<p>PROTEIN TURNOVER AND NITROGEN ECONOMY - proteins metabolism has a balance between body’s energy and synthetic needs - dietary protein required to synthesize endogenous proteins (albumin, myosin, actin) - essential amino acids cannot be synthesize by body; others can be synthesized from carbon sources -table</p><p>- protein balance  relationship between synthesis and degradation (proteolysis) of proteins; 2. roles of proteolysis: activation of enzymes (zymogens), blood clotting cascade, control of organ growth, digestion of dietary protein, fuel supply (starvation), maintain amino acid pools, regulate enzyme activity (removing enzyme from cell, half-lives), remove abnormal proteins, tissue repair</p><p>- starvation  glucose produced from amino acids (muscle proteins serves as fuel supply) - to provide for proper balance during growth, proteolysis counterbalances synthesis to control organ size</p><p>- if dietary intake of amino acids > requirement for protein synthesis  new body protein synthesis (positive balance) or body protein levels maintained at stable level (neutral balance) - positive nitrogen balance  occurs during growth when intake and storage of nitrogen exceed excretion of nitrogen; also associated with restoration of atrophied muscles or body building - if protein intake is insufficient or if balance of amino acids ingested is incorrect for synthetic needs  endogenous protein catabolized to liberate free amino acids for synthesis of essential proteins (negative nitrogen balance); associated with starvation and trauma; occurs when rate of proteolysis exceeds rate of protein synthesis (decrease rate of synthesis or accelerated digestion)</p><p>- elemental constituents of amino acids: carbon  CO2; hydrogen  H2O; nitrogen  urea or 2- ammonia; sulfur  to SO4 </p><p>1. - insulin and glucocorticoids participate in regulation of protein turnover and nitrogen economy - insulin  increase synthesis, decrease degradation of endogenous proteins; favors maintenance of body protein pools; insulin-like growth factor (ILGF) promotes protein synthesis during growth - glucocorticoids (released during stress or starvation)  peripheral tissue catabolism - ala is a precursor for glucose synthesis; glucocorticoid catabolic effect coincides with ability of this class of hormones to promote gluconeogenesis - insulin:glucocorticoid ratio determines net protein turnover; fed state  high ratio  protein formation; fasting  insulin falls, low ratio  protein mobilized via proteolysis; trauma  glucocorticoids increase, low ratio  protein mobilized via proteolysis</p><p>- endogenous protein degradation occurs in lysosome and cytoplasm; membrane/extracellular proteins cycle through lysosome (proteolysis); lysosome is acidic; proteolysis in cytoplasm (calpains, Ca2+ dependent) 3. AMMONIA METABOLISM AND REMOVAL OF NITROGEN WASTE Transamination reactions - 1st step in amino acid degradation is removal of amino nitrogen group by transferring it to alpha-ketoglutarate (alpha-KG) to produce glu; catalyzed by aminotransferase/transaminases (cofactor is pyridoxal phosphate) - pyridoxal phosphate derived from vitamin B6 (also cofactor in glycogen phosphorylase and lysyl oxidase); deficiency  dermatitis, anemia, convulsions</p><p>- transaminases are reversible; alpha-ketoacid accepts amino group from glu to produce new amino acid; most common aminotransferases are for alanine (pyruvate) and aspartate (oxaloacetate); aminotransferases test for liver damage - transaminases transfer nitrogen to glutamate in non-hepatic tissues (muscle) to rid excess nitrogen from those tissues - in liver, nitrogen dumped onto glutamate as an initial step in conversion of nitrogen to excreted form  urea</p><p>Overview of nitrogen excretion - body removes nitrogenous waste; some of these produces are from special starting materials while urea provides a means of removing nitrogen waste in a general manner + - kidney can excrete NH4 as part of acidification mechanism of urine - look at table</p><p>Nitrogen removal from nonhepatic tissues - glutamate dehydrogenase; one direction  reaction involves addition of nitrogen to alpha- ketoglutarate as ammonia (non-hepatic tissues, remove harmful ammonia from these tissues) - glutamate non transported across plasma membrane, but glutamine easily leaves cells - glutamine formed through addition of a second ammonia molecule by glutamine synthetase to produce glutamine; glutamine processed by kidney, which contains glutaminase  (with glutamate dehydrogenase) removes amino groups from glutamine resulting in alpha-KG and ammonia; ammonia released in this manner excreted in urine</p><p>4. UREA CYCLE - liver  glutamate produced by transamination gives up its nitrogen as free ammonia via glutamate dehydrogenase for eventual synthesis of urea (excreted)</p><p>1. Carbamoyl phosphate synthetase-I - urea cycle in liver (kidney) - provides means of ridding body of nitrogen waste as urea - ammonia from amino acids by combined actions of transamination and glutamate dehydrogenase - mitochondria  ammonia incorporated into carbamoyl phosphate via carbamoyl phosphate synthetase-I (CPS-1)  reaction product, carbamoyl phosphate, provides substrate for cycle; reaction requires one ATP molecule providing phosphate that combines with CO2 and ammonia and the other ATP molecule provides driving force for reaction (2 ATP) - carbamoyl phosphate directly introduces the first source of nitrogen for the cycle - CPS-1 is allosterically activated by N-acetylglutamate (produced by enzyme-catalyzed reaction of acetyl CoA + glutamate  N-acetylglutamate + CoA)</p><p>- mitochondrial CPS-1 (CPS1: NH3 nitrogen source) distinguished from cytoplasmic CPS-2 (CPS-2: glutamine nitrogen source) in that CPS-2 is involved with pyrimidine synthesis</p><p>2. Ornithine transcarbamoylase - first reaction of urea cycle: carbamoyl phosphate combines with ornithine  citrulline via ornithine transcarbamoylase (occurs in mitochondrial matrix) - ornithine transported into mitochondria form cytoplasm - citrulline product released from mitochondria to cytoplasm in exchange for ornithine</p><p>3. Arginosuccinate synthetase - cytoplasm  citrulline reacts with aspartate via arginosuccinate synthetase yielding arginosuccinate - aspartate formed by transamination of glutamate with oxaloacetate - aspartate is 2 nd direct source of nitrogen for the cycle - energy requiring reaction that cleaves ATP  AMP + PPi (costs two high-energy P bonds, PPi splits spontaneously into two Pi)</p><p>4. Arginosuccinase - arginosuccinate cleaved by arginosuccinase into fumarate and arginine - fumarate reconverted to oxaloacetate in citric acid cycle  can regenerate aspartate; carbons from aspartate recycled with only nitrogen claimed for urea cycle</p><p>5. Arginase - arginine cleaved to urea and ornithine (into cytoplasm in exchange for citrulline) - urea secreted by liver into blood to be cleared by kidney - when arginase cannot handle accumulation of arginine  arginine stimulates formation of N- acetylglutamate to increase formation of carbamoyl phosphate  reacts with ornithine to produce a mass action effect on arginase reaction thus increasing formation of urea</p><p>5. HYPERAMMONEMIA </p><p>Acquired hyperammonemia - results from collateral circulation of portal system in response to liver damage (cirrhosis); blood flow from intestines bypasses liver - collateral circulation (non cirrhosis) responsible for hyperammonemia; microorganisms in GI tract produce large amount of ammonia absorbed in portal system and sent to liver for detox; portal-systemic shunting  blood flows directly to IVC (bypasses liver)  portal-systemic encephalopathy (PSE) - shunting results in reduction of ammonia detoxification by liver; ammonia from amino acid/protein metabolism cannot be converted to urea to an extent causing blood ammonia to rise - liver transplant; reduce absorption of ammonia using lactulose  fermented by microorganisms to short chain organic acids that lower pH of intestinal lumen: converts NH3 to + NH4 (not readily absorbed across intestinal epithelium and is excreted in stool)</p><p>Inherited hyperammonemia - caused by deficiencies of urea cycle enzymes - severity depends on proximity of defect to point of entry of ammonia in its processing to urea - CPS-1 defects or ornithine transcarbamoylase defects  severe hyperammonemia; these two defects can be distinguished by evaluating appearance of pyrimidines in urine; defect in ornithine transcarbamoylase  CPS-1 accumulates in mitochondria  excess carbamoyl phosphate leaks in to cytoplasm  increases rate of pyrimidine synthesis - X-linked, in males</p><p>- high ammonia leads to mental retardation; possible reasons for neurologic damage:</p><p>1. ammonia reacts with alpha-ketoglutarate to form glutamate thus interfering with ATP production in citric acid cycle</p><p>2. excess glutamate formed undergoes amination to glutamine and then to alpha- ketoglutaramic acid, a neurotoxic compound</p><p>3. high ammonia  increase blood levels of some amino acids; these compete with other amino acids for transport across blood-brain barrier; thus, predominant transport of one or a few amino acids limits availability of other amino acids within the brain  reduction in normal rate of protein synthesis</p><p>Treatment - restrict dietary protein  reduce amount of ammonia that must be detoxified - alternative ammonia excretion mechanisms use body’s detox of exogenous chemicals - benzoic acid  conjugated with glycine to form hippuric acid  readily excreted in urine taking with it the nitrogen from glycine; glycine is synthesized from CO2 and NH3 - phenylacetic acid  conjugated with glutamine forming phenylacetylglutamine  excreted in urine taking two nitrogens per molecule; glutamine continually synthesized in hyperammonemia in peripheral tissues (muscle) via glutamate dehydrogenase and glutamine synthetase reactions</p>