Molecular characterization of alanine racemase in the Kuruma prawn Marsupenaeus japonicus.

Aquatic crustaceans and some bivalve mollusks are known to contain copious amounts of free d-alanine in their tissues. For the first time in the animal kingdom, we have isolated a cDNA clone encoding alanine racemase from the muscle and hepatopancreas of the kuruma prawn Marsupenaeus japonicus. The recombinant enzyme expressed in Escherichia coli exhibited alanine recemase activity. The deduced amino-acid sequence showed only 23-31% identity to bacterial alanine racemases. However, the active site residues and some residues that interact with pyridoxal 5'-phosphate were also conserved in M. japonicus enzyme. There was higher alanine racemase mRNA expression in hepatopancreas than in muscle. In contrast, the d-alanine content in hepatopancreas was lower than that in muscle, suggesting that the physiological functions of free d-alanine may differ among tissues. These data suggest that the alanine racemase gene has been conserved from bacteria to invertebrates throughout a long evolutionary time scale.