Zhu_1998_Pestic.Biochem.Physiol_62_135

The organophosphate-resistant strains, OR-0, OR-1, and OR-2, of the greenbug (Schizaphis graminum) showed 1.6-, 32-, and 42-fold resistance, respectively, to parathion compared with a susceptible (OSS) strain. Enzyme kinetics revealed that esterase activities in the OR-1 and OR-2 strains were 1.9- and 2.4-fold higher, respectively, than that of the OSS strain when alpha-naphthyl acetate (alphaNA) was used as substrate. A good correlation between the alphaNA-hydrolyzing esterase level and parathion resistance among these strains suggested that these esterases played a predominate role in conferring organophosphate resistance. Microassays of the alphaNA-hydrolyzing esterase activity in individual greenbugs indicated that the OR-1 and OR-2 strains were less homogenous than the OSS strain. In contrast, there was no significant difference in the enzyme activity among the OSS, OR-0, and OR-1 strains when phenyl acetate (PA) was used as substrate. However, the PA-hydrolyzing esterase activity was approximately 12-fold higher in the OR-2 strain than in the OSS strain. Approximately 90% of the PA-hydrolyzing esterase activity was inhibited by 1 uM of paraoxon, but only 10% of the activity was inhibited byp-hydroxymercuribenzoic acid at the same concentration. The PA-hydrolyzing esterase(s) appeared to have characteristics of both type A (based on the substrate specificity) and type B (based on the inhibitor specificity) esterases and could be a key factor conferring organophosphate resistance in the OR-2 strain.