Amino Acid Catabolism Disposal of Nitrogen and Carbon Skeletons
Dec 16, 2015
Amino Acid Catabolism
Disposal of Nitrogen and Carbon Skeletons
Clinical Case Study
Male infant, 2.9 kg at birth, healthy Day 3 - seizures Mother with history of aversion to meat
vomiting and lethargy
plasma [NH4+] = 240 uM (25-40 normal)
hyperammonemia mild alkalosis (pH=7.5, normal 7.35-7.45)
Clinical Case Study
Plasma AA gln = 2400 uM (350-650) ala = 750 uM (8-25) arg = 5 uM (30-125) cit = undetectable
Urinary orotic acid = 285 ug/mg creatinine (0.3-10)
Clinical Case Study
Oral therapy initiated EAA + arginine Sodium benzoate
Patient improves after 7 days Plasma [NH4
+] normalized
Overview of Amino Acid Catabolism:Interorgan Relationships
Overview of Amino Acid Catabolism:Interorgan Relationships
Intestine Dietary amino acids absorbed Utilizes glutamine and asparagine as energy
sources Releases CO2, ammonium, alanine, citrulline as
endproducts Utilizes glutamine during fasting for energy
Dietary amino acids and catabolites released to portal blood
Enteral Formulas containing glutamine
JUVEN is a therapeutic nutritional that contains a patented blend of arginine, glutamine, and HMB (beta-hydroxy-beta-methylbutyrate). JUVEN has been clinically shown to help build lean body mass (LBM),1,2 enhance immune response,2 and promote collagen synthesis
Overview of Amino Acid Catabolism:Interorgan Relationships
Liver Synthesis of liver and plasma proteins Catabolism of amino acids
Gluconeogenesis Ketogenesis Branched chain amino acids not catabolized Urea synthesis
Amino acids released into general circulation Enriched (% of total aa) in BCAA (2-3X)
Overview of Amino Acid Catabolism:Interorgan Relationships
Skeletal Muscle Muscle protein synthesis Catabolism of BCAA
Amino groups transported away as alanine and glutamine (50% of AA released)
Alanine to liver for gluconeogenesis Glutamine to kidneys
Kidney Glutamine metabolized to a-KG + NH4
a-KG for gluconeogenesis NH4 excreted or used for urea cycle (arginine synthesis)
Important buffer preventing acidosis [NH4
+] : [NH3] = 100 : 1
Overview of Amino Acid Catabolism:Interorgan Relationships
Vitamin-Coenzymes in Amino Acid Metabolism
Vitamin B-6 (pyridoxal phosphate) Folic acid (tetrahydrofolate) Vitamin B-12
Vitamin-Coenzymes in Amino Acid Metabolism
Vitamin B-6 : pyridoxal phosphate Enzymes that bind amino
acids use PLP as coenzyme for binding
Transaminases Amino acid
decarboxylases Amino acid deaminases
Vitamin-Coenzymes in Amino Acid Metabolism
Folacin: Tetrahydrofolate (THF) Carrier of single
carbons Donor & receptor Glycine and serine Tryptophan degradation Histidine degradation Purine and pyrimidine
synthesis
Vitamin-Coenzymes in Amino Acid Metabolism
Vitamin B-12 Catabolism of BCAA
Methyl-malonyl CoA mutase (25-9 &10)
Vitamin-Coenzymes in Amino Acid Metabolism
Vitamin B-12 Methionine
synthesis/recycling Methionine as a methyl
donor Choline and creatine
synthesis Homocysteine is product HCys -> Met requires B-
12
Figure 26-4
Overview of Amino Acid Catabolism:Interorgan Relationships
How does this occur?
Disposal of Amino Acids Nitrogen: Key reactions
Transamination reactions Deamination reactions
Glutamate dehydrogenase Hydrolytic deamination
Glutaminase
Glutamine synthesis
Disposal of Amino Groups:Transamination Reactions
Often the first step of amino acid degradation Transfer of amino group from many amino acids
to limited number of keto acid acceptors Pyruvate <-> alanine Oxaloacetate <-> aspartate Alpha-keto-glutarate <-> glutamate
Disposal of Amino Groups:Transamination Reactions
Transamination reactions tend to channel amino groups on to glutamate Glutamate’s central role in amino acid N metabolism
Disposal of Amino Groups:Transamination Reactions Transaminase reactions are reversible
ALT = SGOT ALA important in muscle where ~25% of AA-N is transported
out on ALA In liver, reverse reaction moves AA-N back on GLU
AST = SGPT ASP important in liver since half of urea-N is from ASP
Disposal of Amino Groups:Deamination Reactions
Glutamate dehydrogenase oxidative deamination Important in liver where it releases ammonia for urea synthesis
Hydrolytic deamination Glutaminase & asparaginase
Disposal of Amino Groups:Glutamine Synthetase
Important plasma transport form of nitrogen from muscle
Detoxification of ammonia Brain Liver
Removes ammonia intestinal tract Bacterial deamination of amino acids Glutamine utilization in intestinal cells
Overview of Amino Acid Catabolism:Interorgan Relationships
Movement of amino acid nitrogen:post-absorptive and fasting states
From extra-hepatic tissues (muscle) to liver Site of gluconeogenesis and ketogenesis Site of urea synthesis
All amino acids present in plasma but enriched (~50%) in alanine and glutamine
Production of ALA & GLN in extrahepatic tissues
Transamination of AA to form GLU AA + aKG <-> aKA + GLU
Formation of ALA GLU + pyr <-> aKG + ALA
Formation of GLN GLU <-> aKG + NH3
NH3 + GLU -> GLN
Overview of Amino Acid Catabolism:Interorgan Relationships
Detoxification of Ammonia by the Liver: the Urea Cycle
Amino acid N flowing to liver as: Alanine & glutamine Other amino acids Ammonia (from portal blood)
Urea chief N-excretory compound
Detoxification of Ammonia by the Liver: the Urea Cycle
Contains all enzyme of urea cycle
Site of urea synthesis Kidney has all urea
cycle enzymes except arginase
Site of arginine synthesis
Mitochondria CPS regulatory
enzyme
Flow of Nitrogen from Amino Acids to Urea in Liver
Amino acid flow from muscle to liver Alanine & glutamine
Liver Transfers N to GLU
GLN’ase & GDH Transaminases
Transfers GLU-N to: ASP
AST Transamination route
NH3 GDH
Trans-deamination route GLN’ase
Transfers N to urea
Ammonia detoxification by the liver
Liver very effective at eliminating ammonia from blood
Portal blood ammonia = 300 – 1000 uM
Systemic blood ammonia = 20uM
Periportal hepatocytes Urea synthesis Km CPS ~ 1mM
Perivenous hepatocytes Glutamine synthesis
Very low Km for ammonia Removes any NH3 not
removed by periportal hepatocytes
Clinical Case Study
Male infant, 2.9 kg at birth, healthy Day 3 - seizures Mother with history of aversion to meat
vomiting and lethargy
plasma NH4+ = 240 uM (25-40 normal)
hyperammonemia mild alkalosis (pH=7.5, normal 7.35-7.45)
Clinical Case Study
Plasma AA gln = 2400 uM (350-650) ala = 750 uM (8-25) arg = 5 uM (30-125) cit = undetectable
Urinary orotic acid = 285 ug/mg creatinine (0.3-10)
Resolution of Clinical Case
Diagnosis of neonatal hyperammonemia symptoms blood ammonium concentration
Defect in urea cycle elevated glutamine and alanine low or absent arginine and citrulline
Detoxification of Ammonia by the Liver: the Urea Cycle
Resolution of Clinical Case
Genetic deficiency of ornithine transcarbamoylase urinary orotic acid
CP spills into cytosol where enters pyrimidine biosynthetic pathway, orotic acid an intermediate in the pathway
Resolution of Clinical Casesource of orotic acid
Clinical Case Study
Treatment Oral therapy essential amino acids arginine sodium benzoate
@7 days clinically well normal NH4
+
Resolution of Clinical Case:Treatment
Essential Amino Acids Arginine
w/o urea cycle, becomes essential
Benzoic acid conjugates with glycine and excreted in urine as
hippuric acid glycine in equilibrium with ammonia
Glycine synthase CO2 + Me-THF + NADH + NH3 => glycine
removal results in reducing ammonia levels
Resolution of Clinical Case:Genetics
Gene for OTC found on X-chromosome Women are carriers
usually asymptomatic may experience migraines, vomiting, lethargy
when eating high protein meals (meat) OTC deficiency most common (but rare)
disorders of the urea cycle (1: 20-80,000)