The role of amino acids in anaerobiosis and osmoregulation in bivalves

Abstract

Abstract The role of free amino acids as solutes for cell volume regulation in marine molluscs under salinity stress is well established. Under hypoosmotic stress the intracellular concentrations of amino acids decrease by extrusion from the cells, followed by deamination and/or excretion. However, no uncontroversial evidence is available about mechanisms involved in the enlargement of the free amino acid pool during hyperosmotic stress. Since anaerobic metabolism in bivalves can result in the accumulation of the amino acids alanine and glutamate, a possible contribution of the anaerobic metabolic pathways to the osmotic adjustment of the cell during hyperosmotic stress has been considered. The net gain of amino acids implies the fixation of ammonia to 2‐oxo acids. The low activity of glutamate dehydrogenase found in bivalves does not exclude its role in osmoregulation and anaerobiosis. Another ammonia‐fixating enzyme may be alanine dehydrogenase, but so far evidence for its presence in bivalves is lacking. In bivalves the source of ammonia for these enzymes may be found in part in uric acid or other amino compounds that are present in reasonable amounts.