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Inhibition of diazinon metabolism by chlorpyrifos in rat liver microsomes.

Wu H; Timchalk C; Poet T
Toxicologist 2004 Mar; 78(S-1):425
Chlorpyrifos (CPF) and diazinon (DZN) are major organophosphorus pesticides that are structurally and mechanistically similar, but have very different rates of metabolism. Their toxicity is mediated via CYP450 metabolism to oxons. Detoxification of the oxons is mediated by CYP450 and A-esterases, resulting in the formation of trichloropyridinol from CPF and 2-isopropyl-4-methyl-6-hydrox-ypyrimidine (IMHP) from DZN. Since exposures often involve more than one pesticide, this study was conducted to assess the effect of co-incubation with both CPF and DZN on the metabolism of DZN. Substrate concentrations of DZN ranging from 20 to 575 uM were incubated with 0, 100, 300, or 750 uM CPF for 15 min and bioactivation to DZN-oxon and deactivation to IMHP quantified. Data was analyzed using Lineweaver-Burke (1/v; 1/s) plots, by fitting the Michaelis-Menten equation to the data with non-linear regression using SlideWrite Plus, and by fitting the data to a mathematical model written in Simusolv, which employed Michaelis-Menten equations including inhibition constants to describe metabolism. The 1/v; 1/s plots were used to estimate the types of inhibition. Metabolism to IMHP appeared to be competitively inhibited whereas metabolism to the oxon was uncompetitive (or suicide). Fitting the data with non-linear regression, apparent Km concentrations vary for the production of IMHP, but not the apparent Vmax; whereas, the apparent Vmax rates change for the production of oxon, confirming the types of inhibition. Inhibitory rate constants (Ki) for oxon and IMHP metabolism were approximately 250 uM and approximately 50 uM, respectively. Using the different methods to predict Vmax and Km, apparent Vmax and Km with inhibition, and Ki resulted in similar estimations. The mathematical model has the advantage of providing a methodology by which all parameters are considered at once. The relatively high Ki values indicate the importance of metabolic interactions on kinetics of in vivo co-exposures to DZN and CPF may be limited to high acute dose exposures.
Laboratory-animals; Animals; Animal-studies; Liver; Liver-cells; Pesticides; Exposure-levels; Bioactivation; Mathematical-models; In-vivo-studies
2921-88-2; 333-41-5
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Research Tools and Approaches: Exposure Assessment Methods
Source Name
The Toxicologist. Society of Toxicology 43nd Annual Meeting and ToxExpo, March 21-25, 2004, Baltimore, Maryland
Performing Organization
Battelle Memorial Institute, Richland, Washington
Page last reviewed: September 2, 2020
Content source: National Institute for Occupational Safety and Health Education and Information Division