Distribution of Free D-Amino Acids in Bivalve Mollusks and the Effects of Physiological Conditions on the Levels of D- and L-Alanine in the Tissues of the Hard Clam, <i>Meretrix lusoria</i>

Abstract

The distribution of D-amino acids was examined on the tissues of seven species of marine bivalve mollusks belonging to two different subclasses, Pterimorphia and Heterodonta. High concentration of D-alanine was found in all tissues of only Heterodonta and not in the tissues of Pterimorphia except in gills and midgut gland. Several other D-amino acids including D-arginine also occurred in the several tis sues of both groups of bivalves.Along with the salinity stress on the hard clamMeretrix lusoria in 150% seawater, only D and L alanine increased in adductor muscle, gills, and midgut gland. The incorporation of D-alanine from ex ternal seawater was found onlyin gills, hemolymph, and midgut gland of hard clam, and the levels of D and L-alanine inadductor and foot muscles decreased in the seawater containing 50 HIM D-alanine and returned to the control level after recovery innormal seawater.During the starvation of hard clam for 22 days,D and L-alanine as well as other free amino acids decreased considerably in most tissues. The anoxia of hard clam for a week duration gave no large effect on the levels of free amino acids but resulted in the slight increases of D and L-alanine levels after sevendays.

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“…Of these species used in this study, all crustaceans and some mollusks were reported to contain some Damino acid, especially D-alanine, in their tissues 17 . Muscle, gill, and hepatopancreas of kuruma prawn and crayfish contained considerable amounts of Dalanine and small amounts of D-arginine, D-aspartate, and D-glutamate 21 , but no activity of these enzymes was detected in muscle and gill except for DAO activity in the hepatopancreas of kuruma prawn and both enzyme activities in crayfish hepatopancreas and antenal gland.…”

“…Muscle, gill, and hepatopancreas of kuruma prawn and crayfish contained considerable amounts of Dalanine and small amounts of D-arginine, D-aspartate, and D-glutamate 21 , but no activity of these enzymes was detected in muscle and gill except for DAO activity in the hepatopancreas of kuruma prawn and both enzyme activities in crayfish hepatopancreas and antenal gland. Several crustaceans contained a sizable amount of D-alanine in muscle, gill, nervous tissues, and hepatopancreas 17 . These crustaceans have also been reported to have alanine racemase in these tissues 22 .…”

“…Thus, at least in hepatopancreas of crustaceans, DAO and/or DDO might be expected to have some roles in the metabolism of D-amino acids. Adductor, foot, and mantle muscles and mid gut gland of hard clam and geoduck contained copious amounts of D-alanine and small amounts of D-arginine, Daspartate, and D-proline 17 . In contrast, almost no activity of DAO and/or DDO was detected in these organs.…”

“…Ark shell, scallop, and oyster as well as squid and octopus contained only trace amounts of D-amino acids in the muscle tissues 14,17 . All organs of octopus showed substantial activities of both enzymes.…”

“…Recently, significant quantities of D-amino acids have been detected in various animals including humans. For instance, a large amount of D-alanine has been found in various aquatic invertebrates such as crustaceans and bivalve mollusks 17 . Thus, aquatic animals are considered to have a high chance of intake of D-amino acids through predation compared with terrestrial animals where the D-amino acid source is rather limited to bacteria in foods and intestine or food consisting of aquatic invertebrates.…”