Detoxification of Detoxification of ammonia and ammonia and

biosynthesis of biosynthesis of ureaurea..

The basic features of nitrogen metabolism were elucidated initially in pigeons

AMMONIA METABOLISMThe ways of ammonia formation

1. Oxidative deamination of amino acids

2. Deamination of physiologically active amines and nitrogenous bases.

3. Absorption of ammonia from intestine (degradation of proteins by intestinal microorganisms results in the ammonia formation).

4. Hydrolytic deamination of AMP in the brain (enzyme – adenosine deaminase)

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 Terrestrial vertebrates synthesize urea (excreted by the kidneys) - ureotelic organisms

Birds, reptiles synthesize uric acid

Urea formation takes place in the liver

Peripheral Tissues Transport Nitrogen to the Liver

Two ways of nitrogen transport from peripheral tissues (muscle) to the liver:

1. Alanine cycle. Glutamate is formed by transamination reactions

Glutamate is not deaminated 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 ATP-dependent reaction:

Urea cycle - a cyclic pathway of urea synthesis first postulated by H.Krebs

THE UREA CYCLE

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 phosphate donates carbamoyl group to ornithine

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)

The Linkage between Urea Cycle, Citric Acid Cycle and Transamination of Oxaloacetate

Fumarate formed in urea cycle enters citric acid cycle and is converted to oxaloacetate.

Fates of oxaloacetate: (1) transamination to aspartate, (2) conversion into glucose,(3) condensation with acetyl CoA to form citrate,(4) conversion into pyruvate.