Role of D-amino acid in adaptation of organisms to environmental change

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After the finding that D-serine serves as an agonist of NMDA receptor of a mammalian brain, role of D-amino acids in eucaryotes has been received an increased attention. The fresh water crayfish has been reported to accumulate total free D-alanine by more than 30-fold in muscle after seawater acclimation. We are interested in the realtionship between D-amino acids and the environmental stress, and studied the enzymes related to the D-amino acid metabolism in eucaryotes. We found the alanine racemase from the hepatopancreas of the black tiger prawn, Penaeus monodon. The prawn enzyme is distinguishable immunochemically from bacterial enzymes and differs from fungal enzymes in the moleular weight and subunit structure. Monovalent anions including chloride are involved in the regulation of the enzyme through stabilization and activation. This is consistent with the hypothesis that D-alanine serves as an osmoregulator in marine and euryhaline animals. We also found that the silkworm has serine racemase which has similar properties to the rat brain serine racemase in pH optimum, substrate specificity, and behaviors upon chromatographies. At the stage of pupation, the contents of D-serine increased concomitantly with the increase in this enzymatic activity. In this work, we also studied the three amino acid racemases of Schizosaccharomyces pombe, Alr1p, Alr2p, and Srf1p. Alr1p is a homologs of bacterial alanine racemase. A1r2p is a fission yeast counterpart to bacterial arginine racemase, and Srf1p resembles a rat serine racemase in low catalytic efficiency. These racemases are probably involved in the detoxification of D-amino acids. In this work, we also performed the screening of new enzymes involved in the D-amino acid metabolism in marine and euryhaline animals in Australia and the United States.