A role of highly active carboxylesterase in an organophosphorus insecticide resistant strain of Culex pipiens molestus

Abstract

In order to elucidate the role of highly active carboxylesterases (CE's) in organophosphorus insecticide resistance, characteristics of CE's in a resistant strain of Culex pipiens molestus were investigated with special reference to their susceptibility to insecticides. Two CE's named E1 and E2 were separated from whole body larvae of a resistant strain by polyacrylamide gel electrophoresis (PAGE). E1 was found specifically in alimentary canal of the larva and E2 was distributed in every tissue of the body. The activities of E1 and E2 in the resistant strain were several and 25-30 times higher than the corresponding CE in a susceptible strain, respectively. E1 was strongly inhibited by most of the organophosphates tested and was moderately inhibited by carbamates. On the other hand, E2 was inhibited by limited types of organophosphates and was not inhibited by S-n-propyl phosphorothioate and carbamates. Insecticides which strongly inhibited E2 gave a high resistance ratio or a strong synergistic effect. Very low dissociation constant of the CE to insecticides indicated that the CE's have no potential to hydrolyze the insecticides. These results support that CE's conjugate with the insecticide uptaken into the body cavity, and consequently prevent the access of the insecticide to the target.