amino acid catabolism disposal of nitrogen and carbon skeletons

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)