BIOCHEMISTRY TRANS 7b - Nitrogen Disposal.pdf

1A BIOCHEMISTRY AMINO ACIDS: NITROGEN DISPOSAL (Part 2) DR. JANDOC...

1A BIOCHEMISTRY AMINO ACIDS: NITROGEN DISPOSAL (Part 2) DR. JANDOC Normal Protein Protein-Poor Protein-Rich Diet Intake Diet g %N g %N g %N Total urinary 13.2 100.0 23.28 100.0 42 100.0 nitrogen Protein represented by total 82.5 … 145.5 … 26.25 … urinary nitrogen Urea nitrogen 11.36 86.1 20.45 87.9 2.9 69.1 Ammonia 0.4 3.0 0.82 3.5 0.17 4.0 nitrogen Creatinine 0.61 4.6 0.64 2.7 0.6 14.3 nitrogen Uric acid 0.21 1.6 0.3 1.3 0.11 2.6 nitrogen Undetermined 0.62 4.7 1.07 4.6 0.42 10.0 nitrogen 2. OTHERS  Skin – sweat  Feces – small amount  70 kg individual – losses of about 2 gm per day B. POSITIVE NITROGEN BALANCE  Total Daily Nitrogen Daily Loss < Total Daily Nitrogen Intake  In healthy, growing children  Convalescing adults C. NEGATIVE NITROGEN BALANCE  Total Daily Nitrogen Losses > Total Daily Nitrogen Intake  In diseases involving tissue wasting  Starvation  Prolonged negative balance is dangerous  One – third of total body protein loss – fatal UREA CYCLE NITROGEN BALANCE  Occurs exclusively in the liver  Urea  Healthy, adequately fed adult  Major disposal form of amino groups derived  Total Daily Nitrogen Losses = Total Daily Nitrogen Intake from amino acids  90 % of nitrogen-containing components of urine A. ROUTES OF NITROGEN  Produced in the liver 1. URINARY EXCRETION  Dispose about 90% of nitrogen loss in the body  Urinary Urea – 70 – 85% of total urinary nitrogen depending on nitrogen intake Trans Finals 7b | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 1 of 5  1A BIOCHEMISTRY AMINO ACIDS: NITROGEN DISPOSAL (Part 2) DR. JANDOC  Nitrogen supplied by:  free NH3  aspartate  Glutamate  immediate precursor of both ammonia (through oxidative deamination by glutamate dehydrogenase) and aspartate nitrogen (through transamination of oxaloacetate by aspartate 2. CITRULINE FORMATION aminotransferase)  release of high-energy phosphate of carbamoyl A. CYCLE REACTIONS phosphate as Pi drives the reaction in the forward reaction  1st 2 reactions – occur in mitochondria  citrulline is transported into the cytosol  Rest of reactions – occur in cytosol  Ornithine and Citrulline  Glutamate dehydrogenase  basic amino acids in the urea cycle  Also occurs in the mitochondria providing  not incorporated into cellular proteins because ammonia for incorporation into carbamoyl there are no codons for these amino acids phosphate  Ornithine - regenerated with each turn of the cycle 1. CARBAMOYL PHOSPHATE FORMATION 3. ARGININOSUCCINATE SYNTHESIS  Driven by cleavage of 2 molecules of ATP  Enzyme  driven by cleavage of ATP (3rd and final ATP molecule  Carbamoyl phosphate synthetase I – requires N- consumed in urea formation)  AMP + acetylglutamate for activity pyrophosphate (PPi)  Carbamoyl Phosphate Synthetase II  aspartate for this reaction arises from transamination  For biosynthesis of pyrimidines reaction between oxaloacetate and glutamate  Does not require N-acetylglutamate  Occurs in the cytosol 4. ARGININOSUCCINATE CLEAVAGE  Allosteric effector  N-acetyl glutamate  Products: - essential activator for carbamoyl phosphate  Arginine - immediate urea precursor synthetase I (rate-limiting step in the urea  Fumarate cycle) - hydrated to malate link with several - regulate supply of carbamoyl phosphate to metabolic pathways the urea cycle - malate shuttle mitochondria TCA cycle - Acetylglutamate Synthetase - cytosolic malate oxidized to oxaloacetate - synthesize N-acetylglutamate from - aspartate acetyl CoA and glutamate - glucose - activity is markedly increased by amino acids particularly arginine - 5. ARGININE CLEAVAGE TO UREA AND ORNITHINE protein-rich meal intrahepatic concentration increases increased  Arginase rate of urea synthesis  occurs almost exclusively in the liver  Urea  highly soluble  nontoxic compound  enters the blood excreted in the urine  Ornithine  may enter mitochondria Trans Finals 7b | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 2 of 5  1A BIOCHEMISTRY AMINO ACIDS: NITROGEN DISPOSAL (Part 2) DR. JANDOC  may continue to act as urea cycle intermediate C. UREA CYCLE REGULATION 6. FATE OF UREA  N-Acetylglutamate (NAG)  essential activator for CPS I, the rate-  Urea limiting step in the urea cycle.  diffuses from the liver  increases the affinity of CPS I for ATP.  transported in the blood to the kidney – then  NAG - synthesized from acetyl CoA and filtered and excreted in the urine. glutamate by N acetylglutamate synthase  Portion of the urea - diffuses from the blood into the  in a reaction for which arginine is intestine - cleaved to CO2 and NH3 by bacterial an activator. urease.  Cycle is also regulated by substrate  This ammonia - partly lost in the feces and partly availability (short-term regulation) and reabsorbed into the blood. enzyme induction (long term).  Patients with kidney failure - plasma urea levels are elevated D. UREA CYCLE ENZYMES COMPARTMENTALIZATION  promote greater transfer of urea from blood into. UREA CYCLE REGULATIC. UREA CYCLE REGULA the gut.  Mitochondria  intestinal action of urease on this urea becomes a  Carbamoyl Phosphate Synthetase clinically important source of ammonia,  Ornithine Transcarbamoylase contributing to the hyperammonemia  Cytosol  Oral administration of antibiotics reduces the number  Argininosuccinate Synthetase of intestinal bacteria responsible for NH3 production.  Argininosuccinate Lyase  Arginase B. OVERALL STOICHIOMETRY OF UREA CYCLE E. GENETIC DEFECTS Aspartate + NH3 + HCO3– + 3 ATP + H2O → urea + fumarate + 2 ADP + AMP + 2 Pi + PPi  for each of the urea cycle enzymes   Nitrogen of Urea neonatal period  1 supplied by free NH3  Diseases  1 supplied by aspartate  Type I Hyperammonemia  Glutamate  carbamoyl phosphate synthetase  immediate precursor of defect  Ammonia - through oxidative  Type II Hyperammonemia deamination by glutamate  ornithine transcarbamoylase dehydrogenase defect  Aspartate Nitrogen - through  Citrullinuria transamination of oxaloacetate by  argininosuccinate synthase defect aspartate aminotransferase  Argininosuccinic Acidemia  both nitrogen atoms of urea arise from glutamate  argininosuccinate lyase defect (gathers nitrogen from other amino acids)  Hyperargininemia  Four High-Energy Phosphates  arginase defect  consumed in the synthesis of each molecule  Symptoms of urea  Hyperammonemia  2 ATPs needed to restore 2 ADPs to  blood ammonia accumulation 2 ATPs  high ammonia level  2 ATPs to restore AMP to ATP - toxic  urea synthesis is irreversible with a large - cause brain damage energy expenditure  Episodic Encephalopathies  Convulsions  Ataxia Trans Finals 7b | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 3 of 5  1A BIOCHEMISTRY AMINO ACIDS: NITROGEN DISPOSAL (Part 2) DR. JANDOC  in children with partial deficiencies  cease when protein intake is  From Amines restricted  from diet  critical very early diagnosis (to  monoamines (as hormones or prevent mental retardation) neurotransmitters) amine oxidase  Treatment ammonia  Low-Protein Diet  From Purines and Pyrimidines  supplemented with arginine or  amino groups attached to the rings citrulline ensure adequate released as ammonia arginine levels for protein synthesis  Sodium Benzoate and Sodium Phenylacetate Administration  reduce serum ammonia level  react with glutamine and glycine excreted in the urine  some of the serum ammonia must be used to synthesize more of these nonessential amino acids help lower overall ammonia level AMMONIA METABOLISM  blood ammonia level must be kept low (toxic to the central nervous system) A. AMMONIA SOURCES  From Amino Acids  by the liver from amino acids  Reactions  Aminotransferase  Glutamate Dehydrogenase A. AMMONIA SOUR Reactions B. AMMONIA TRANSPORT IN THE CIRCULATION  From Glutamine  Renal Glutaminase Ammonia  Ammonia  mostly excreted in the urine as  constantly produced in the tissues NH4+ (important mechanism for  present at very low levels in blood due to maintaining acid-base balance) rapid removal of blood ammonia by the  Intestinal Glutaminase glutamine liver hydrolysis ammonia  Muscle - release amino acid nitrogen in the  intestinal mucosal cells obtain form of glutamine or alanine, rather than glutamine from free ammonia - blood  Urea - digestion of dietary  Formation of urea in the liver is protein quantitatively the most important disposal  From Intestinal Bacterial Action route for ammonia.  bacterial urea degradation by urease in the  Travels in the blood from the liver to the intestinal lumen ammonia kidneys, where it passes into the glomerular reabsorption  portal system - filtrate. removed by the liver via conversion to urea Trans Finals 7b | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 4 of 5  1A BIOCHEMISTRY AMINO ACIDS: NITROGEN DISPOSAL (Part 2) DR. JANDOC  Glutamine liver) impaired ammonia  amide of glutamate detoxification elevated ammonia  provides a nontoxic storage and transport concentrations form of ammonia. 2. Hereditary Hyperammonemia  ATP-requiring formation of glutamine from  can result in mental retardation glutamate and ammonia by glutamine  Genetic Deficiencies synthetase occur primarily in skeletal  of each of the 5 enzymes of the muscle and liver but is also important in the urea cycle failure of urea CNS, where it is the major mechanism for synthesis  hyperammonemia in the removal of ammonia in the brain. the 1st week post-natally  found in plasma at concentrations higher  Ornithine Transcarbamoylase than other amino acids, a finding consistent Deficiency - x-linked with its transport function. - most common - affects males *Note: The liver keeps blood ammonia levels low through predominantly glutaminase and the urea cycle in periportal (close to inflow - female carriers of blood) hepatocytes and via glutamine synthetase as an  All the Other Urea Cycle ammonia “scavenger” in the perivenous hepatocytes. Disorders - autosomal recessive C. HYPERAMMONEMIA  Treatment  Dietary Protein Limitation  Hepatic Urea Cycle Capacity > Normal Ammonia  Binding Compounds Generation Rates - bind covalently to amino  low serum ammonia levels (5- acids production of  Liver Function Compromise nitrogen-containing  increased blood ammonia concentration excreted in the symptoms of ammonia intoxication urine (direct neurotoxic effects to the CNS) - Phenylbutyrate  tremors  given orally  slurring of speech  converted to  somnolence phenylacetate  vomiting condenses with  cerebral edema glutamine   blurring of vision phenylacetylgluta mine urinary  Liver Function Compromise Secondary To excretion  Genetic Urea Cycle Defects  Liver Disease D. MECHANISM OF AMMONIA TOXICITY (IN PART)  At High Concentrations  coma and death a-Ketoglutarate + NADPH + H+ + NH3 Glutamate + NADP+ 1. Acquired Hyperammonemia  Liver Disease  shift in the equilibrium of the glutamate  Viral hepatitis dehydrogenase reaction toward the direction of  Ischemia glutamate formation depletes a-ketoglutarate  Hepatotoxins (essential TCA cycle intermediate) decreased  Alcoholism, Hepatitis, Biliary Obstruction cellular oxidation and ATP production Alcoholism, Hepatitis, Biliary Obstruction  liver cirrhosis formation of  brain (high energy production rate by the TCA cycle) collateral circulation around the liver vulnerable to hyperammonemia portal blood shunting into the systemic circulation (no access to the Trans Finals 7b | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 5 of 5  1A BIOCHEMISTRY AMINO ACIDS: NITROGEN DISPOSAL (Part 2) DR. JANDOC Trans Finals 7b | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 6 of 1