Oxidation of trichloroethylene by liver microsomal cytochrome P-450: evidence for chlorine migration in a transition state not involving trichloroethylene oxide

Abstract

Trichloroethylene (TCE) [a suspected carcinogenic agent] was metabolized by cytochrome P-450 containing mixed-fuction oxidase systems to chloral (2,2,2-trichloroacetaldehyde), glyoxylic acid, formic acid, CO and TCE oxide. TCE oxide was synthesized, and its breakdown products were analyzed. Under acidic aqueous conditions the primary products were glyoxylic acid and dichloroacetic acid. The primary compounds formed under neutral or basic aqueous conditions were formic acid and CO. TCE oxide did not form chloral in any of these or other aqueous systems, even when Fe salts, ferriprotoporphyrin IX or purified cytochrome P-450 was present. Ferric iron salts catalyzed the rearrangement of TCE oxide to chloral only in CH2Cl2 or CH3CN. A 500-fold excess of Fe was required for complete conversion. A kinetic model involving the zero-order oxidation of TCE to TCE oxide by cytochrome P-450 and 1st-order degradation of the epoxide was used to test the hypothesis that TCE oxide was an obligate intermediate in the conversion of TCE to other metabolites. Kinetic constants for the breakdown of TCE oxide and for the oxidative metabolism of TCE to stable metabolites were used to predict epoxide concentrations required to support the obligate intermediacy of TCE oxide. The maximum levels of TCE oxide detected in systems using microsomal fractions and purified cytochrome P-450 were 5- to 28-fold lower than those predicted from the model. The kinetic data and the discrepancies between the observed metabolites and TCE oxide breakdown products supported the view that the epoxide was not an obligate intermediate in the formation of chloral, and an alternative model was presented in which Cl migration occurred in an oxygenated TCE-cytochrome P-450 transition state.