Biochemical studies of aminopeptidase polymorphism in Mytilus edulis. II. Dependence of reaction rate on physical factors and enzyme concentration

Enzymatic parameters of aminopeptidase-I that may be sensitive to temperature and solute variations were investigated to provide a functional explanation for specific activity differences among genotypes in natural populations. The effect of temperature on the apparent K _m of l-leucyl-4-methoxy-2-naphthylamide and the dipeptide phenylalanyl-glycine was small, especially between 10 and 25 C. The apparent K _m varied only between 36.7 and 49.8 µM at these temperatures and the six common genotypes did not differ in temperature-dependent substrate affinities. While pH had a significant effect on K _m , no differences among genotypes were observed. Activation enthalpies were also identical among genotypes. Thermal inactivation was slowest at 15 C and the same for all genotypes. Of 18 tested amino acids, only phenylalanine inhibited aminopeptidase-I; K _I values ranged from 1.2 to 0.8 mM and were the same for all genotypes. Small differences among genotypes were detected in the inhibitory effect of zinc. The concentration of aminopeptidase-I enzyme was the same for all genotypes in a population exposed to oceanic salinity, but the concentration of Lap ^94/94was 15% lower than that of other genotypes in a population experiencing estuarine salinity. Genotypes with the Lap ⁹⁴allele exhibited higher apparent k _cat values in all population samples. The probable genotype-dependent effects of enzyme concentration and k _cat differences are discussed with regard to maintenance of the polymorphism and genetic differences among populations.