Abstract
Carboxylesterase of the rice stem borer, Chilo suppressalis WALKER, was studied, whether or not the enzyme has a sequestering activity to fenitroxon. Both the carboxylesterase activity (substrate: α-naphthyl acetate) and the fenitroxon sequestering activity of C. suppressalis closely correlated with the degree of resistance to fenitrothion. The fenitroxon sequestering activity was localized mainly in the midgut, and the Scatchard analysis showed that the KD and Bmax values of organophosphate (OP)-resistant strain were 0.65μM and 1.42nmol/mg protein, respectively, and those of OP-susceptible strain were 0.66μM and 0.20nmol/mg protein, respectively. The carboxylesterase isozyme patterns separated by IEF were clearly different between the two strains, i. e., the OP-resistant strain has only one highly active isozyme (pI=5.2), whereas the isozyme (pI=5.2) was scarcely in the OP-susceptible strain. In the OP-resistant strain, the carboxylesterase activity of the isozyme (pI=5.2) evaluated by a red coloration on IEF gel was significantly inhibited by the preincubation with fenitroxon. Furthermore, the fenitroxon sequestering activity was markedly reduced by coexistence with an excess amount of α-naphthyl acetate. Based on these results, it was suggested that the carboxylesterase of C. suppressalis has a role in fenitrothion resistance as a sequestering protein.
