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-betamethylbutyrate). 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 B12
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


Formation of ALA


AA + aKG <-> aKA + GLU
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



Periportal hepatocytes



Portal blood ammonia = 300 –
1000 uM
Systemic blood ammonia =
20uM
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 Case
source 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)