Amino acid metabolism in euryhaline bivalves: Regulation of glycine accumulation in ribbed mussel gills

Glycine levels in isolated ribbed mussel (Modiolus demissus) gill tissue increased slightly and decreased markedly when incubated at high and low salinities, respectively. Low levels of the enzymes involved in the biosynthesis of serine from triose phosphate intermediates, the serine hydroxy‐methyltransferase, and serine dehydrase were detected in gill tissue homogenates. Experiments using gill tissue incubated with (U‐14C)‐glycine and (U‐14C)‐serine indicated interconversion between serine and glycine and transfer of label to alanine, asparate, glutamate, CO2, organic acids, and protein. Glyoxylate was metabolized more slowly than glycine and was probably converted to glycine for catabolism. Studies using (1‐14C)‐glycine and (2‐14C)‐glycine with isolated gill tissue and mitochondria indicated that the mitochondrial glycine cleavage enzyme was the major route of glycine catabolism. Metabolic controls activating or inhibiting the glycine cleavage enzyme regulate tissue glycine accumulation and catabolism during hypersalinity or hyposalinity stress.