Kinetic properties and variability of esterases in organophosphate-susceptible and -resistant greenbugs, Schizaphis graminum (Homoptera : Aphididae)

The organophosphate-resistant strains, OR-0, OR-1, and OR-2, of the greenbu g (Schizaphis graminum) showed 1.6-, 32-, and 42-fold resistance, respectiv ely, 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- a nd 2.4-fold higher, respectively, than that of the OSS strain when or-napht hyl acetate (alpha NA) was used as substrate. A good correlation between th e alpha NA-hydrolyzing esterase level and parathion resistance among these strains suggested that these esterases played a predominate role in conferr ing organophosphate resistance. Microassays of the alpha NA-hydrolyzing est erase activity in individual greenbugs indicated that the OR-1 and OR-2 str ains were less homogenous than the OSS strain. In contrast, there was no si gnificant difference in the enzyme activity among the OSS, OR-0, and OR-1 s trains when phenyl acetate (PA) was used as substrate. However, the PA-hydr olyzing esterase activity was approximately 12-fold higher in the OR-2 stra in than in the OSS strain. Approximately 90% of the PA-hydrolyzing esterase activity was inhibited by 1 mu M Of paraoxon, but only 10% of the activity was inhibited by p-hydroxymercuribenzoic acid at the same concentration. T he 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. (C) 1998 Academic Press.