Inhibition Kinetics of 16 Organophosphorus Pesticides or Their Active Metabolites on Erythrocyte Acetylcholinesterase from Humans and Rats.

Inhibition kinetics assays were conducted with 16 commercial organophosphate (OP) pesticides or their metabolites on acetylcholinesterase (AChE) in erythrocyte "ghost" preparations from 18 individual humans (both sexes; adults, juveniles and cord blood samples; mixed races/ethnicities) and pooled samples from adult rats (both sexes). A well established spectrophotometric assay using acetylthiocholine as substrate and a chromogen was employed. The kinetic parameters bimolecular rate constant (ki), dissociation constant (KI) and phosphorylation constant (kp) were calculated for each compound. As expected, a wide range of potencies were displayed among the tested compounds. Statistical analysis of the resultant data indicated no differences in sex, age or race/ethnicity among the human samples that are unexpected based on chance (4.2% statistically significant out of 48 parameters calculated) and no differences between the sexes in rats. The bimolecular rate constants for 10 of the compounds were not statistically different between rats and humans. The data indicate that, consistent with the high level of conservation of AChE among species and the fact that AChE at different locations within a species arises from the same gene, the inhibition kinetic parameters calculated from rat erythrocyte ghost preparations should be useful in estimating potencies of OP compounds on target AChE in humans. Additionally the data indicate that differences in sensitivities among individual humans were not apparent. Impact Statement: These data are expected to be useful in consideration of the intraspecies and interspecies uncertainty factors in OP pesticide risk assessment.