Abstract: Malathion is an efficient but low toxic organophosphate insecticide with a large molecular weight and a special structure. It is widely used in the prevention and control of various agricultural pests. Mutation in carboxylesterases is one of important metabolic resistance mechanisms to organophosphate insecticides in insects. In a previous study, seven non-specific carboxylesterase genes from Aphis gossypii, Nilaparvata lugens, Spodoptera litura, Bombyx mori, Harmonia axyridis, Tribolium castaneum and Apis mellifera, respectively, were cloned, mutated at position 151 or 271 and expressed in Escherichia coli. In this experiment, the hydrolysis of the purified recombinant proteins of the seven insects was further examined towards malathion. The results showed that the wild-type carboxylesterases from A. gossypii, A. mellifera, S. litura and T. castaneum were capable of degrading malathion and the two single mutations did not improve their hydrolysis activity. The wild-type carboxylesterases from B. mori, H. axyridis and N. lugens could not degrade malathion while the G/A151D and/or W271L mutation made these esterases acquire malathion carboxylesterase activity, implying that these two single mutations could confer resistance to malathion in the three insects. Malathion carboxylesterase activity in different species had large difference. S. litura had the highest malathion carboxylesterase activity, and its K_cat/K_m was 1.8-1.9 L/μmol·min, followed by T. castaneum, with the K_cat/K_m of 0.87-0.95 L/μmol·min. Malathion carboxylesterase activity in other insects was relatively low, with ten-fold difference.

Key words: Carboxylesterase, insecticide resistance, mutation, malathion, enzymatic dynamics