d-Alanine content in the marine edible bivalve Panopea japonica and evaluation of its associated enzyme activities

Amino acids play essential roles in various biological processes. In humans, most amino acids are present in the L-form; however, small amounts of D-amino acids also exist and have significant physiological roles, highlighting the importance of dietary intake from foods or drinks. In this study, we investigated the amino acid composition of the geoduck clam Panopea japonica, emphasizing its remarkably high D-alanine (D-Ala) content in the siphon tissue. The D-Ala content (6.99-14.2 mmol/100 g-wet) amounted to 91-94% of the total Ala, far exceeding that of other bivalves such as Tresus keenae (74%). Enzyme assays revealed alanine racemase and D-amino acid oxidase activities, suggesting active D-Ala biosynthesis and metabolism. The high concentrations of D-Ala enhance its value as a delicacy owing to its unique sweetness. This study provides new insights into the biosynthesis and metabolic characteristics of D-Ala in bivalves, highlighting its physiological and food ingredient significance.