic acid, with iodine chloride serving Wolfson's method An analytical procedure for as an accelerator of the reaction. determining gamma globulins in serum by precipitating them with ammonium sulfate that of a random coil. The wormlike coil mod- and sodium chloride. el can be considered to be a limiting case of a Wolman's disease A genetically inherited freely jointed chain. See also freely jointed metabolic defect in humans that is due to a chain. deficiency of a lysosomal lipase (cholesterol wound tumor virus A plant virus, containing ester hydrolase) that normaly catalyzes the double-stranded RNA, that infects a large hydrolysis of cholesterol esters carried by low- number of unrelated plants. density lipoproteins; one of the lysosomal woven bone Bone that consists of random, diseases. nonparallel collagen fibers; found in embry- wood alcohol Methanol; methyl alcohol. onic life and during bond repair in adults. wood sugar D-XYLOSE. W reactivation Weigle reactivation; see UV WOOl fat LANOLIN. reactivation. Woolf-Augustinsson plot A single-reciprocal writhe WRITHING NUMBER. plot of the Michaelis-Menten equation; a plot writhing number The number of superhelical of v as a function of v/[S], where v is the turns in DNA; the number of turns that the velocity of the reaction and [S] is the substrate axis of the double-stranded helix makes in concentration. Aka Woolf-Augustinsson- space in the process of forming the superhelix. Hofstee plot. The writhing number (W) is a measure of WOOl Wax LANOLIN. the extent of supercoiling of the DNA. It is word CODON. related to the linking number (L) as follows: working hypothesis A hypothesis the formula- L = W + T, where T is the twisting number. tion of which provides a basis for further ex- Sym W. periments. W strand Watson strand. wormlike coil model A model that is frequently wt Weight. invoked to describe the DNA molecule in w/v The concentration of a solution that is ex- solution. The model is characterized by a con- pressed in terms of weight per unit volume. tour length and a persistence length; the latter w/w The concentration of a solution that is increases with increasing stiffness of the mole- expressed in terms of weight per unit weight. cule but is independent of the former. wyosine A hypermodified nucleoside, related According to this model, the DNA molecule to guanosine, that is found in tRNA; the base is best approximated as a rod having a con- consists of three nitrogen-containing, fused tinuum of flexible distortions; such a structure rings. Abbr W. is intermediate between that of a rigid rod and X

X 1. Xanthine. 2. Xanthosine. 3. An uniden- anesthetics, and petroleum products are some tified amino acid in an amino acid sequence. examples. 2, adj Of, or pertaining to, a com- Xan Xanthine. pound that is foreign to living systems. xanthine A purine that is formed in catabolism xenogeneic Referring to a transplant from one by the deamination of guanine, and that does species to another. not occur in nucleic acids. Abbr Xan; X. xenograft HETEROGRAFT. xanthine oxidase An enzyme of purine catabol- xenoplastic Referring to a transplant from one ism that catalyzes the oxidation of xanthine to genus to another. uric acid and the oxidation of hypoxanthine xenosome Proposed term for intracytoplasmic to xanthine; a molybdenum-containing flavo- symbiotic bacteria found in the marine proto- protein that also catalyzes the oxidation of zoan Parauronema acutum. These symbionts aldehydes. Abbr XO. are obligate parasites that grow and divide in xanthine oxidase factor Obsolete designation synchrony with the host and cannot be cul- for inorganic molybdate. tured outside the protozoan. xanthinuria A genetically inherited metabolic xenotropic virus An endogenous retrovirus defect in humans that is characterized by the that does not replicate in animal cells in which presence of excessive amounts of xanthine in it is endogenous but can grow in cells of other the urine and that is due to a deficiency of the species. enzyme xanthine oxidase. xero- Prefix meaning dry. xanthoma A deposit of lipid in the skin; usual- xeroderma pigmentosum A genetically inher- ly yellow-orange in color due to the presence ited metabolic defect in humans that is due to of lipid-soluble pigments such as carotenes. a deficiency in the excision-repair of pyrimi- xanthophyll An oxygenated carotenoid. dine dimers in DNA. Patients with this dis- xanthoproteic reaction A reaction for the qual- ease are abnormally sensitive to sunlight and itative determination of proteins that is based show a high incidence of skin cancer. on the successive production of a white, yel- xerogel A gel in which the removal of the dis- low, and orange precipitate upon treatment of persing agent (the solvent) results in the struc- the sample with nitric acid, followed by heat- ture shrinking to an unswollen state. ing and the addition of alkali. xerophthalmia A pathological change in the xanthopterin A pterin that functions as a pig- eye that results from a deficiency of vitamin ment in insects. A; involves an extreme dryness of the con- xanthosine The ribonucleoside of xanthine. junctiva which loses its luster and becomes Xanthosine mono-, di-, and triphosphate are skin-like from a lack of intrinsic secretion. abbreviated, respectively, as XMP, XDP, and xerophthol VITAMIN A. XTP. The abbreviations refer to the 5'- xerophyte A plant growing in an arid environ- nucleoside phosphates unless otherwise indi- ment; a drought-resisting plant. Var sp cated. Abbr Xao; X. serophyte. xanthurenic aciduria A genetically inherited Xis Excision ase. metabolic defect in humans due to a defici- X linkage SEX LINKAGE. ency of the enzyme kynureninase, an enzyme XMDIC Xylylene-w-diisocyanate; a bifunc- of tryptophan metabolism. tional reagent used to label antibodies with xanthylic acid The ribonucleotide of xanthine. ferritin. Xao Xanthosine. XMP 1. Xanthosine monophosphate (xanthylic X body VIROPLAST. acid). 2. Xanthosine-5'-monophosphate (5'- X cell See XYZ cell theory. xanthylic acid). X chromosome A sex chromosome of which XO Xanthine oxidase. the female generally carries two and the male x ray An electromagnetic radiation that is pro- carries one per cell. duced when high-speed electrons are suddenly XDP 1. Xanthosine diphosphate. 2. Xan- stopped by a metal target. The impinging thosine-5' -diphosphate. electrons raise target electrons to higher ener- xeno- Prefix meaning foreign. gy levels, and when these electrons fall back xenobiotic 1. n A synthetic compound that is to lower energy levels the excitation energy foreign to living systems; drugs, insecticides, is given off in the form of x rays. The wavelength of x rays extends from about 10 10 X2 test Chi-square test. to 2.5 x 10~6 cm; x rays are a form of ionizing XTP 1. Xanthosine triphosphate. 2. radiation. See also hard x rays; soft x rays. Xanthosine-5' -triphosphate. x-ray analysis X-RAY CRYSTALLOGRAPHY. XuI Xylulose. x-ray crystallography The study of the three- XyI Xylose. dimensional structure of molecules in a crystal xylan A homopolysaccharide of xylose that by means of x-ray diffraction. occurs in plants. x-ray diffraction The diffraction patterns xylem A group of specialized plant cells that obtained when x rays are reflected from the are dead and that have thickened cell walls. atoms in a crystal; a useful method for deter- They are aligned to form tubes and function mining the structure of macromolecules. The in the transport of water and inorganic ions in x rays are scattered by the extranuclear elec- the plant. See also phloem. trons of the atoms as opposed to neutrons xylitol A sugar alcohol derived from xylose which are scattered by the atomic nuclei. that can be used as a sugar substitute in the Abbr XRD. Aka x-ray scattering. treatment of human diabetes. x-ray diffraction pattern The pattern of spots, xylose A five-carbon aldose. Abbr XyI. Aka arcs, and rings that is produced by x-ray dif- wood sugar. fraction. xylulose A five-carbon ketose that is an inter- x-ray film A photographic film that is coated mediate in the hexose monophosphate shunt. with a sensitive emulsion on both sides and Abbr XuI. that is used in x-ray crystallography. X-Y recorder A recorder in which two signals x-ray microanalysis A technique for determin- are recorded simultaneously by one pen; the ing the relative and absolute concentrations pen is driven in one direction, the X axis, in of elements within cells and tissues. Based response to one signal, and it is driven in the on bombarding freeze-dried specimen sections other direction, the Y axis, in response to the with electrons (as in the electron microscope) second signal. and then analyzing the x rays produced by XYZ cell theory A theory according to which various metallic and nonmetallic elements in immunocytes are classified into three categor- the specimen. ies: X cells are immunologically competent x-ray structure The structural arrangement of cells that are not yet engaged in any specific the atoms in a molecule or in a crystal as immunological response; Y cells are immuno- deduced from x-ray diffraction patterns. logically activated, or primed, cells as a result XRD X-ray diffraction. of an interaction between X cells and antigen; XRF X-ray fluorescence; a spectrophotometric and Z cells are antibody-producing cells, method in which fluorescence is produced by formed as a result of a second stimulation of means of x rays. the Y cells by antigen. Aka Sterzl theory.