Biochemical studies of aminopeptidase polymorphism in Mytilus edulis. I. dependence of enzyme activity on season, tissue, and genotype.

Aminopeptidase-I is polymorphic in the marine bivalve Mytilus edulis and catalyzes the liberation of neutral an aromatic N-terminal amino acids from oligopeptides. The enzyme is abundant in the digestive gland, where it is lysosomal, but is present in several other tissues. Temporal variation in enzyme activity was monitored for 2.5 years in two natural populations. The temporal pattern of variation was similar in gill, mantle, and digestive gland tissues; variations occurred over both short and long time periods. Enzyme activity under ambient temperature conditions was seasonally related to temperature in gill and digestive gland, but varied with reproductive cycle in mantle tissue. In the last, maximum activity corresponded to the postreproductive period in each population. Enzyme activity varies in response to tissue-specific metabolic demands. Population difference in enzyme activity are due to both genotype-dependent enzyme activity, since allele frequencies differ between populations, and environmental salinity. High salinity induces high activity, which is a response to the need for higher intracellular concentrations of free amino acids for cell volume regulation. Salinity has comparable effects on enzyme activity in natural and experimental populations. Genotype-dependent specific activities are a consequence of both differing kinetic properties among genotypes [Koehn, R. K., and Siebenaller, J. S. (1981). Biochem. Genet. 19: 1143] and genotype-specific concentration of enzyme protein that change in response to environmental salinity.