Physiology of Finfish and Shellfish (2+1)

4.2.1. Ammonia production

Ammonia is the major end product of N-metabolism in almost all osteichthyes, agnathans, and freshwater chondrichthyes, but less important than urea in marine chondrichthyes, dipnoans during estivation, and a very few teleosts living in extreme environment .

The largest source of ammonia is catabolism of dietary or structural protein . Essential amino acids in fish are the same as those in mammals: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (Halver and Shanks, 1960); all others can be synthesized. Fish have a remarkable capacity to utilize amino acids both as a metabolic fuel and as precursors for protein, lipid, and carbohydrate synthesis. In actively growing fish fed a balanced diet, greater than 50% of dietary N-content is incorporated into structural growth, and most of the remainder is used for energy production. Dietary carbohydrate does not appear to be an important aerobic fuel, but there is nevertheless a high capacity for gluconeogesis from exogenous amino acids. However, dietary lipid certainly is important, especially in carnivores. If the d i et is deficient in lipid, then a greater proportion of dietary protein is metabolized for energy or deaminated for conversion to fat and carbohydrate, more ammonia is excreted, and a lower percentage of dietary N is retained for growth.

All fish exhibit a marked ability to consume their own struc­tural protein during prolonged starvation, but the size of the endogenous fraction as a proportion of metabolic rate varies depending on the extent of fat reserves. Glycogen reserves tend to be spared during starvation, probably because of their survival value as an anaerobic fuel during burst exercise and hypoxia. The other important source of ammonia is the deamination of adenylates .

When fish are feeding and growing, absorbed amino acids in excess of those needed for protein synthesis are deaminated and then oxidized in the Kreb's cycle or converted to fat and carbohydrate. When fish are starving, or energetic expenditure exceeds intake, amino acids released by muscle proteolysis are similarly deaminated. T he most important pathway (Walton and Cowey, 1977) is thought to be th e transamination system. Various aminotransferases transfer the amino group of L­amino acids to alpha-ketoglutarate to form glutamate, w ich is subsequently deaminated by glutamate dehydrogen ase . This occurs in many tissues, including muscle, gill, and kidney, but enzyme activities are generally highest in liver. The other major pathway is probably the direct hydrolysis of the amide groups on glutamine and asparagine, catalyzed by glutaminase and asparaginase . These enzymes occur in liver, kidney, and gills, and at least glutaminase is present in both red and white muscle.

The purine nucleotide cyclel of Lowenstein (1972) has been suggested as another deamination route, albeit energetically more expensive. Here, the amino group on glutamate is transferred to oxaloacetate by aspartate aminotransferase to form aspartate and regenerate alpha-ketoglutarate. Aspar­tate then reacts with inosine monophosphate (IMP) and guanosine triphosphate (GTP) to form adenylosuccinate, mediated by adenylosuccinate synthetase, followed by conversion to fumarate and adenosine monophosphate (AMP), mediated by adenylosuccinate lyase. The deamination of AMP by AMP deami­nase liberates ammonia and regenerates IMP, thereby completing the cycle. In addition, fumarate may be converted to malate by fumarase, and then back to oxaloacetate by malate dehydrogenase, thereby regenerating the recipient of the amino group. While the enzymes of this cycle are present in the rver, it does not appear to be the main hepatic pathway of amino acid deamination . Illustration