Study on Activation of Organothiophosphorus Insecticides

Abstract

Organophosphorus insecticides are one of the largest groups of pesticides and most of them have one or two sulfur directly connected with phosphorus atom. Thiono or thiolothiono types of organophosphorus insecticides exert their insecticidal activity by inhibiting acetylcholinesterase (AChE) after activation. This activation is mediated by the mixed function oxidase (mfo), and thus the reaction is regarded as one of the most important ones. Studies on the origin of oxygen incorporated into the phosphinyl disulfide resulted from peracid oxidation indicated that the occurrence of phosphinyl disulfide was direct evidence for the phosphorus oxythionate intermediate as the initial oxidation product of phosphorothiolothionate compounds. An analytical procedure was developed to derive the unstable phosphinyl disulfide into the stable S-thiomethyl phosphorothiolates. By using this technique, the occurrence of the phosphorus oxythionate intermediate at mfo oxidation became evidenced.Phosphorothionate compounds are often isomerized to the thiolo isomers of higher toxicity by heat or light. The occurrence of such isomerization by mfo system was first indicated. S-Alkyl phosphorothiolates are insecticidal but do not inhibit AChE in vitro. The type of activation was shown to be the production of the unstable sulfoxide which inhibits AChE. Evidences were provided to indicate the involvement of another activated form, the glutathione conjugate of the sulfoxide which inhibits AChE. From a S-alkyl phosphorothiolothionate, the phosphinyl disulfide was obtained by peracid oxidation, which gave the unstable sulfoxide, then its glutathione conjugate, both being the potential activated forms.