PROTEIN AS A SOURCE OF AMINO NITROGEN DURING HYPEROSMOTIC VOLUME REGULATION IN THE MUSSEL MYTILUS-EDULIS

The proteolytic enzyme aminopeptidase-I (AM-I) is the product of the polymorphic Lap locus in the mussel M. edulis. Genotypes with the Lap94 allele have a 20% greater rate of substrate turnover than other genotypes. If protein hydrolysis is a source of the free amino acids (FAA) which accumulate during cellular adjustment to high salinity, Lap94 genotypes should accumulate FAA faster than other genotypes. Transfer of mussels from 15.permill.-30.permill. seawater (SW) resulted in genotype-dependent rates of FAA accumulation in the digestive gland. The digestive gland, mantle and adductor muscle of individuals with the Lap94 allele accumulated, respectively, 38%, 31%, and 6% more FFA than these tissues from mussels without the Lap94 allele after 70 h at high salinity. The magnitude of the genotype-dependent difference in FAA accumulation in each tissue was strongly correlated with the AM-I activity of the tissue. In the digestive gland, alanine and glycine accounted for most of the increase in FAA; there also were genotype-dependent differences in their rates of accumulation. Mussels returned to 15.permill. SW after a 70-h exposure to 30.permill. SW had elevated rates of FAA and ammonia excretion. Individuals with the Lap94 allele excreted 50% more ammonia and FAA than mussels without the Lap94 allele. There were no genotype-dependent differences in the excretion rates of animals acclimated to 15.permill. SW. Protein hydrolysis probably is a major source of FAA during hyperosmotic cellular volume regulation.