Mitochondrial ammoniagenesis in liver of the channel catfish Ictalurus punctatus.

Rates of ammonia formation from six amino acids by hepatocytes and liver mitochondria were compared with the rate of ammonia excretion by individual fish. Glutamine and asparagine are the most ammoniagenic substrates for hepatocytes and glutamine and glutamate, for isolated liver mitochondria. The main site of ammonia release is the mitochondrion. Ammonia is formed mainly via deamidation or transdeamination. Relatively small amounts of aspartate are formed from glutamate and glutamine by the fish liver mitochondria. This apparent "uncoupling" of glutamate transamination may represent an adaptation of carnivorous fish to the utilization of amino acids as a major energy source. More than 90% of the ammonia formed from glutamate and glutamine by isolated mitochondria are elaborated into the medium. Exiting ammonia is accompanied by a proton, but the source of this proton appears not to be the electrogenic proton pump or the carboxylate of exiting glutamic acid.