Organophosphorus pesticides: do they all have the same mechanism of toxicity?

Organophosphorus (OP) pesticides are used extensively to control agricultural, household and structural pests. These pesticides constitute a diverse group of chemical structures exhibiting a wide range of physicochemical properties, with their primary toxicological action arising from inhibition of the enzyme acetylcholinesterase (AChE, EC 3.1.1.7). Historically, risk characterizations for these toxicants have been based on hazard and exposure data pertaining to individual chemicals. The Food Quality Protection Act of 1996 now requires, however, that combined risk assessments be performed with pesticides having a common mechanism of toxicity. It is therefore critical to consider whether OP pesticides all exert toxicity through a common mechanism. This brief review evaluates the comparative toxicity of the 38 OP AChE inhibitors currently registered for use as pesticides in the United States and examines the data which suggest that some OP pesticides have toxicologically relevant sites of action in addition to AChE. It is concluded that all OP anticholinesterases potentially have a mechanism of toxicity in common, that is, phosphorylation of AChE causing accumulation of acetylcholine, overstimulation of cholinergic receptors, and consequent clinical signs of cholinergic toxicity. Additional macromolecular targets for some OP pesticides, however, may alter the cascade of events following AChE phosphorylation and thereby modify that common mechanism. Furthermore, other macromolecular targets of some OP pesticides appear capable of altering noncholinergic neurochemical processes. These additional actions may contribute to qualitative and quantitative differences in toxicity sometimes noted in the presence of similar levels of AChE inhibition induced by different OP pesticides. Further investigation of these additional sites of action may allow subclassification and influence the decision to perform combined risk assessments on this class of pesticides based on common mechanism of toxicity.