Analysis of the time-dependent acute aquatic toxicity of organophosphorus pesticides: The critical target occupation model

A model is presented for the acute toxicity of organophosphorus (OP) pestic ides belonging to the class of phosphorothionates. The acute toxicity of th ese pesticides is governed by the irreversible inhibition of the enzyme ace tylcholinesterase (AChE), after their meta belie activation to oxon analogu es. The model is based on the idea that, for chemicals exhibiting an irreve rsible receptor interaction, mortality is associated with a critical amount of "covalently occupied" target sites, i.e., the "critical target occupati on" (CTO). For a given compound and species, this CTO is associated with a critical time-integrated concentration of the oxon analogue in the target t issue, which can be modeled by the critical area under the curve (CAUC) tha t describes the time-concentration course of the phosphorothionate in the a queous phase or in the entire aquatic organism. In contrast to the classica l critical body residue (CBR) model, the CTO model successfully describes t he 1-14-d LC50(t) data of several phosphorothionates in the pond snail and guppy. Furthermore, the time dependency of lethal body burdens (LBBs) of ph osphorothionates is explained by the model. Although the CTO model is speci fically derived for OP pesticides, it can be applied to analyze the acute t oxicity and to estimate incipient LC50 values of organic chemicals that exe rt an irreversible receptor interaction in general.