Comparison of chlorpyrifos-oxon and paraoxon acetylcholinesterase inhibition dynamics: potential role of a peripheral binding site.
Kousba-A; Sultatos-LG; Poet-T; Timchalk-C
Toxicologist 2004 Mar; 78(S-1):425
The primary mechanism of action for organophosphorus (OP) insecticides involves the inhibition of acetylcholinesterase (AChE) by active oxon metabolites. This inhibition has been attributed to the phosphorylation of the serine hydroxyl group in the active site of AChE. Inhibition is described by the bimolecular rate constant (Ki), which is used to assess OP inhibitory capacity. It has been reported that the Ki for inhibition of AChE by certain oxons changes as a function of oxon concentration, and that this phenomenon could be explained by the presence of a peripheral binding site. When such a putative site would be occupied, the capacity of additional oxon molecules to phosphorylate the active site is reduced. Therefore, the objective of the current study was to evaluate the in vitro inhibition of AChE by a wide range of chlorpyrifos oxon (CPO) and paraoxon (PO) concentrations (0.5 pM -100 nM) using the Ellman assay combined with a dynamic model. The results indicated that the Ki is inversely changed in proportion to the oxon concentrations. The Ki determined using high oxon concentrations, were 238 and 22 uM-1h-1 and the spontaneous reactivation rates were 0.087 and 0.078 h-1 for CPO and PO, respectively. While the estimated Ki using high oxon concentrations were similar to those reported previously, the Ki estimates following 1 pM oxon, which have not been reported previously, were >1000-fold higher. Surprisingly, the Ki estimated using 1 pM CPO was similar to the Ki estimate using an equivalent PO concentration (0.18 and 0.25 pM-1h-1, respectively). This implies that both oxons exhibit similar inhibitory potency at the lower concentration, in contrast to the marked difference following higher concentrations, which was explained by considering a peripheral binding site. The potential of a peripheral site should be considered when modeling AChE kinetics particularly at low environmentally relevant concentrations.
Organo-phosphorus-compounds; Organo-phosphorus-pesticides; Pesticides; In-vitro-studies; Models; Environmental-factors
Research Tools and Approaches: Exposure Assessment Methods
The Toxicologist. Society of Toxicology 43nd Annual Meeting and ToxExpo, March 21-25, 2004, Baltimore, Maryland
Battelle Memorial Institute, Richland, Washington