Chlorine-substituted transformation products generated during chlorination of the organophosphorus insecticide disulfoton induce anti-acetylcholine esterase activity

Abstract

Global concern regarding transformation products (TPs) derived from contaminants, including pesticides, in the environment and during water treatment has been growing markedly. In the present study, we investigated the anti-acetylcholinesterase (AChE) activity of an aqueous solution of the organophosphorus insecticide disulfoton, a toxicological endpoint for determining the acceptable daily intake of disulfoton, both in the presence and the absence of metabolism during chlorination. Disulfoton rapidly reacted with free chlorine and completely disappeared within 0.25 h. Although the aqueous disulfoton solution did not induce anti-AChE activity before chlorination, the chlorinated samples did induce anti-AChE activity, both with (indirect toxicity) and without (direct toxicity) metabolism. These observations clearly indicated that disulfoton was converted into toxic TPs through reactions with free chlorine. Liquid chromatographic fractionation followed by an anti-AChE activity assay revealed that three TPs were responsible for the observed direct toxicity. Further mass spectrometric analyses showed that these TPs were disulfoton-oxon-sulfone, and mono- and dichloro-substituted derivatives of disulfoton-oxon-sulfoxide (O-(1-chloroethyl) S-[2-(ethanesulfinyl)ethyl] O-ethyl phosphorothioate and O-(1,2-dichloroethyl) S-[2-(ethanesulfinyl)ethyl] O-ethyl phosphorothioate, respectively), none of which were simply oxon. Results of the anti-AChE activity assay on the chemical standard of disulfoton-oxon-sulfone after metabolism and quantification of the disulfoton-oxon-sulfone in the chlorinated samples revealed that the observed indirect toxicity was solely induced by this TP. It is recommend that drinking water treatment plants that use free chlorine as a disinfectant monitor the concentrations of at least disulfoton-oxon-sulfone, which is commercially available, in finished water in addition to disulfoton itself, to ensure the safety of tap water.