Biochemical and physiological differences in the malathion carboxylesterase activities of malathion-susceptible and -resistant lines of the sheep blowfly, Lucilia cuprina

Abstract

Three malathion carboxylesterase (MCE) phenotypes have been described previously in Lucilia cuprina adults: low, intermediate, and high. The MCE specific activities of adults with the intermediate and high phenotypes are 21- and 33-fold higher than those with the low phenotype, but the high phenotype is almost 1000-fold more resistant to malathion than either of the other phenotypes. Here we show that MCE activity also peaks in Day 4 larvae from three lines representative of these phenotypes. The MCE activity of the low-line larvae is only 2.7- and 4-fold lower than those of the intermediate and high-line larvae, respectively. The relatively high MCE activity of the low-line larvae is largely explained by their crop activity. Assays in the presence of esterase inhibitors reveal three distinct types of MCE activity across the three lines and two developmental stages. Activity in the low- line adults is not completely inhibited by paraoxon and is classified as a subclass II carboxylesterase. The MCE activities in the larvae of the low and intermediate lines and adults from the intermediate line are inhibited by low concentrations of paraoxon, classifying them as subclass I carboxylesterases. The MCE activities in the high-line larvae and adults are also classified as subclass I carboxylesterases, but they are more sensitive to inhibition by triphenyl phosphate than those in the other two lines. These data suggest that MCE in the malathion-resistant high line may be structurally different from the MCEs in the malathion-susceptible intermediate and low lines.