Distribution, Metabolism, and Physiological Functions of Free D-Amino Acids in Invertebrates

Abstract

Aquatic crustaceans and some bivalve mollusks contain a copious amount of free D-alanine (3-100 umol/g wet wt.) in their tissues. Under high osmotic stress, D-and L-alanine were largely accumulated In these animals and were proven to be one of the major compatible osmolytes responsible for the intracellular isoosmotic regulation. cDNAs of alanlne racemase catalyzing the interconversion of D-and L-alanine were cloned from the muscle and hepatopancreas of kuruma prawn. Carp which feeds on invertebrates contained only a trace amount of D-alanine and showed high activities of D-amino acid and D-aspartate oxidases in some tissues. A cDNA of D-amino acid oxidase was cloned from the hepatopancreas of carp. During adminlstration of D-alanine to carp, mRNA and enzyme activity of D-amino acid oxidase were largely increased in intestine followed by hepatopancreas and kidney, suggesting the inducible nature of the enzyme.