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Characterization and Determination of Muscle Connective Tissue Components

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Characterization and Determination of Muscle Connective Tissue Components 242. CHARACTERIZATION AN0 OETERMINA TlON OF MUSCLE CONtdEC T I YE T I'SSUE COMPONNZN TS The muscle connective tissue components have long been linked with meat tenderness and for this reason their determination and, more re- cently, their characterization have received considerable attention from meat researchers. Already in the 1930rs, Helser, Bull, Mitchell, Hamilton, and others reported a negative correlation between the quantity of connec- tive tissue and meat tenderness. However, these early studies were handi- capped by the lack of adequate techniques for the quantitative determination of connective tissue. In 1941, I~wryand comrkera (15) published a tech- nique for collagen and elastin determination Based on the insolubility or" these proteins. Neuman and Logan (22) presented the first adequate method for hydroxyproline determination in 1950. Although both of these techniques are still widely used today in modified forms, several studies have shown that neither is completely satisfactory for the determination of connective tissue in meat (1, 12, 16, 39). bbre recently, the chemical makeup and the three-dimensional structure of connective tissue components have been extensively investi- gated (6). These studies have revealed mrked differences in the chemical structure of connective tissue isolated from animals of different ages. It io not unreasonable to wsume that the three-dimensional structure as well as the amounts of the connective tissue cornnents might be involved in meat tenderness. In fact, this disregard of the three-dimensional struc- ture may be responsible for some of the conflicting reports regarding the importance of connective tissue in meat tenderness. In order to make an intelligent analysis of the problems of con- nective tissue determination and characterization, it is necessary to be familiar with its chemical composition and the chemical and physical prop- erties of its components. Connective tissue consists of three morphologi- cally distinct constituents. his paper was written during the tenure of a Predoctoral Fellowship from the Division of General Medical Sciences, United States Rblic Health Service. 243. Slide 1 Constituents of Connective Tissue *--- *--- Cells ----Ground Substance Proteins 1. Fibroblasts 1. Hyaluronic acid 1, Collagen (embryonic cells), 2. Fat cells 2. Keratosulfate 2. Elastin 3. blast cells 3. Chondroitin sulfate A 3, I?e-ticulin 4. Ivlacrophages 4. Chondroitin sulfate B 5. Mesenchymal 5. Chondroitin sulfate C cells 6. Heparin (undifferentiated) 7, Reparitin’ SUI-fate -- The connective tissue cells will not be discussed since Dr. Nullins has already discussed these components, particularly the mast cella, at last year’s Reciprocal Meats Conference (20). Although up to now the gmund aubstance has not been extensively investigated, recent studies have provided ~omeevidence on the chemical structure of the mucopolysaccharides found in the ground substance (19). Slide 2 bbcopolysaccharides of Connective Tissue Mucopolysaccharide Constituents Linkage Hyaluronic acid N-acetylglucosamine (1) (1 -+3) glucuronido- glucosamine Glucuronic acid (1) (1-+4) glucosarninido- glucuronic acid Keratosulf ate N-acetylglucosamine (1) Galactose (1) Sulfate (1) Hepa rin Glucosamine (1) Glucuronic acid (1) Sulfate (3) Heparitin sulfate Glucosamine (1) Glucuronic acid (1) Sulfate (1) Slide 3 Mucopolysaccharides from Connective Tissue - Mucopolysaccharide Constituents Linkage Chondroitin sulfate A N-acetylgalactosamine (1-+3) glucuronido- galactoeamine Glucuronic acid (1) (1-4) galac tosaminido- glucuronic acid Sulfate (1) Chondroitin sulfate B N-acetylgalactosamine (143) iduronido- galac tosarnine L-idumnic acid (1) (1- 4) galacto saminido- iduronic acid Sulfate (1) Chondroitin sulfate C N-acetylgalactoaamine (l-+ 3) glucuronido- galactosamine Glucuronic acid (1) (144) wactosaminido- glucuronic acid Sulfate (1) In vivo, these mcopolysaccharides are presumed to be complexed with a noncollagenous protein to form various mucoprotein substances (25). Although these mucoproteins will not be discussed firther because of the dearth of information concerning their role in meat tenderness, their im- portance should not be underestimated. Further studies on the relationship between these components and meat tenderness are indicated by McIntosh's recent report of a negative correlation between the mucoprotein content and tenderness of skeletal muscle (21). The ratios of the contents of the dif- ferent mucopolysaccharides components have also been shown to differ among different sources and this my have aome importance in meat tenderness. While the three fibrous connective tissue proteins belong to the insoluble class of proteins called the scleroproteins, their chemical be- havior is much more complex than this classification might indicate. 245, Slide 4 Proteins from Connective Tissue Collagen Raqtin Reticulin 1. Banded under the 1, Not banded under the 1. Precollagen fibers 8 electron microscope EM-700 .. 2. Not stretched 1% by 2. Easily stretched 2. Highly branched 1,000 times its own weight 3. Solubilized to 3. Not affected by hot 3. Partially hydro- gelatin by hot . aqueous extraction lyzed by trypsin aqueous extraction Not hydrolyzed by 4. 4, Not hydrolyzed by 4. Contains some trypsin trypsin lipid 5. Eydrolyzed by 5. Hydrolyzed by collagenase, elastase, ficin, ficin, papain, and and papain some bacterial enzymes 6. Crystalline 6. Amorphous structure structure Reticulin is a poorly characterized protein which greatly re- sembles collagen, although it is not as widespread. it possesses dif- ferent staining properties than collagen, contains a Also,tightly-bound, myristic acid-rich Z.ipid, ad some kinds of it appear to be resistant to collagenase. Since it is not present in large amounts, it should not be a major factor in meat tenderness. Ivbscle is reported to contain about 1/3 as much elastin as collagen. Elastin isn't markedly affected by hot aqueous solvents and thus, it shou3.d play an important role in meat tenderness. The amino acid com- position of elastin may explain its insolubility in aqueous solvents since it contains over 90s nonpolar amino acids by weight. It is similar to collagen in its glycine and proline content, containing 27% and 13$ of these two amino acids respectively. These nonpolar amino acids together with the presence of a lipid which appears to be tightly bound to the elastin mole- cule make it quite refractory to aqueous solvents. This same unique amino acid composition may also explain the resistance of elastin to many enzymes. Very little is known about the three-dimensional structure of elastin due to the difficulty of obtaining good x-ray diffraction patterns. Recent investigation using elastase as a probe into the chemical structure of elastin may help to clarify the role of this protein in meat tenderness. Collagen, like elastin, has an unique amino acid composition. It is the only protein known to contain hydroxylysine, containing 6-7 residues 26-6. of this amino acid per 1,000 amino acid residues. Also, it is the only protein which contains an appreciable amount of hydroxyproline although elastin contains 2$ by weight of this amino acid. Slide 5 shows the seven most abundant amino acids in collagen. Slide 5 Amino Acid Composition of Collagen Glycine 33.5$, proline 13 .l$, alanine 10.5$, hydroxyproline 9.5$, glutamic acid 7.1$, aspartic acid 4.75 and arginine 4.55 Although over 70$ of the amino acids in collagen are nonpolar in character, the high dibasic acid and arginine content prevent collagen from being as nonpolar as elastin, Very few aromatic or sulfur containing amino acids are found in collagen and tryptophan is absent entirely. This latter fact has been used as a test for the purity of collagen preparations. Although collagen has traditionally been thought to be an insoluble protein, small fractions of it can be extracted by aqueous buffers. These fractions have been termed neutral salt 60lUble or acid soluble collagen depending upon whether pH 7 phosphate buffer or pH 4 citrate or acetate buffers were used in their extraction, physicochemical. studies show these fractions to be monodisperse containing protein molecules with a molecular weight of nearly 350,000. These protein molecules are thought to re resent monomers and have been named tropocollagen. They are 2800 fl long in diameter appearing in the form of a long rod in the native state. I Electron micrograph of tropocollagen molecules magnified 100,000 X I Rectron micrograph of heat denatured tropocollagen mlecules magnified 100,000 X A Slide 8 I Left-handed helical peptide chains I Slide 9 Formation of the collagen I and I1 structures by the coiling of the three left-handed helical peptide chains into a right-handed super coil 1 247. Slide 10 1 The triple helix I Slide U. I Cross-sectional view of the triple helix I When the peptide chains are coiled into a triple helix of this type, every third position along an individual peptide chain is identical; that is, it has an identical environment. These positions may be numbered sequentially 1, 2, and 3. Because of conditions of steric hindrance, there are certain restrictions concerning the nature of the amino acids which may occupy these positions. Slide 12 The Possible Positions of Side Chains Collagen I Position Undef ormed De f o rtned Collagen I1 Gly only Other residues Gly only possible except Pro and Eypro I 2 Any residue Any residue Any residue Gly only Any residue Any residue except Val and Ileu H bonding of -" Bonds to a Cannot bond the
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