Effect of an allozyme polymorphism on regulation of cell volume

Abstract

Biochemical differences have been described among allozymes^1–4, but the physiological consequences (that is, physiological phenotypes) of such differences have only rarely been demonstrated^5–7. Such a relationship is necessary both to the argument that natural selection maintains allozymic diversity in natural populations and for understanding biochemical mechanisms of adaptation⁸. We report here a genotype-dependent difference in the rate of cellular free amino acid accumulation during adjustment to hyperosmotic conditions in the mussel Mytilus edulis. The product of the Lap locus in M. edulis is the lysosomal enzyme aminopeptidase-I (AM-I; E.C.3.4.11.-) which hydrolyses oligopeptides to their constituent amino acids⁹. Total AM-I activity is positively correlated with salinity; a 120% increase in salinity increases AM-I activity twofold^10–13. Adjustment to hyperosmotic stress in osmoconforming marine bivalves, including M. edulis, involves rapid accumulation of cellular free amino acids^14,15. The biochemical properties of AM-I, and its activation by salinity changes, suggest that it is important for providing cellular free amino acid pools during adjustment to hyperosmotic stress¹¹. We show that individuals carrying the allele for high catalytic efficiency (k_cat) accumulated cellular amino acids more rapidly than other genotypes. The difference in accumulation rate was also demonstrated by an interruption of hyperosmotic adjustment, which resulted in genotype-dependent rates of amino acid excretion. These results demonstrate the role of AM-I in the physiological processes that regulate cell volume. Thus, differing catalytic properties of allozymes are manifested as phenotypic differences at the physiological level and provide a mechanism for the known selective mortality of mussels in natural populations⁷.