Distribution, metabolism and physiological functions of free D-amino acids in aquatic invertebrates

Abstract

Several aquatic crustaceans and bivalve molluscs contain a large amount of free D-alanine (3-50 μmol/g wet wt.) in their tissues. Under high salinity stress, crustaceans and bivalve molluscs accumulate much D- and L-alanine irrespective of species, together with L-glutamine, L-proline, and glycine, the increases of which are species dependent. These data indicate that D-alanine is one of the major compatible osmolytes responsible for the intracellular isosmotic regulation in the tissues of crustaceans and bivalves. Under hypoxia stress, crayfish increased D- and L-alanine in muscle and hepatopancreas in addition to the increase of lactate. The increase was much higher in seawater than in freshwater. Thus, D- and L-alanine are possible anaerobic end products during prolonged hypoxia of this species and other invertebrates. Alanine racemase has been proved to catalyze the interconversion of D- and L-alanine in crustaceans and bivalve molluscs. This enzyme has been isolated to homogeneity from the muscle of black tiger prawn, crayfish and a brackish water clam. It has also been clarified that D-amino acid and D-aspartate oxidases which catalyze the decomposition of D-amino acids to corresponding α-keto acids distributed widely in some tissues of invertebrates and fish species. In this review, the author also describes the recent advances of D-amino acid researches on invertebrates and other animals.