Whyard_1994_Pestic.Biochem.Physiol_50_198

Resistance to malathion in a strain of the Australian sheep blowfly is due to a 10-fold increase in malathion carboxylesterase (MCE) activity relative to a more susceptible strain. MCE was purified to apparent homogeneity from these two strains and was shown to be a monomer of 60,500, with a pI of 5.5 in both strains. Purified MCE from both populations had identical Km and Vmax values for the hydrolysis of malathion as well as for three other esterase substrates. Similarly, the kinetics of inhibition by several inhibitors were the same for the MCE from each strain. These data therefore suggest that resistance to malathion is due to a quantitative rather than a qualitative change in the MCE of the two strains. Estimation of the total MCE content in each strain showed that the resistant blowflies had nine times more MCE than the more susceptible insects. Although blowfly MCE showed greater specificity for naphthyl esters over malathion, it nevertheless hydrolyzes malathion faster than any other esterase yet isolated from an insect. This is in sharp contrast to previously studied insect strains in which organophosphate resistance has been attributed to large increases in nonspecific esterases that show very slow or no hydrolysis of the insecticides.