KINETIC CHARACTERIZATION OF CYP2E1 INHIBITION IN-VIVO AND IN-VITRO BYTHE CHLOROETHYLENES

Trans- and cis-1,2-dichloroethylene (DCE) isomers inhibit their own me tabolism in vivo by inactivation of the metabolizing enzyme, presumabl y the cytochrome P450 isoform, CYP2E1. In this study, we examined cyto chrome P450 isoform-specific inhibition by three chloroethylenes, cis- DCE, trans-DCE, and trichloroethylene (TCE), and evaluated several kin etic mechanisms of enzyme inhibition with physiological models of inhi bition. Trans-DCE was more potent than cis-DCE, and both were much mor e effective than TCE in inhibiting CYP2E1. The kinetics of in vitro lo ss 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 metabolizi ng enzyme (CYP2E1) was presumed to be related to interaction of a reac tive DCE metabolite with remaining substrate-bound, active CYP2E1. The kinetics of inhibition by TCE, a weak inhibitor in vitro, were very d ifferent from that of the dichloroethylenes. With TCE, parent compound concentrations influenced enzyme loss. Trans-DCE was a more potent in hibitor of CYP2E1 than cis-DCE based on both in vivo and in vitro stud ies. Quantitative differences in the inhibitory properties of the 1,2- DCE isomers may be due to the different stability of epoxides formed f rom bioactivation by CYP2E1. Epoxide intermediates of DCE-metabolism, reacting by water addition, would yield dialdehyde, a potent cross-lin king reagent.