Methabolism and functional role of D-amino acid in marine invertebrates.

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D-Amino acids, which were considered as natural oddities, have been found in various animals, plants and bacteria. D-Aspartic acid (D-Asp) has been also detected in human proteins like eye lens, tooth enamel and brain. D-Asp was proposed to be causes and indicators of diseases and aging. Occurrence of D-asp has therefore gained increasing attention in a variety of research fields. In marine invertebrates, several investigators have described either isolated occurrences of individual D-amino acids or have measured the unspecified presence of D-amino acid using enzyme assay. This study was aimed to obtain the basic information on the distribution, metabolism and physiological function of D-amino acids in marine invertebrates.Among fourteen DL-amino acids which compose protein, except DL-lysine, thirteen DL-amino acids were possibly analyzed by optical-resolution HPLC method. This method was however able to detect nine D-amino acids such as Asp, Glu, Thr, Arg, Ala, Tyr, Val, Ile and Leu, in biological tissue extracts. Several marine invertebrates were analyzed for D-amino acids using this method. Only two D-amino acids (D-asp and D-ala) were confirmed in several marine invertebrates examined. High concentration of D-Asp was detected in the blood shell Scapharca broughtonii. The concentration ratio of D- to L-isomer was below one. Low concentration of D-asp was detected in all other species especially in its gill. Significant concentration of D-Ala was detected in a clam Meretrix lusoria. The concentration of D-ala was three times higher than that of L-ala in the adductor muscle of this species. D-Ala was also detected in high concentration in shrimp Crangon affinis and in sea urchin Strongylocentrotus nudus. When C. affinis, which was kept in 50% sea water for one week, was transferred to 0, 25, 75 and 100% sea water and analyzed the concentration of DL-alanine. D-Ala changed more intensely than L-ala under both low and high salinity conditions.