1. Detoxification of ammonia and biosynthesis of urea. The basic features of nitrogen metabolism were elucidated initially in pigeons

2. AMMONIA METABOLISM The 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)

3. 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

4. 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

5. 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.

6. 2. Nitrogen can be transported as glutamine. Glutamine synthetase catalyzes the synthesis of glutamine from glutamate and NH 4 + in an ATP- dependent reaction:

7. 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; - NH 4 +. Carbon atom comes from CO 2.

8. 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)

9. 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

10. In the cytosol citrulline in the presence of ATP reacts with aspartate to form argininosuccinate Enzyme: argininosuccinate synthetase

11. Argininosuccinate is cleaved to free arginine and fumarate Enzyme: argininosuccinate lyase The fumarate enters the tricarboxylic acid cycle

12. 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)

14. 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.