AMONYAK
DETOXİFİKASYONU

Doç Dr Osman EVLİYAOĞLU
Ammonia is a toxic substance to plants and animals
(especially for brain)
Normal concentration: 25-40 mol/l (0.4-0.7 mg/l)
Ammonia must be removed from the organism
Peripheral Tissues Transport Nitrogen to the Liver
Two ways of nitrogen transport from peripheral
tissues (muscle) to the liver:
Glutamate is
not deaminated
1. Alanine cycle. Glutamate is
formed by transamination reactions in peripheral
tissues
Nitrogen is then transferred to pyruvate to
form alanine, which is released into the blood.
The liver takes up the alanine and converts it back
into pyruvate by transamination.
The glutamate formed in the liver is deaminated
and ammonia is utilized in urea cycle.
2. Nitrogen can be transported as glutamine.
Glutamine synthetase catalyzes the synthesis of
glutamine from glutamate and NH4+ in an ATPdependent reaction:
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
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
Disposal of Amino Acids Nitrogen:
Key reactions


Transamination reactions
Deamination reactions


Glutamate dehydrogenase
Hydrolytic deamination


Glutaminase
Glutamine synthesis
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
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
THE UREA CYCLE
Urea cycle - a cyclic pathway of urea synthesis
first postulated by H.Krebs
The sources of
nitrogen atoms in
urea molecule:
- aspartate;
- NH4+.
Carbon atom
comes from CO2.
The free ammonia is coupling with carbon dioxide to
form carbamoyl phosphate
Two molecules of ATP are required
Reaction takes place in the matrix of liver
mitochondria
Enzyme: carbamoyl phosphate synthetase (20 % of
the protein of mitochondrial matrix)
Carbamoyl
ornithine
phosphate
donates
carbamoyl
group
to
The product - citruilline
Enzyme: ornithine carbamoyltransferase
Reaction takes place in the mitochondrial matrix
Citrulline leaves the matrix and passes to the cytosol
In the cytosol citrulline in the presence of ATP
reacts with aspartate to form argininosuccinate
Enzyme: argininosuccinate synthetase
Argininosuccinate is cleaved to free arginine and
fumarate
Enzyme: argininosuccinate lyase
The fumarate enters the tricarboxylic acid cycle
Arginine is hydrolyzed to generate urea and ornithine
Enzyme: arginase (present only in liver of ureotelic
animals)
Ornithine is transported back into the mitochondrion
to begin another cycle
Urea is excreted (about 40 g per day)