The studies on the mechanism of D-alanine accumulation in bivalves

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Traditionally, it has been assumed that D-amino acids are not present in the free amino acid pools or proteins of higher organisms. Author showed that the content of D-alanine is high level in bivalves belong to Subclass Eulamellibranchia and low level in other bivalves, gastropods and cephalopods. The present studies were planned to accumulate more information on distribution and physiological role of D-alanine in bivalves.1) Improved HPLC method for the determination of D and L-alanine pre-labeled with accety-L-cysteine-omicron-phalaldehyde was invetigated.2) The content of D-alanine was determined in the tissues of a marine bivalve Barnea dilatata and two species of fresh water bivalves Unio douglasiae and Anodonta woodiana which belog to Subclass Eulamellibranchia. D-alamine contents was high level in the muscle and viscera of Barnea dilatata and D/D+L ratio was 0.4 and 0.5, respectively. On the other hand, D-alanine content in the tissues of Unio douglasiae and Anodonta woodiana was low level. But D/D+L ratio was 0.22-0.47 and 0.05-0.27, respectively.3) Thirty specimens of hard clam Meretrix Iusoria were acclimated in the artificial seawater (35%, 100% seawater) for two days. After 7 days salinity in the rearing water was gradually increased to 53% (150% seawater) usingan artificial seawater salt mixture. D and L-alanine were deterined in the tissues of 100% and 150% specimens. The amounts of D-alanine in the adductor muscle, gills and mid-gut gland of 100% specimens were 29.4,7.3 and 3.0 mumol/g respectively. After acclimation of 150% seawater the contents of D-alanine were increased to 45.6,15.1 and 13.1 mumol/g, respectively. The content of L-alanine was also increased in all tissues of 150% seawater specimens. These results suggest that D-alanine together with L-alanine is the osmolyte for osmoregulation in hard clam.