The 1,2-dichloroethylenes: Their metabolism by hepatic cytochrome P-450 in vitro

Cis- and trans-1,1-dichloroethylene bound to the active site of hepatic microsomal cytochrome P-450 with the production of a Type I difference spectrum and stimulated CO-inhibitable hepatic microsomal NADPH oxidation. Incubation of cis- and trans-1,2-dichloroethylene plus hepatic microsomes, NADPH-generating system-EDTA resulted in the production of measurable levels of 2,2-dichloroethanol and dichloroacetaldehyde but not of 2-chloroethanol, chloroacetaldehyde or chloroacetic acid and, also, resulted in decreased levels of hepatic microsomal cytochrome P-450 and heme. In addition, dichloroacetic acid was produced from trans-dichloroethylene under these experimental conditions. The omission of any component of the incubation mixture eliminated the above effects, while the inclusion of SKF-525A, metyrapone or CO: O₂ (80, v/v) diminished these effects. The effects of β-naphthoflavone and phenobarbital pretreatment on the values of K_s, ΔA_max, K_m and V_mam for the binding and metabolism of the 1,2-dichloroethylenes are reported. The binding and metabolism of the 1,2-dichloroethylenes and the 1,2-dichloroethylene-mediated inactivation of cytochrome P-450 were enhanced per mg of microsomal protein, but generally not per nmole of cytochrome P-450 by prior induction with β-naphthoflavone or phenobarbital. It is concluded that multiple forms of hepatic microsomal cytochrome P-450 bind and metabolize the 1,2-dichloroethylenes. The role of cytochrome P-450 in the metabolic activation of the dichloroethylenes is considered.