Food Chemistry and Fish in Nutrition

8.2.2.3. Ammonia formation and Transport

1. Ammonia Transport by L-amino acid oxidases

L-amino acid oxidase is present in liver and kidney tissue. These autoxidizable flavoproteins oxidize amino acids to an α-imino acid that adds water and decomposes to the corresponding α-keto acid with release of ammonium ion. The reduced flavin is reoxidized directly by molecular oxygen, forming hydrogen peroxide (H 2 O 2 ), which is split to O 2 and H 2 O by the enzyme catalase present in many tissues, especially liver.

2. Formation of ammonia and transport by Glutamine synthetase

While ammonia is constantly produced in the tissues, it is rapidly removed from the circulation by the liver and converted to glutamate, glutamine, and ultimately to urea. Formation of glutamine is catalyzed by glutamine synthetase. Synthesis of the amide bond of glutamine is accomplished at the expense of hydrolysis of one equivalent of ATP to ADP and Pi.

3.Action of Glutaminase and asparaginase Hydrolytic release of the amide nitrogen of glutamine as ammonia, catalyzed by glutaminase, strongly favours glutamate formation. Glutamine synthetase and glutaminase thus catalyze inter conversion of free ammonium ion and glutamine. An analogous reaction is catalyzed by L-asparaginase. These enzymes occur in liver, kidney and gills and also in red and white muscles.