Abstract
Organophosphorus pesticides (OPs) remain a potential concern to human health because of their continuing worldwide use. Thiophosphorus OPs, once bioactivated by cytochromes P450 (P450s), form oxon metabolites, which are potent acetylcholinesterase inhibitors. This study investigated the rate of desulfation (activation) and dearylation (detoxification) of parathion and chlorpyrifos in human liver microsomes. In addition, recombinant human P450s were used to quantify, for the first time, the P450-specific kinetic variables (Km and Vmax) for each compound for future use in refining human physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) models of OP exposure. CYP1A2, 2B6, 2C9, 2C19, 3A4, 3A5, and 3A7 were found to be active to a widely varying degree in parathion metabolism, whereas all, with the exception of CYP2C9, were also found to be active in chlorpyrifos metabolism. CYP2B6 and CYP2C19 demonstrated low Km and high Vmax values for the metabolism of both model compounds, which supports their role as the primary enzymes that regulate metabolism at low-level human exposures to OPs. With Km and Vmax values of 0.61 μM, 4827 pmol/min/nmol P450 and 0.81 μM, 12,544 pmol/min/nmol for formation of paraoxon and chlorpyrifos-oxon, respectively, CYP2B6 favored the desulfation reaction. CYP2C19 activity favored dearylation with Km and Vmax values of 0.60 μM, 2338 pmol/min/nmol P450 and 1.63 μM, 13,128 pmol/min/nmol for formation of p-nitrophenol and 3,4,5-tricholorpyrindinol, respectively. P450-specific kinetic parameters for OP metabolism will be used with age-dependent hepatic P450 content to enhance PBPK/PD models so that OP exposures can be modeled to protect human health in different age groups.
Footnotes
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Supported by U.S. EPA STAR (Environmental Protection Agency Science to Achieve Results) Grant R-83068301.
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Part of this work was presented at the 2006 Society of Toxicology Annual Meeting, San Diego, CA [The Toxicologist (supplement to Toxicological Sciences) 90:S-1, March 2006].
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
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doi:10.1124/dmd.106.012427.
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ABBREVIATIONS: OP, organophosphorus pesticide; P450, cytochrome P450; PBPK/PD, physiologically based pharmacokinetic/pharmacodynamic; TCP, 3,4,5-trichloropyrindinol; PNP, p-nitrophenol; iso-OMPA, tetraisopropyl pyrophosphoramide; HPLC, high-performance liquid chromatography.
- Received August 9, 2006.
- Accepted October 25, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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