Kinetic analysis of species difference in acetylcholinesterase sensitivity to organophosphate insecticides.

Abstract Previous in vitro studies have elucidated species differences in the concentration of organophosphates required for 50% inhibition (IC 50 ) of brain acetylchoinesterase (AChE). In this study, we tested the hypothesis that the differences in IC 50 s between species might be due to variability in the kinetic parameters of AChE from different species; i.e., different affinities ( K a ) to the binding of organophosphate inhibitors and/or different rates of phosphorylation of the enzyme. Brain AChEs from four classes of animals were used: mammalian (monkey, rat, and guinea pig), avian (chicken), piscine (catfish), and amphibian (frog). The kinetic parameters of inhibition of five organophosphates were determined. The observed differences in IC 50 values can be satisfactorily explained on the basis of different affinities and/or different rates of phosphorylation of AChEs from various sources. For example, in the IC 50 study, chicken brain AChE was 103-fold more sensitive to inhibition by methyl paraoxon than frog brain AChE. This difference could be explained on the basis that chicken brain AChE had a 7.1-fold higher affinity and a 15.5-fold faster rate of phosphorylation. The correlation of observed p I 50 (−log[IC 50 ]) with p I 50 calculated from kinetic data was almost ideal. The data from studies on homologs of paraoxon and Gutoxon indicated that increasing the alkyl chain from methyl to ethyl results in a 2.4-fold increase in the apparent affinity, while the rate of phosphorylation was not significantly changed. These kinetic studies suggest that both affinity and rate of phosphorylation are important in determining the sensitivity of AChEs from different species.

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