A novel function of housefly glutathione S-transferase 6B—Its effect on the retention and increase of insecticidal activity of the insecticide prothiofos
Masayuki Sue, Tsuyoshi Mikawa, Takashi Ueda, Youhei Nomoto, Toru Miyamoto
Journal of pesticide science
Introduction Glutathione S-transferase (GST) conjugates many chemicals to glutathione (GSH) and is generally an enzyme in detoxification metabolism. In a living organism, increase of GST and its activity is, in many cases, one of the causes of the development of resistance to chemicals. In the case of organophosphorus (OP) insecticide, most of them are exposed to desalkylation and desarylation by GST possessed by OP-resistant insects, and their insecticidal activities are lost. However,
... os-like S-propyl phosphorothiolate-type OP insecticides maintain high insecticidal activity against resistant insects. This paper is concerned with the elucidation of this mechanism. In general, thiono-type OP insecticides are oxidatively converted into their oxon to inhibit acetylcholinesterase (AChE), thus showing insecticidal activity, but certain oxons, i.e., phosphorothiolates such as metamidofos, profenofos, and pyraclofos and prothiofos oxon, are weak AChE inhibitors and their S-oxide is an activated product which gives insecticidal activity. We 1) indicated that the active intermediate pro-thiofos oxon S-oxide ( Fig. 1 ) not only inhibits AChE but is also further activated by GST, showing high insecticidal activity to highly resistant insects; however, we could not identify the type of reaction and the activated product. In the previous study, 2) to elucidate this question, two new activated products were predicted by computational chemistry which differ from the known activated S-oxide, and of the two, desethyl S-oxide was shown to possess insecticidal activity and AChE inhibitory activity. By further detecting its hydrolysate 2,4dichlorophenyl phosphate in an in vitro approach, it became clear that desethyl S-oxide was produced from prothiofos oxon by partially purified housefly GST under oxidation (oxidative GSH conjugation), thus indicating the involvement of desethyl S-oxide along with S-oxide in the insecticidal activity of prothiofos. In this study, we cloned housefly GST isozymes, both from susceptible Takatsuki and resistant Yachiyo strains, ascertained which isozymes were involved in the desethylation and oxidative conjugation, and aimed to explain the activation mechanism of prothiofos oxon (LD 50 by injection: 55 ng/fly for Takatsuki, 110 ng/fly for Yachiyo) from a GST standpoint. Housefly glutathione S-transferases 1, 3, 4, 6A and 6B were obtained from organophosphorus (OP)-resistant Yachiyo and susceptible Takatsuki strains, respectively. Over 90% homology was found between isozymes 6A and 6B but their functions differed in desethylation metabolism. Yachiyo-6A produced more desethyl product of diazinon oxon than Takatsuki-6A, thus suggesting that it plays a central role in the development of O-alkyl phosphate-type OP resistance. On the other hand, unlike Takatsuki-6B, Yachiyo-6B barely achieved the desethylation of prothiofos oxon and the resultant effect was more in the direction of resistance suppression. However, the chief role played by 6B with regard to prothiofos was the exclusive desethylation of S-oxide, which is the oxidative product of prothiofos oxon, thus giving desethyl S-oxide with insecticidal activity and bringing forth a novel active intermediate along with S-oxide. © Pesticide Science Society of Japan Keywords: housefly glutathione S-transferase, GST6A and 6B, resistance suppression, oxidative glutathione conjugation of prothiofos oxon, desethyl S-oxide of prothiofos oxon.