In this study, acetylcholinesterase (AChE) was extracted from two field-collected populations of Oxya chinensis (XinxiangCity, Henan Province and Changzhi City, Shanxi Province). AChE activities were decreased when concentrations of ATCincreased, showing a characteristic phenomenon of substrate inhibition at high concentration in both populations. Suchinhibition occurred at relatively low concentration for AChE from Xinxiang population but relatively high for AChE fromChangzhi population. The kinetic study showed that there were no significant differences between the two populations inthe Km values. The Km value in Changzhi population was only 1.09-fold higher than that in Xinxiang population. However,significant differences were observed between the two populations in Vmax values. The Vmax value in Changzhi populationwas 1.32-fold higher than that in Xinxiang population. The inhibition study in vitro showed that the AChE from bothpopulations exhibited similar rank order in sensitivity to inhibition by three OPs, as determined by comparison of theirbimolecular rate constants (ki), from the most potent inhibition to the least was chlopyrifos-oxon > paraoxon >demeton-s-methyl for AChE from the two populations and that the ki values in Xinxiang population were lower than those in Changzhipopulation. The I50 values of AChE from Xinxiang population were 4.84-, 2.66-, and 1.92-fold less sensitive to inhibition byparaoxon, chlopyrifos-oxon, and demeton-s-methyl. These results were consistent with the results in bioassay. It isinferred that AChE insensitivity to OP insecticides plays an important role in the differences of insusceptibility of Oxyachinensis to malathion between the two populations.
