Evaluation of the Role of Boll WeevilAliesterases in Noncatalytic Detoxication of Four Organophosphorus Insecticides

Abstract

Noncatalytic detoxication of the oxons of four phosphorothionate insecticides (parathion, methyl parathion, chlorpyrifos, and chlorpyrifos methyl) by boll weevil aliesterase was determined. All the oxons except methyl paraoxon were potent inhibitors of aliesterase (I50s in 1–100 nMrange). Paraoxon and chlorpyrifosoxon were more potent inhibitors of aliesterase than of acetylcholinesterase, while the reverse was true for methyl paraoxon and chlorpyrifos-methyl-oxon. Boll weevil homogenates (containing EDTA and no Triton X-100, to eliminate possible interference by A-esterase and acetylcholinesterase) significantly increasedI50s of four oxons to bovine brain acetylcholinesterase, indicating detoxication of the oxons by the homogenates. The degree of detoxication (% detoxication/mgweevil/ml) for the oxons was chlorpyrifos-oxon > chlorpyrifos-methyl-oxon > paraoxon > methyl paraoxon, which correlated with weevil aliesterase sensitivity to inhibition by these oxons.In vivotreatments of weevils with any one of three aliesterase inhibitors, DEF (S,S,S-tributyl phosphorotrithioate), DAPDT (S,S-diamyl phenylphosphonodithioate), or BPNxn (n-butyl 4-nitrophenyl phenylphosphonate), all caused an appreciable reduction of thein vitrodetoxication. Using chlorpyrifos-oxon as a representative insecticide, the detoxication by homogenates of DEF-treated weevils was a function of residual aliesterase activity.In vivostudies indicated a trend of an inverse relationship betweenin vivotoxicities and antialiesterase activities of the four phosphorothionate insecticides. Substrate specificity of aliesterase for four esters was α-naphthyl propionate > α-naphthyl acetate > α-naphthyl butyrate > nitrophenyl butyrate. The more toxic insecticides, chlorpyrifos methyl and methyl parathion, showed little or no inhibition of any of the substrate hydrolases while the less toxic chlorpyrifos and parathion and the two aliesterase inhibitors (DEF, BPNxn) all selectively and strongly inhibited aliesterase activities toward nitrophenyl butyrate and α-naphthyl butyrate. Theses results suggest that low toxicities of the insecticides may have resulted from higher affinity for specific aliesterases, and their greater synergism by aliesterase inhibitors demonstrated previously is because of selective blocking of the specific aliesterase activities by the synergists. Therefore,in vitroandin vivostudies both demonstrated that aliesterase played an important role in noncatalytic detoxication of organophosphorus insecticides in boll weevils. Acting as an alternative phosphorylation site, aliesterase reduces the concentration of organophosphorus insecticides reaching acetylcholinesterase and, thus, provides a protection to this target. The level of this detoxication depends on relative sensitivities or affinities of the oxons for aliesterase and acetylcholinesterase.