METABOLISM OF D-ALANINE AND L-ALANINE AND REGULATION OF INTRACELLULAR FREE AMINO-ACID LEVELS DURING SALINITY STRESS IN A BRACKISH-WATER BIVALVE CORBICULA-JAPONICA

1. High salinity-induced accumulation of free amino acids (FAA) in tissues of a brackish-water clam (Corbicula japonica) was compared between animals incubated in the presence or absence of externally added amino acids, of which concentration and composition roughly mimicked those occurring in the habitat. 2. Most FAA were accumulated without significant difference between the two groups with or without added FAA. 3. Incubation with 40-mu-M L-alanine under a condition without salinity stress did not affect significantly the tissue levels of any major FAA, though the significant net uptake of the amino acid was detected by measuring disappearance of L-alanine from the external medium (HPLC). 4. The incorporation of radioactivity from [C-14]L- or [C-14]D-alanine into 5% trichloroacetic acid (TCA)-50% ethanol-precipitated fractions of tissue homogenate was compared between the two isomers. A major part of radioactivity recovered in the TCA-ethanol soluble fractions was detected as alanine, even after 24 hr of chase period. 5. D-Alanine appeared to be metabolized through conversion to L-alanine by the action of alanine racemase.