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 Mytilus 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‰ to 30‰ 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 FAA 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, and there were genotype-dependent differences in their rates of accumulation. Mussels returned to 15‰ SW after a 70-h exposure to 30‰ 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‰ SW. These results strongly suggest that protein hydrolysis is a major source of FAA during hyperosmotic cellular volume regulation.