title: Kinetic characterization of CYP2E1 inhibition in vivo and in vitro by the chloroethylenes.
abstract: Trans- and cis-1,2-dichloroethylene (DCE) isomers inhibit their own metabolism in vivo by inactivation of the metabolizing enzyme, presumably the cytochrome P450 isoform, CYP2E1. In this study, we examined cytochrome P450 isoform-specific inhibition by three chloroethylenes, cis-DCE, trans-DCE, and trichloroethylene (TCE), and evaluated several kinetic mechanisms of enzyme inhibition with physiological models of inhibition. Trans-DCE was more potent than cis-DCE, and both were much more effective than TCE in inhibiting CYP2E1. The kinetics of in vitro loss of p-nitrophenol hydroxylase (pNP-OH) activity (a marker of CYP2E1) in microsomal incubations and of the in vivo gas uptake results were most consistent with a mechanism in which inhibition of the metabolizing enzyme (CYP2E1) was presumed to be related to interaction of a reactive DCE metabolite with remaining substrate-bound, active CYP2E1. The kinetics of inhibition by TCE, a weak inhibitor in vitro, were very different from that of the dichloroethylenes. With TCE, parent compound concentrations influenced enzyme loss. Trans-DCE was a more potent inhibitor of CYP2E1 than cis-DCE based on both in vivo and in vitro studies. Quantitative differences in the inhibitory properties of the 1,2-DCE isomers may be due to the different stability of epoxides formed from bioactivation by CYP2E1. Epoxide intermediates of DCE metabolism, reacting by water addition, would yield dialdehyde, a potent cross-linking reagent.
authors.affiliation: Curriculum in Toxicology, University of North Carolina-Chapel Hill, USA.
authors.forename: P D
authors.initials: PD
authors.lastname: Lilly
authors.forename: J R
authors.initials: JR
authors.lastname: Thornton-Manning
authors.forename: M L
authors.initials: ML
authors.lastname: Gargas
authors.forename: H J
authors.initials: HJ
authors.lastname: Clewell
authors.forename: M E
authors.initials: ME
authors.lastname: Andersen
mesh_terms: D000818:Animals; D019392:Cytochrome P-450 CYP2E1 / Q000378:metabolism; D065691:Cytochrome P-450 CYP2E1 Inhibitors; D004000:Dichloroethylenes / Q000494:pharmacology*; D066298:In Vitro Techniques; D008297:Male; D008956:Models, Chemical; D015394:Molecular Structure; D051381:Rats; D011916:Rats, Inbred F344; D013997:Time Factors; D014241:Trichloroethylene / Q000494:pharmacology*; D014752:Vinyl Chloride / Q000494:pharmacology*
chemical_list: D065691:Cytochrome P-450 CYP2E1 Inhibitors; D004000:Dichloroethylenes; D014241:Trichloroethylene; D019392:Cytochrome P-450 CYP2E1; D014752:Vinyl Chloride