Carboxylesterase of the Rice Stem Borer, Chilo suppressalis Walker (Lepidoptera: Pyralidae), Responsible for Fenitrothion Resistance as a Sequestering Protein

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 K_D and B_max values of organophosphate (OP)-resistant strain were 0.65 _µm and 1.42 nmol/mg protein, respectively, and those of OP-susceptible strain were 0.66 _µm and 0.20 nmol/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.