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Urea Cycle

The document discusses the urea cycle and its role in removing toxic ammonia from the body. The urea cycle occurs primarily in the liver and involves several steps to convert ammonia and carbon dioxide into urea, which is excreted in urine. First, carbamoyl phosphate is formed in the mitochondria using ammonia, bicarbonate, and ATP. Carbamoyl phosphate then reacts with ornithine to form citrulline. Further reactions involving aspartate, ATP hydrolysis, and other enzymes ultimately yield arginine and then urea, which is excreted, along with regenerated ornithine to continue the cycle. Defects in this cycle can cause toxic hyperammonemia in

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Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle A.N. Emami R.
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle amino acids The carbon chains are broken Most mammals convert amino- acid nitrogen to urea for down to molecules that feed excretion into the TCA cycle. NH4+ Some animals excrete NH4+ or uric acid. most terrestrial fish & other aquatic birds & reptiles vertebrates vertebrates O O NH4+ H HN N H2N-C-NH2 ammonium ion O urea N N uric acid O H H June 26, 2012 Total slide : 50 3
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Major fate of waste nitrogen O H2N C NH2 urea June 26, 2012 Total slide : 50 4
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Why Urea? Non toxic Water soluble Combines two waste products into one molecule: CO2 NH3 June 26, 2012 Total slide : 50 5
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Ammonia is highly toxic Main reason to form urea is to reduce levels of ammonia “Ammonia” often refers to (NH3 + NH4+) NH3 is really ammonia NH4+ is the ammonium ion pKa = 9.3 NH4+ NH3 + H+ June 26, 2012 Total slide : 50 6
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Ammonia rapidly equilibrates across membranes NH4+ NH4+ pKa = 9.3 NH3 NH3 Lipid Bilayer June 26, 2012 Total slide : 50 7
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Production of ammonia Amino acids α-Ketoglutarate α-Keto acids Glutamate GDH Other reactions Oxaloacetate NH+ α-Ketoglutarate 4 Aspartate Urea Urea cycle June 26, 2012 Total slide : 50 8
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Reaction catalyzed by glutamate dehydrogenase (GDH). COO- COO- + NAD(P)+ NAD(P)H H3 N C H C O CH2 CH2 + CH2 NH4 CH2 COO- COO- June 26, 2012 Total slide : 50 9
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Most of the ammonia is produced by other reactions… H H N N N N H H H H CH2 C H C NH3 histidase C H + NH4 COO C L-histidine O O urocanate June 26, 2012 Total slide : 50 10
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Most of the ammonia is produced by other reactions… COO serine COO H dehydratase C O + NH4 H3N C CH2OH CH3 L-serine pyruvate COO COO serine H 3N C H dehydratase C O + NH4 H C OH CH2 CH3 CH3 L-threonine 2-ketobutyrate June 26, 2012 Total slide : 50 11
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle O C NH2 COO O CH2 H2O NH4 CH2 C NH2 H2O NH4 COO CH4 CH4 CH4 CH4 glutaminase asparaginase H C NH3 H C NH4 H C NH4 H CH4NH4 COO COO CHOO CHOO Gln Synthetase L-glutamine L-glutamate L-asparagine L-aspartate glutamine synthetase ATP ADP +Pi + NH4 + glutamate glutamine glutaminase June 26, 2012 Total slide : 50 12
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Summary of sources of ammonia for urea cycle June 26, 2012 Total slide : 50 13
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Closer look at transport of waste N from peripheral tissue to liver via alanine and glutamine Waste N funnelled to pyruvate Glucose – Alanine Cycle via transaminations Net: N (muscle)  Urea (liver) June 26, 2012 Total slide : 50 14
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Synthesis of Glutamine in Peripheral Tissue and Transport to the Liver June 26, 2012 Total slide : 50 15
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle UREA FORMATION June 26, 2012 Total slide : 50 16
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Overview Occurs primarily in liver; excreted by kidney Principal method for removing ammonia Hyperammonemia: Defects in urea cycle enzymes (CPS, OTC, etc.) Severe neurological defects in neonates Treatment: Stop protein intake Dialysis Increase ammonia excretion: Na benzoate, Na phenylbutyrate, L-arginine, L-citrulline June 26, 2012 Total slide : 50 17
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Overview Key reaction: hydrolysis of arginine Arginine + H2O ==> urea + ornithine arginase Resynthesis of Arginine June 26, 2012 Total slide : 50 18
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Blood Urea Nitrogen Normal range: 7-18 mg./dL Elevated in amino acid catabolism Glutamate N- acetylglutamate CPS-1 activation Elevated in renal insufficiency Decreased in hepatic failure June 26, 2012 Total slide : 50 19
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Urine urea NH2 C O NH2 NH2 H2O Mitochondrion C NH 5 CO2 + NH4 + Cytosol CH2 NH CH2NH2 2 ATP carbamoyl CH2 phosphate CH2 synthetase I CH2 CH2 CH2NH2 2ADP + Pi (CPSI) H C NH2 H C NH2 CH2 1 COOH COOH COOH CH2 O O Arginine Ornithine HC H C NH2 H2N C O P O- 2 ornithine COOH CH - transcarbamoylase 4 Carbamoyl O Ornithine COOH NH2 argininosuccinase phosphate NH2 Fumarate C O C O Pi CH2 NH NH COOH CH2 NH CH2 C NH CH CH2 CH2 CH2 NH CH2 CH2 H C NH2 CH2 H C NH2 COOH CH2 COOH COOH 3 Citrulline argininosuccinate H C NH2 Citrulline synthetase COOH COOH Argininosuccinate ATP AMP + PPi H2N C H CH2 COOH Aspartate June 26, 2012 Total slide : 50 20
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle The urea cycle HCO3- 2 ATP 2 ADP + Pi carbamoyl mitochondria O O phosphate NH4+ H2N-C-O-P-O- O- citrulline ornithine NH3+ Pi O NH3+ H2N-CH2CH2CH2CH-CO2- H2N-C-NH-CH2CH2CH2CH-CO2- cytosol ATP CO2- Asp O H2N-C-NH2 - O2C-CH2CH-NH3+ AMP + PPi urea CO2- NH2+ NH3+ H2O - O2C-CH2CH-NH-C-NH-CH2CH2CH2CH-CO2- argininosuccinate arginine NH2+ NH3+ H2N-C-NH-CH2CH2CH2CH-CO2- - O2C-CH=CH-CO2- fumarate June 26, 2012 Total slide : 50 21
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Incorporation of ammonia into urea begins with formation of carbamoyl phosphate O O NH4+ + HCO3- H2N-C-O-P-O- carbamoyl O- phosphate 2 ATP 2 ADP + Pi This occurs in the mitochondrial matrix. Carbamoyl-phosphate synthetase-1 catalyzes the reaction in three steps, using two molecules of ATP: (1) O O carbonic- HCO3- HO-C-O-P-O- phosphoric acid O- anhydride ATP ADP NH4+ (2) Pi ATP ADP O O O carbamate H2N-C-O- H2N-C-O-P-O- (3) O- June 26, 2012 Total slide : 50 22
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Carbamoyl phosphate reacts with ornithine to form citrulline O O NH3+ H2N-C-O-P-O- + H3N-CH2CH2CH2CH-CO2- ornithine O- carbamoyl phosphate O NH3+ Pi H2N-C-NH-CH2CH2CH2CH-CO2- + H+ citrulline This step also occurs in the mitochondrial matrix. June 26, 2012 Total slide : 50 23
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Combination of citrulline with aspartate to form argininosuccinate is driven by breakdown of ATP to AMP CO2- O NH3+ - O2C-CH2CH-NH3+ H2N-C-NH-CH2CH2CH2CH-CO2- ATP citrulline aspartate AMP + PPi + H2O argininosuccinate CO2- NH2+ NH3+ - O2C-CH2CH-NH-C-NH-CH2CH2CH2CH-CO2- This reaction occurs only in the cytosol, so citrulline first must leave the mitochondria. A transporter exchanges ornithine for citrulline plus a proton across the mitochondrial inner membrane. June 26, 2012 Total slide : 50 24
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Argininosuccinate splits into arginine and fumarate CO2- NH2+ NH3+ - O2C-CH2CH-NH-C-NH-CH2CH2CH2CH-CO2- argininosuccinate - O2C-CH=CH-CO2- fumarate NH2+ NH3+ H2N-C-NH-CH2CH2CH2CH-CO2- arginine This reaction occurs in the cytosol. June 26, 2012 Total slide : 50 25
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Hydrolysis of arginine releases urea and regenerates ornithine NH2+ NH3+ H2N-C-NH-CH2CH2CH2CH-CO2- arginine H2O O NH3+ H2N-C-NH2 H2N-CH2-CH2-CH2-CH-CO2- urea H+ ornithine This reaction occurs in the cytosol. To continue the cycle, ornithine must return to a mitochondrion. June 26, 2012 Total slide : 50 26
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle HCO3- 2 ATP 2 ADP + Pi carbamoyl O O phosphate NH4+ H2N-C-O-P-O- O- Pi citrulline O NH3+ ornithine H2N-C-NH-CH2CH2CH2CH-CO2- 2 Pi ATP CO2- O - O2C-CH2CH-NH3+ H2N-C-NH2 PPi + AMP Asp urea H2O CO2- NH2+ NH3+ - O2C-CH2CH-NH-C-NH-CH2CH2CH2CH-CO2- argininosuccinate Formation of urea consumes 4 phosphate anhydride bonds June 26, 2012 Total slide : 50 27
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Input-Output 1. NH4+ + CO2 + 2 ATP carbamoyl phosphate + 2 ADP + Pi 2. carbamoyl phosphate + ornithine citrulline + Pi 3. citrulline + aspartate + ATP argininosuccinate + AMP + PPi 4. AMP + ATP 2 ADP 5. PPi + ATP 2 ADP 6. argininosuccinate arginine + fumarate 7. arginine + H2O urea + ornithine SUM: NH4+ + CO2 + 4 ATP + aspartate urea + fumarate + 4 ADP + 4 Pi June 26, 2012 Total slide : 50 28
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle The aspartate consumed in the urea cycle can be regenerated from the fumarate that is produced 2 ATP 2 ADP + Pi HCO3- + carbamoyl phosphate α-keto acids amino acids NH4+ Pi aspartate- oxaloacetate ornithine aminotransferase citrulline Urea oxaloacetate ATP cycle urea aspartate AMP + PPi malate dehydrogenase arginine argininosuccinate NADH malate NAD+ fumarate This process also uses H2O both cytosolic and mitochondrial enzymes June 26, 2012 Total slide : 50 29
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Oxidation of malate in mitochondria generates ATP 2 e- to O2 via NADH dehydrogenase 2 ATP 2 ADP + Pi generates ~ 2.5 ATP HCO3- carbamoyl NADH mitochondrion oxaloacetate phosphate + NH4+ NAD Pi aspartate glutamate ornithine citrulline malate α-ketoglutarate ornithine citrulline α-ketoglutarate glutamate ATP aspartate urea AMP + PPi amino acids α-ketoacids arginine argininosuccinate malate cytosol fumarate H2O NADH, NAD and oxaloacetate can’t cross the mitochondrial inner membrane, but + there are transporters for malate, aspartate, glutamate and α-ketoglutarate. June 26, 2012 Total slide : 50 30
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Transport systems in the mitochondrial inner membrane exchange aspartate for glutamate and a-ketoglutarate for malate mitochondrion aspartate- glutamate- + H+ α-ketoglutarate malate aspartate- glutamate- + H+ α-ketoglutarate malate cytosol Because the Asp/Glu transporter also moves a proton across the membrane, it can be driven by an electrochemical potential gradient. Mutations in this transporter have been linked to autism. June 26, 2012 Total slide : 50 31
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Alpha--ketoglutarate/malate and aspartate/glutamate transporters also participate in oxidation of cytosolic NADH mitochondrion 2 e- to electron- transport chain NADH NAD+ oxaloacetate aspartate glutamate α-ketoglutarate malate aspartate glutamate α-ketoglutarate malate oxaloacetate cytosol NADH NAD+ glycolysis June 26, 2012 Total slide : 50 32
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Well Fed State a.a.'s Asp a.a.'s αKG urea fumarate αKAs Glu NH+ 4 cycle urea OAA Mal malate CO2 + H2O Net: 2 NH4+ + CO2 + 4 ATP  urea + 4 ADP + 4 Pi June 26, 2012 Total slide : 50 33
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Fasted State ala Asp ala αKG Gluconeogenesis urea fumarate pyr Glu NH+4 cycle urea OAA malate OAA 2 ala + CO2  1 urea + 1 glucose Glucose June 26, 2012 Total slide : 50 34
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Balancing the levels of ammonia and aspartate for entry into urea cycle (a) NH3 in excess (b) Aspartate in excess NH3 NH3 NADH α-Ketoglutarate NADH Glutamate Glutamate α-Ketoglutarate dehydrogenase dehydrogenase NAD NAD Glutamate Glutamate Aspartate Aspartate transaminase transaminase Oxaloacetate Aspartate Oxaloacetate Aspartate Citrulline Citrulline Carbamoyl Carbamoyl phosphate phosphate Urea Urea Cycle Cycle Urea Urea Η 2Ο Η 2Ο June 26, 2012 Total slide : 50 35
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle The urea cycle is regulated in two ways 1. Allosteric activation of carbamoylphosphate synthetase-1 by N-acetylglutamate CO2- CO2- + H3N C H + acetyl-CoA CH3CO-NH C H N-acetylglutamate CH2 CH2 CH2 CoA-SH CH2 Glu CO2- CO2- In mammals, N-acetylGlu appears to play only a regulatory role. Carbamoylphosphate synthetase-1 is completely inactive in its absence. A genetic carbamoyl- deficiency in the enzyme that forms N-acetylGlu 2 ATP 2 ADP + Pi phosphate can cause a lethal defect in the urea cycle. O O NH4+ + HCO3- H2N-C-O-P-O- O- 2. A high-protein diet or starvation leads to increased synthesis of all five enzymes used in the urea cycle, including carbamoylphosphate synthetase-1. Expression of the enzyme that synthesizes N-acetylglutamate also increases. June 26, 2012 Total slide : 50 36
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders Deficiency of any of the five enzymes in the urea cycle results in the accumulation of ammonia and leads to encephalopathy. Episodes of encephalopathy and associated systems are unpredictable and, if untreated, are lethal or produce devastating neurologic sequelae in long-term survivors. Although these disorders do not produce liver disease, the consequences of hyperammonemia resemble those seen in patients with hepatic failure or in a transient interference with the urea cycle, as seen in some forms of organic acidemias. Investigate for hyperammonemia in any infant or child with altered mental status June 26, 2012 Total slide : 50 37
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle The urea cycle Asterisk = N-acetyl glutamate synthetase; 1 = carbamyl phosphate synthetase; 2 = ornithine transcarbamylase; 3 = argininosuccinate synthetase; 4 = argininosuccinate lyase; 5 = arginase June 26, 2012 Total slide : 50 38
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle UREA CYCLE DISORDERS Disorder Deficient Enzyme Inheritance Pattern Carbamyl phosphate Carbamyl phosphate Autosomal recessive synthetase deficiency synthetase Ornithine Ornithine X-linked transcarbamylase transcarbamylase deficiency Citrullinemia Argininosuccinate Autosomal recessive synthetase Argininosuccinic aciduria Argininosuccinate lyase Autosomal recessive Argininemia Arginase Autosomal recessive June 26, 2012 Total slide : 50 39
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Case The patient is a full-term newborn boy from a normal vaginal delivery. The pregnancy was uncomplicated. At 36 hours the baby became lethargic, irritable, and was hyperventilating. Over the next 24 hours lethargy increased and progressed to coma requiring mechanical ventilation. Hemodialysis was started at 5 days. Patient died at one week of age. Laboratory Results At 36 hours arterial blood pH was 7.50 (7.35-7.45), carbon dioxide was 25 torr (35-45), and blood urea nitrogen was 2 mg/dl (5-20). Sepsis workup was negative. On day 5 plasma ammonium was 1800 :mol/l (<35). Plasma glutamine was 1500 :mol/l (550-650),arginine was below normal, and citrulline undetectable. Orotic acid in the urine was extremely elevated. Family History Two of the mother’s four brothers had died shortly after birth. Cause of death was given as encephalitis. Biochemical Basis of Disorder , same as.. Diagnosis: ornithine transcarbamoylase deficiency June 26, 2012 Total slide : 50 40
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Biochemical explanations for ornithine transcarbamoylase deficiency Low BUN Low blood arginine Undetectable blood citrulline Elevated blood ammonia Elevated blood glutamine Elevated orotic acid June 26, 2012 Total slide : 50 41
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Urine NH4+ + Glu  Gln urea NH2 C O NH2 NH2 H2O Mitochondrion C NH 5 CO2 + NH4+ Cytosol CH2 NH CH2NH2 2 ATP carbamoyl CH2 phosphate CH2 CH2 synthetase I CH2 (CPSI) CH2NH2 ADP + Pi H C NH2 H C NH2 CH2 1 COOH COOH COOH CH2 O O Arginine Ornithine HC H C NH2 H2N C O P O- 2 ornithine COOH CH - transcarbamoylase 4 Carbamoyl O Ornithine COOH NH2 argininosuccinase phosphate NH2 Fumarate C O C O Pi CH2 NH NH COOH CH2 NH CH2 C NH CH CH2 CH2 CH2 NH CH2 CH2 H C NH2 CH2 H C NH2 COOH CH2 COOH COOH 3 Citrulline argininosuccinate H C NH2 Citrulline synthetase COOH COOH Argininosuccinate ATP AMP + PPi H2N C H Carbamoyl P  orotic acid CH2 COOH Aspartate June 26, 2012 Total slide : 50 42
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Carbamoyl P synthetase deficiency June 26, 2012 Total slide : 50 43
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Urine NH4+ + Glu  Gln urea NH2 C O NH2 NH2 H2O Mitochondrion C NH 5 CO2 + NH4 + Cytosol CH2 NH CH2NH2 2 ATP carbamoyl CH2 phosphate CH2 synthetase I CH2 CH2 CH2NH2 ADP + Pi (CPSI) H C NH2 H C NH2 CH2 1 COOH COOH COOH CH2 O O Arginine Ornithine HC H C NH2 H2N C O P O- 2 ornithine COOH CH - transcarbamoylase 4 Carbamoyl O Ornithine COOH NH2 argininosuccinase phosphate NH2 Fumarate C O C O Pi CH2 NH NH COOH CH2 NH CH2 C NH CH CH2 CH2 CH2 NH CH2 CH2 H C NH2 CH2 H C NH2 COOH CH2 COOH COOH 3 Citrulline argininosuccinate H C NH2 Citrulline synthetase COOH COOH Argininosuccinate ATP AMP + PPi H2N C H CH2 Carbamoyl P  orotic acid COOH Aspartate June 26, 2012 Total slide : 50 44
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Autism is a neurodevelopmental genetic disorder  Deficits in verbal & nonverbal communication and social interactions  Repetitive or stereotyped behaviors  Incidence ~1 per 1000 people (possibly higher)  Strong evidence for heritability  Polygenic - between 5 & 10 genes may be involved Single-nucleotide polymorphisms (SNPs) in the gene for a mitochondrial, Ca2+-dependent Asp/Glu exchanger increase the risk by a factor of 3 to 4. This is the main form of the Asp/Glu exchanger that is expressed in the brain. Mutations in the gene impair the urea cycle. N. Ramoz et al., Am. J. Psychiatry 161: 662 (2004) L. Palmieri et al., EMBO J. 20: 5060 (2001) June 26, 2012 Total slide : 50 45
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders (Diagnosis) Cultured skin fibroblasts may be desirable if prenatal diagnosis is considered in future pregnancies. Carbamyl phosphate synthetase I and ornithine transcarbamylase (OTC) are not expressed in cultured fibroblasts. The enzymatic diagnosis of CPSD and OTCD requires liver biopsy. Biopsy should be done when establishing the diagnosis of the first case in a family. June 26, 2012 Total slide : 50 46
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders (Treatment) Once hyperammonemia is demonstrated in an infant, protein-containing feedings should be discontinued immediately, appropriate supportive care, (mechanical ventilation) Maximal calories should be provided in the form of intravenous glucose and lipids in an effort to reduce catabolism. Plans should be immediately made to initiate hemodialysis in infants who are encephalopathic and have plasma ammonia levels over 10 times the upper limit of normal. June 26, 2012 Total slide : 50 47
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders (Treatment) Maintenance therapy dietary protein restriction+supplementation with citrulline or arginine+ the use of drugs The primary drug now used( provides an alternate pathway for waste nitrogen excretion) for maintenance therapy in patients with urea cycle disorders is sodium phenylbutyrate (Buphenyl). The drug is typically administered four times a day in a dose of 0.4 to 0.6 g/kg/day. It is supplied as a powder, which can be mixed with food or formula, or as a tablet. June 26, 2012 Total slide : 50 48
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders (Treatment) Liver transplantation for Severe neonatal OTC and CPS deficiency. Liver failure and cirrhosis in ASL deficiency. Failed medical-pharmacologic treatment. Pretransplant care by aggressively managing intercurrent hyperammonemia, vaccinations and prophylaxis are given against infectious appropriate caloric intake Gene replacement June 26, 2012 Total slide : 50 49
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle Genetic deficiencies in some of the urea-cycle enzymes can be treated pharmacologically CO2- benzoate CO2- phenylacetate ATP + CoA-SH ATP + CoA-SH AMP + PPi AMP + PPi O S-CoA S- benzoyl-CoA CoA O phenylacetyl-CoA glycine glutamine CoA-SH O CoA-SH H N CO2- N CO2 - phenylacetyl- hippurate H O glutamine (benzoylglycine) O NH2 The amide products of these reactions (hippurate and phenylacetylglutamine) are excreted in the urine. Replenishing the Gly or Gln removes ammonia. June 26, 2012 Total slide : 50 50
Metabolic & Molecular Basis of Inherited Disease Metabolic Disorders of Urea Cycle THE END

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Urea Cycle

  • 1.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle A.N. Emami R.
  • 2.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle
  • 3.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle amino acids The carbon chains are broken Most mammals convert amino- acid nitrogen to urea for down to molecules that feed excretion into the TCA cycle. NH4+ Some animals excrete NH4+ or uric acid. most terrestrial fish & other aquatic birds & reptiles vertebrates vertebrates O O NH4+ H HN N H2N-C-NH2 ammonium ion O urea N N uric acid O H H June 26, 2012 Total slide : 50 3
  • 4.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Major fate of waste nitrogen O H2N C NH2 urea June 26, 2012 Total slide : 50 4
  • 5.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Why Urea? Non toxic Water soluble Combines two waste products into one molecule: CO2 NH3 June 26, 2012 Total slide : 50 5
  • 6.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Ammonia is highly toxic Main reason to form urea is to reduce levels of ammonia “Ammonia” often refers to (NH3 + NH4+) NH3 is really ammonia NH4+ is the ammonium ion pKa = 9.3 NH4+ NH3 + H+ June 26, 2012 Total slide : 50 6
  • 7.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Ammonia rapidly equilibrates across membranes NH4+ NH4+ pKa = 9.3 NH3 NH3 Lipid Bilayer June 26, 2012 Total slide : 50 7
  • 8.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Production of ammonia Amino acids α-Ketoglutarate α-Keto acids Glutamate GDH Other reactions Oxaloacetate NH+ α-Ketoglutarate 4 Aspartate Urea Urea cycle June 26, 2012 Total slide : 50 8
  • 9.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Reaction catalyzed by glutamate dehydrogenase (GDH). COO- COO- + NAD(P)+ NAD(P)H H3 N C H C O CH2 CH2 + CH2 NH4 CH2 COO- COO- June 26, 2012 Total slide : 50 9
  • 10.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Most of the ammonia is produced by other reactions… H H N N N N H H H H CH2 C H C NH3 histidase C H + NH4 COO C L-histidine O O urocanate June 26, 2012 Total slide : 50 10
  • 11.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Most of the ammonia is produced by other reactions… COO serine COO H dehydratase C O + NH4 H3N C CH2OH CH3 L-serine pyruvate COO COO serine H 3N C H dehydratase C O + NH4 H C OH CH2 CH3 CH3 L-threonine 2-ketobutyrate June 26, 2012 Total slide : 50 11
  • 12.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle O C NH2 COO O CH2 H2O NH4 CH2 C NH2 H2O NH4 COO CH4 CH4 CH4 CH4 glutaminase asparaginase H C NH3 H C NH4 H C NH4 H CH4NH4 COO COO CHOO CHOO Gln Synthetase L-glutamine L-glutamate L-asparagine L-aspartate glutamine synthetase ATP ADP +Pi + NH4 + glutamate glutamine glutaminase June 26, 2012 Total slide : 50 12
  • 13.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Summary of sources of ammonia for urea cycle June 26, 2012 Total slide : 50 13
  • 14.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Closer look at transport of waste N from peripheral tissue to liver via alanine and glutamine Waste N funnelled to pyruvate Glucose – Alanine Cycle via transaminations Net: N (muscle)  Urea (liver) June 26, 2012 Total slide : 50 14
  • 15.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Synthesis of Glutamine in Peripheral Tissue and Transport to the Liver June 26, 2012 Total slide : 50 15
  • 16.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle UREA FORMATION June 26, 2012 Total slide : 50 16
  • 17.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Overview Occurs primarily in liver; excreted by kidney Principal method for removing ammonia Hyperammonemia: Defects in urea cycle enzymes (CPS, OTC, etc.) Severe neurological defects in neonates Treatment: Stop protein intake Dialysis Increase ammonia excretion: Na benzoate, Na phenylbutyrate, L-arginine, L-citrulline June 26, 2012 Total slide : 50 17
  • 18.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Overview Key reaction: hydrolysis of arginine Arginine + H2O ==> urea + ornithine arginase Resynthesis of Arginine June 26, 2012 Total slide : 50 18
  • 19.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Blood Urea Nitrogen Normal range: 7-18 mg./dL Elevated in amino acid catabolism Glutamate N- acetylglutamate CPS-1 activation Elevated in renal insufficiency Decreased in hepatic failure June 26, 2012 Total slide : 50 19
  • 20.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Urine urea NH2 C O NH2 NH2 H2O Mitochondrion C NH 5 CO2 + NH4 + Cytosol CH2 NH CH2NH2 2 ATP carbamoyl CH2 phosphate CH2 synthetase I CH2 CH2 CH2NH2 2ADP + Pi (CPSI) H C NH2 H C NH2 CH2 1 COOH COOH COOH CH2 O O Arginine Ornithine HC H C NH2 H2N C O P O- 2 ornithine COOH CH - transcarbamoylase 4 Carbamoyl O Ornithine COOH NH2 argininosuccinase phosphate NH2 Fumarate C O C O Pi CH2 NH NH COOH CH2 NH CH2 C NH CH CH2 CH2 CH2 NH CH2 CH2 H C NH2 CH2 H C NH2 COOH CH2 COOH COOH 3 Citrulline argininosuccinate H C NH2 Citrulline synthetase COOH COOH Argininosuccinate ATP AMP + PPi H2N C H CH2 COOH Aspartate June 26, 2012 Total slide : 50 20
  • 21.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle The urea cycle HCO3- 2 ATP 2 ADP + Pi carbamoyl mitochondria O O phosphate NH4+ H2N-C-O-P-O- O- citrulline ornithine NH3+ Pi O NH3+ H2N-CH2CH2CH2CH-CO2- H2N-C-NH-CH2CH2CH2CH-CO2- cytosol ATP CO2- Asp O H2N-C-NH2 - O2C-CH2CH-NH3+ AMP + PPi urea CO2- NH2+ NH3+ H2O - O2C-CH2CH-NH-C-NH-CH2CH2CH2CH-CO2- argininosuccinate arginine NH2+ NH3+ H2N-C-NH-CH2CH2CH2CH-CO2- - O2C-CH=CH-CO2- fumarate June 26, 2012 Total slide : 50 21
  • 22.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Incorporation of ammonia into urea begins with formation of carbamoyl phosphate O O NH4+ + HCO3- H2N-C-O-P-O- carbamoyl O- phosphate 2 ATP 2 ADP + Pi This occurs in the mitochondrial matrix. Carbamoyl-phosphate synthetase-1 catalyzes the reaction in three steps, using two molecules of ATP: (1) O O carbonic- HCO3- HO-C-O-P-O- phosphoric acid O- anhydride ATP ADP NH4+ (2) Pi ATP ADP O O O carbamate H2N-C-O- H2N-C-O-P-O- (3) O- June 26, 2012 Total slide : 50 22
  • 23.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Carbamoyl phosphate reacts with ornithine to form citrulline O O NH3+ H2N-C-O-P-O- + H3N-CH2CH2CH2CH-CO2- ornithine O- carbamoyl phosphate O NH3+ Pi H2N-C-NH-CH2CH2CH2CH-CO2- + H+ citrulline This step also occurs in the mitochondrial matrix. June 26, 2012 Total slide : 50 23
  • 24.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Combination of citrulline with aspartate to form argininosuccinate is driven by breakdown of ATP to AMP CO2- O NH3+ - O2C-CH2CH-NH3+ H2N-C-NH-CH2CH2CH2CH-CO2- ATP citrulline aspartate AMP + PPi + H2O argininosuccinate CO2- NH2+ NH3+ - O2C-CH2CH-NH-C-NH-CH2CH2CH2CH-CO2- This reaction occurs only in the cytosol, so citrulline first must leave the mitochondria. A transporter exchanges ornithine for citrulline plus a proton across the mitochondrial inner membrane. June 26, 2012 Total slide : 50 24
  • 25.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Argininosuccinate splits into arginine and fumarate CO2- NH2+ NH3+ - O2C-CH2CH-NH-C-NH-CH2CH2CH2CH-CO2- argininosuccinate - O2C-CH=CH-CO2- fumarate NH2+ NH3+ H2N-C-NH-CH2CH2CH2CH-CO2- arginine This reaction occurs in the cytosol. June 26, 2012 Total slide : 50 25
  • 26.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Hydrolysis of arginine releases urea and regenerates ornithine NH2+ NH3+ H2N-C-NH-CH2CH2CH2CH-CO2- arginine H2O O NH3+ H2N-C-NH2 H2N-CH2-CH2-CH2-CH-CO2- urea H+ ornithine This reaction occurs in the cytosol. To continue the cycle, ornithine must return to a mitochondrion. June 26, 2012 Total slide : 50 26
  • 27.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle HCO3- 2 ATP 2 ADP + Pi carbamoyl O O phosphate NH4+ H2N-C-O-P-O- O- Pi citrulline O NH3+ ornithine H2N-C-NH-CH2CH2CH2CH-CO2- 2 Pi ATP CO2- O - O2C-CH2CH-NH3+ H2N-C-NH2 PPi + AMP Asp urea H2O CO2- NH2+ NH3+ - O2C-CH2CH-NH-C-NH-CH2CH2CH2CH-CO2- argininosuccinate Formation of urea consumes 4 phosphate anhydride bonds June 26, 2012 Total slide : 50 27
  • 28.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Input-Output 1. NH4+ + CO2 + 2 ATP carbamoyl phosphate + 2 ADP + Pi 2. carbamoyl phosphate + ornithine citrulline + Pi 3. citrulline + aspartate + ATP argininosuccinate + AMP + PPi 4. AMP + ATP 2 ADP 5. PPi + ATP 2 ADP 6. argininosuccinate arginine + fumarate 7. arginine + H2O urea + ornithine SUM: NH4+ + CO2 + 4 ATP + aspartate urea + fumarate + 4 ADP + 4 Pi June 26, 2012 Total slide : 50 28
  • 29.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle The aspartate consumed in the urea cycle can be regenerated from the fumarate that is produced 2 ATP 2 ADP + Pi HCO3- + carbamoyl phosphate α-keto acids amino acids NH4+ Pi aspartate- oxaloacetate ornithine aminotransferase citrulline Urea oxaloacetate ATP cycle urea aspartate AMP + PPi malate dehydrogenase arginine argininosuccinate NADH malate NAD+ fumarate This process also uses H2O both cytosolic and mitochondrial enzymes June 26, 2012 Total slide : 50 29
  • 30.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Oxidation of malate in mitochondria generates ATP 2 e- to O2 via NADH dehydrogenase 2 ATP 2 ADP + Pi generates ~ 2.5 ATP HCO3- carbamoyl NADH mitochondrion oxaloacetate phosphate + NH4+ NAD Pi aspartate glutamate ornithine citrulline malate α-ketoglutarate ornithine citrulline α-ketoglutarate glutamate ATP aspartate urea AMP + PPi amino acids α-ketoacids arginine argininosuccinate malate cytosol fumarate H2O NADH, NAD and oxaloacetate can’t cross the mitochondrial inner membrane, but + there are transporters for malate, aspartate, glutamate and α-ketoglutarate. June 26, 2012 Total slide : 50 30
  • 31.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Transport systems in the mitochondrial inner membrane exchange aspartate for glutamate and a-ketoglutarate for malate mitochondrion aspartate- glutamate- + H+ α-ketoglutarate malate aspartate- glutamate- + H+ α-ketoglutarate malate cytosol Because the Asp/Glu transporter also moves a proton across the membrane, it can be driven by an electrochemical potential gradient. Mutations in this transporter have been linked to autism. June 26, 2012 Total slide : 50 31
  • 32.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Alpha--ketoglutarate/malate and aspartate/glutamate transporters also participate in oxidation of cytosolic NADH mitochondrion 2 e- to electron- transport chain NADH NAD+ oxaloacetate aspartate glutamate α-ketoglutarate malate aspartate glutamate α-ketoglutarate malate oxaloacetate cytosol NADH NAD+ glycolysis June 26, 2012 Total slide : 50 32
  • 33.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Well Fed State a.a.'s Asp a.a.'s αKG urea fumarate αKAs Glu NH+ 4 cycle urea OAA Mal malate CO2 + H2O Net: 2 NH4+ + CO2 + 4 ATP  urea + 4 ADP + 4 Pi June 26, 2012 Total slide : 50 33
  • 34.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Fasted State ala Asp ala αKG Gluconeogenesis urea fumarate pyr Glu NH+4 cycle urea OAA malate OAA 2 ala + CO2  1 urea + 1 glucose Glucose June 26, 2012 Total slide : 50 34
  • 35.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Balancing the levels of ammonia and aspartate for entry into urea cycle (a) NH3 in excess (b) Aspartate in excess NH3 NH3 NADH α-Ketoglutarate NADH Glutamate Glutamate α-Ketoglutarate dehydrogenase dehydrogenase NAD NAD Glutamate Glutamate Aspartate Aspartate transaminase transaminase Oxaloacetate Aspartate Oxaloacetate Aspartate Citrulline Citrulline Carbamoyl Carbamoyl phosphate phosphate Urea Urea Cycle Cycle Urea Urea Η 2Ο Η 2Ο June 26, 2012 Total slide : 50 35
  • 36.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle The urea cycle is regulated in two ways 1. Allosteric activation of carbamoylphosphate synthetase-1 by N-acetylglutamate CO2- CO2- + H3N C H + acetyl-CoA CH3CO-NH C H N-acetylglutamate CH2 CH2 CH2 CoA-SH CH2 Glu CO2- CO2- In mammals, N-acetylGlu appears to play only a regulatory role. Carbamoylphosphate synthetase-1 is completely inactive in its absence. A genetic carbamoyl- deficiency in the enzyme that forms N-acetylGlu 2 ATP 2 ADP + Pi phosphate can cause a lethal defect in the urea cycle. O O NH4+ + HCO3- H2N-C-O-P-O- O- 2. A high-protein diet or starvation leads to increased synthesis of all five enzymes used in the urea cycle, including carbamoylphosphate synthetase-1. Expression of the enzyme that synthesizes N-acetylglutamate also increases. June 26, 2012 Total slide : 50 36
  • 37.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders Deficiency of any of the five enzymes in the urea cycle results in the accumulation of ammonia and leads to encephalopathy. Episodes of encephalopathy and associated systems are unpredictable and, if untreated, are lethal or produce devastating neurologic sequelae in long-term survivors. Although these disorders do not produce liver disease, the consequences of hyperammonemia resemble those seen in patients with hepatic failure or in a transient interference with the urea cycle, as seen in some forms of organic acidemias. Investigate for hyperammonemia in any infant or child with altered mental status June 26, 2012 Total slide : 50 37
  • 38.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle The urea cycle Asterisk = N-acetyl glutamate synthetase; 1 = carbamyl phosphate synthetase; 2 = ornithine transcarbamylase; 3 = argininosuccinate synthetase; 4 = argininosuccinate lyase; 5 = arginase June 26, 2012 Total slide : 50 38
  • 39.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle UREA CYCLE DISORDERS Disorder Deficient Enzyme Inheritance Pattern Carbamyl phosphate Carbamyl phosphate Autosomal recessive synthetase deficiency synthetase Ornithine Ornithine X-linked transcarbamylase transcarbamylase deficiency Citrullinemia Argininosuccinate Autosomal recessive synthetase Argininosuccinic aciduria Argininosuccinate lyase Autosomal recessive Argininemia Arginase Autosomal recessive June 26, 2012 Total slide : 50 39
  • 40.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Case The patient is a full-term newborn boy from a normal vaginal delivery. The pregnancy was uncomplicated. At 36 hours the baby became lethargic, irritable, and was hyperventilating. Over the next 24 hours lethargy increased and progressed to coma requiring mechanical ventilation. Hemodialysis was started at 5 days. Patient died at one week of age. Laboratory Results At 36 hours arterial blood pH was 7.50 (7.35-7.45), carbon dioxide was 25 torr (35-45), and blood urea nitrogen was 2 mg/dl (5-20). Sepsis workup was negative. On day 5 plasma ammonium was 1800 :mol/l (<35). Plasma glutamine was 1500 :mol/l (550-650),arginine was below normal, and citrulline undetectable. Orotic acid in the urine was extremely elevated. Family History Two of the mother’s four brothers had died shortly after birth. Cause of death was given as encephalitis. Biochemical Basis of Disorder , same as.. Diagnosis: ornithine transcarbamoylase deficiency June 26, 2012 Total slide : 50 40
  • 41.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Biochemical explanations for ornithine transcarbamoylase deficiency Low BUN Low blood arginine Undetectable blood citrulline Elevated blood ammonia Elevated blood glutamine Elevated orotic acid June 26, 2012 Total slide : 50 41
  • 42.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Urine NH4+ + Glu  Gln urea NH2 C O NH2 NH2 H2O Mitochondrion C NH 5 CO2 + NH4+ Cytosol CH2 NH CH2NH2 2 ATP carbamoyl CH2 phosphate CH2 CH2 synthetase I CH2 (CPSI) CH2NH2 ADP + Pi H C NH2 H C NH2 CH2 1 COOH COOH COOH CH2 O O Arginine Ornithine HC H C NH2 H2N C O P O- 2 ornithine COOH CH - transcarbamoylase 4 Carbamoyl O Ornithine COOH NH2 argininosuccinase phosphate NH2 Fumarate C O C O Pi CH2 NH NH COOH CH2 NH CH2 C NH CH CH2 CH2 CH2 NH CH2 CH2 H C NH2 CH2 H C NH2 COOH CH2 COOH COOH 3 Citrulline argininosuccinate H C NH2 Citrulline synthetase COOH COOH Argininosuccinate ATP AMP + PPi H2N C H Carbamoyl P  orotic acid CH2 COOH Aspartate June 26, 2012 Total slide : 50 42
  • 43.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Carbamoyl P synthetase deficiency June 26, 2012 Total slide : 50 43
  • 44.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Urine NH4+ + Glu  Gln urea NH2 C O NH2 NH2 H2O Mitochondrion C NH 5 CO2 + NH4 + Cytosol CH2 NH CH2NH2 2 ATP carbamoyl CH2 phosphate CH2 synthetase I CH2 CH2 CH2NH2 ADP + Pi (CPSI) H C NH2 H C NH2 CH2 1 COOH COOH COOH CH2 O O Arginine Ornithine HC H C NH2 H2N C O P O- 2 ornithine COOH CH - transcarbamoylase 4 Carbamoyl O Ornithine COOH NH2 argininosuccinase phosphate NH2 Fumarate C O C O Pi CH2 NH NH COOH CH2 NH CH2 C NH CH CH2 CH2 CH2 NH CH2 CH2 H C NH2 CH2 H C NH2 COOH CH2 COOH COOH 3 Citrulline argininosuccinate H C NH2 Citrulline synthetase COOH COOH Argininosuccinate ATP AMP + PPi H2N C H CH2 Carbamoyl P  orotic acid COOH Aspartate June 26, 2012 Total slide : 50 44
  • 45.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Autism is a neurodevelopmental genetic disorder  Deficits in verbal & nonverbal communication and social interactions  Repetitive or stereotyped behaviors  Incidence ~1 per 1000 people (possibly higher)  Strong evidence for heritability  Polygenic - between 5 & 10 genes may be involved Single-nucleotide polymorphisms (SNPs) in the gene for a mitochondrial, Ca2+-dependent Asp/Glu exchanger increase the risk by a factor of 3 to 4. This is the main form of the Asp/Glu exchanger that is expressed in the brain. Mutations in the gene impair the urea cycle. N. Ramoz et al., Am. J. Psychiatry 161: 662 (2004) L. Palmieri et al., EMBO J. 20: 5060 (2001) June 26, 2012 Total slide : 50 45
  • 46.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders (Diagnosis) Cultured skin fibroblasts may be desirable if prenatal diagnosis is considered in future pregnancies. Carbamyl phosphate synthetase I and ornithine transcarbamylase (OTC) are not expressed in cultured fibroblasts. The enzymatic diagnosis of CPSD and OTCD requires liver biopsy. Biopsy should be done when establishing the diagnosis of the first case in a family. June 26, 2012 Total slide : 50 46
  • 47.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders (Treatment) Once hyperammonemia is demonstrated in an infant, protein-containing feedings should be discontinued immediately, appropriate supportive care, (mechanical ventilation) Maximal calories should be provided in the form of intravenous glucose and lipids in an effort to reduce catabolism. Plans should be immediately made to initiate hemodialysis in infants who are encephalopathic and have plasma ammonia levels over 10 times the upper limit of normal. June 26, 2012 Total slide : 50 47
  • 48.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders (Treatment) Maintenance therapy dietary protein restriction+supplementation with citrulline or arginine+ the use of drugs The primary drug now used( provides an alternate pathway for waste nitrogen excretion) for maintenance therapy in patients with urea cycle disorders is sodium phenylbutyrate (Buphenyl). The drug is typically administered four times a day in a dose of 0.4 to 0.6 g/kg/day. It is supplied as a powder, which can be mixed with food or formula, or as a tablet. June 26, 2012 Total slide : 50 48
  • 49.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Urea Cycle Disorders (Treatment) Liver transplantation for Severe neonatal OTC and CPS deficiency. Liver failure and cirrhosis in ASL deficiency. Failed medical-pharmacologic treatment. Pretransplant care by aggressively managing intercurrent hyperammonemia, vaccinations and prophylaxis are given against infectious appropriate caloric intake Gene replacement June 26, 2012 Total slide : 50 49
  • 50.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle Genetic deficiencies in some of the urea-cycle enzymes can be treated pharmacologically CO2- benzoate CO2- phenylacetate ATP + CoA-SH ATP + CoA-SH AMP + PPi AMP + PPi O S-CoA S- benzoyl-CoA CoA O phenylacetyl-CoA glycine glutamine CoA-SH O CoA-SH H N CO2- N CO2 - phenylacetyl- hippurate H O glutamine (benzoylglycine) O NH2 The amide products of these reactions (hippurate and phenylacetylglutamine) are excreted in the urine. Replenishing the Gly or Gln removes ammonia. June 26, 2012 Total slide : 50 50
  • 51.
    Metabolic & MolecularBasis of Inherited Disease Metabolic Disorders of Urea Cycle THE END

Editor's Notes

  • #19 June 26, 2012
  • #22 June 26, 2012 Historical note: Hans Krebs discovered the urea cycle some years prior to his discovery of the TCA cycle.
  • #32 June 26, 2012 N. Ramoz et al. “Linkage and association of the mitochondrial aspartate/glutamate carrier SLC25A12 gene with autism,” Am. J. Psychiatry 161: 662-669 (2004)
  • #46 June 26, 2012 The SNPs are intronic. Deficiencies in the Asp/Glu exchanger could have other effects on mitochondrial metabolism in addition to impairing the urea cycle.