Analysis of the additivity of in vitro inhibition of cholinesterase by mixtures of chlorpyrifos-oxon and azinphos-methyl-oxon.

Organophosphorus (OP) insecticides or their active metabolites act through a common mechanism of toxicity, the inhibition of cholinesterase (ChE). The effects of in vitro exposure of brain (target) and serum (biomarker) ChE to chlorpyrifos-oxon (C horizontal lineO) and azinphos-methyl-oxon (AZM horizontal lineO), the active metabolites of the insecticides chlorpyrifos and azinphos-methyl, respectively, were investigated to determine if simultaneous or sequential exposure to these two OP compounds results in purely additive effects. Additive was defined by the theoretical calculated percent inhibition (dose additivity), which takes into account the fraction of ChE molecules assumed to be available for inhibition by the second compound following inhibition by the first compound, not simple mathematical summation of percent inhibition (response additivity). Brain ChE simultaneously exposed to the two compounds resulted in additive effects, which were less than the simple mathematical summation of percent inhibition. However, serum ChE simultaneously exposed to the two compounds resulted in a nonlinear response, presumably due in part to the presence of detoxifying enzymes in the serum. Sequential exposure of both brain and serum ChE to the two compounds resulted in greater than additive effects at the higher concentrations of each compound. There was no departure from additivity at the lower concentrations of the two compounds. These data suggest that simple mathematical summation of percent inhibitions, i.e., response additivity, is not the appropriate method for describing the combined effects of C horizontal lineO and AZM horizontal lineO on ChE in vitro. In addition, there are other mechanisms involved, such as the presence of detoxication enzymes, that must be taken into account when analyzing the effects of combined exposure of ChE to these two compounds.