Reductive metabolism of 1,1,1,2-tetrachloroethane and related chloroethanes by rat liver microsomes.

The susceptibility of polychlorinated ethanes to reductive metabolism was evaluated by measuring the amount of each compound consumed during anaerobic incubations with rat live microsomes; 1,1,1,2-tetrachloroethane, pentachloroethane and hexachloroethane were metabolized extensively, 1,1,1,2-tetrachloroethane and the trichloroethanes were metabolized very slowly and the dichloroethanes were not metabolized at a detectable rate. The electron affinity of the chloroethanes was determined by measuring electrochemical half-wave reduction potentials. Chloroethanes with an E1/2 of - 1.35 V or less negative were reduced readily in microsomes while those with an E1/2 equal to or more negative than -1.90 V were not good substrates for enzymatic reduction. Metabolites produced from 1,1,1,2-tetrachloroethane in vitro were 1,1-dichloroethylene (DCE) and 1,1,2-trichloroethane (TCEA) and the ratio DCE/TCEA was about 25:1. These conversions were NADPH-dependent and were inhibited by air, CO and metyrapone. In the presence of SKF 525-A, DCE formation was inhibited by 47%. Microsomes from untreated or beta-naphthoflavone-treated rats were 70-90% less active than microsomes from phenobarbital-treated rats. The Km was 0.50 mM and the Vmax was 66 nmol min-1 mg-1 protein for DCE formation. The results are consistent with the proposal that 1,1,1,2-tetrachloroethane is reduced by hepatic cytochrome(s) P-450 to a free radical intermediate which, for the most part, remains closely associated with the enzyme, is reduced further and undergoes beta-elimination of a chloride ion to form DCE. The occurrence of this reductive pathway in vivo was demonstrated by the quantitation of DCE and TCEA in blood from rats treated with 1,1,1,2-tetrachloroethane.