Oxo-Type Organophosphate-Resistant Acetylcholinesterase from Organophosphate-Unsusceptible Culex tritaeniorhynchus

A highly resistant strain of Culex tritaeniorhynchus was employed to clarify the mechanisms of the acquired resistance of mosquitoes to organophosphorus and carbamate insecticides by an enzymological approach. Carboxylesterase (CE) activity was higher in the abdomen than in the head and thorax, and was higher in every part of the body of the resistant strain as compared with the susceptible strain. Acetylcholinesterase (AChE) activity was high in the head, and was higher in every part of the body in the susceptible strain as compared with the resistant strain.Greater inhibition of CE by oxo-type organophosphorus insecticides was observed in the susceptible strain (11-, 8.2- and 22.3-fold greater inhibitions by fenitrooxon, malaoxon and dichlorvos, respectively) than in the resistant strain. Much stronger inhibition of AChE was observed in the susceptible strain (1227-, 76.5- and 183-fold inhibitions by fenitrooxon, malaoxon and dichlorvos, respectively) than in the resistant strain. These results suggest that both AChE and CE are involved in the development of the acquired resistance to organophosphorus and carbamate insecticides, and that the former enzyme plays the major role.