Some Properties of Carboxylesterase from Aphis gossypii GLOVER (Homoptera: Aphididae)

Effects of temperature, hydrogen ion concentration and substrate concentration on hydrolysis of 1-naphthyl acetate by carboxylesterase from Aphis gossypii, as well as the activation and inhibition of the enzyme by divalent cations, were evaluated. Optimum ranges of these parameters read as; 40-45°C for temperature, 6.8 for pH, and 5×10^-3 M-5×10^-2 M for substrate concentration. The Michaelis constant and maximal velocity, read from the double reciprocal plot which was determined with 1-naphthyl acetate at 40°C for 10 min, were 1.0×1^-3 M and 62.5 nmol/10 min/μg protein, respectively. Among 13 divalent cations tested, mercury and copper served as potential inhibitors of the enzyme, while cadmium partially inhibited the hydrolytic reaction at higher concentrations. The remaining 10 cations exhibited neither activation nor inhibitory effect at all levels of concentrations tested. Of four subcellular fractions, the 100, 000×g supernatant fraction (soluble fraction) gave the highest enzyme activity.