Use of Kinetic and Mechanistic Data in Species Extrapolation of Bioactivation: Cytochrome P‐450 Dependent Trichloroethylene Metabolism at Occupationally Relevant Concentrations

Abstract

Use of Kinetic and Mechanistic Data in Species Extrapolation of Bioactivation: Cytochrome P‐450 Dependent Trichloroethylene Metabolism at Occupationally Relevant Concentrations: John C. Lipscomb, et al. United States Air Force, Armstrong Laboratory, Toxicology Division—Trichloroethylene (TRI) is an industrial solvent and environmental contaminant; therefore exposure to TRI occurs in diverse human populations. TRI causes hepatocellular carcinoma in B6C3F1 mice, but not rats; this suggests that TRI may be metabolized differently in the two species. We investigated the metabolism of TRI and the effect of TRI on enzymatic activities indicative of specific cytochrome P450 (CYP) forms in hepatic microsomes from mice, rats and humans. Studies in microsomes estimated Michaelis‐ Menten kinetic parameters by saturation analysis. Km values were 35.4, 55.5 and 24.6 μM and V values were 5,425, 4,826 and 1,440 pmol/min/mg in pooled mouse, rat and human microsomes, respectively. TRI (1,000 ppm) inhibited CYP2E1 dependent activity in all three species and BROD activity in mice and rats; TRI (1,000 ppm) increased CYP1A1/1A2 activity, and had no effect on CYP2A activity. Inhibition studies with mouse hepatic microsomes demonstrated that TRI was a competitive inhibitor of CYP2E1, with K of 50 ppm. TRI noncompetitively inhibited CYP2B‐dependent activities in the rat and mouse. Preincubation of microsomes with TRI and NADPH decreased the absorbence of CO‐bound CYP in all three species, but the dose‐dependence was most evident in mouse hepatic microsomes. These results have quantified the interspecies difference in CYP‐dependent TRI bioactivation and indicate that under both equivalent and occupationally relevant (hepatic) exposure conditions the human is at less risk of forming toxic CYP‐derived TRI metabolites.