Vinylidene chloride: Its metabolism by hepatic microsomal cytochrome P-450 in vitro

Abstract

The effects of inducing agents on the binding and metabolism of vinylidene chloride by hepatic microsomal cytochrome P-450 are reported. Hanes plots for the Type I binding of vinylidene chloride to cytochrome P-450 were biphasic with hepatic microsomes from untreated and β-naphthoflavone- or phenobarbital-treated male rats. Neither pretreatment affected the value of the K s ( ca. 0.22 mM) for the high-affinity binding site for vinylidene chloride, while phenobarbital induction, but not β-naphthoflavone treatment, decreased the value of the K s for the low-affinity site by 3-fold to ca. 1.6 mM. The maximum extents of binding ( ΔA max or ΔA max/nmole cytochrome P-450) of vinylidene chloride were decreased or not affected by β-naphthoflavone induction, while ΔA max but not ΔA max/ nmole cytochrome P-450 was elevated following phenobarbital induction. The rate of vinylidene chloride stimulated CO-inhibitable hepatic microsomal NADPH oxidation was not affected by β-naphthoflavone induction, but was increased significantly following phenobarbital induction. Vinylidene chloride was converted to monochloroacetate and to the previously unreported metabolite, dichloroacetaldehyde, by hepatic microsomes plus NADPH-generating system. Measurable levels of 2-mono- and 2,2-dichloroethanol, and of chloroacetaldehyde and dichloroacetic acid, were not produced from vinylidene chloride under these conditions. SKF-525A and CO:O 2 (80:20, v/v) inhibited the conversion of vinylidene chloride to monochloroacetate and dichloroacetaldehyde by approximately 60%. The rates of production of monochloroacetate and dichloroacetaldehyde in the presence of NADH were ca. 15% of the rates seen with NADPH-generating system. The rate of monochloroacetate production per mg microsomal protein was not affected by β-naphthoflavone induction but was increased slightly following phenobarbital induction. In contrast, the V max values per mg microsomal protein for the metabolism of vinylidene chloride to dichloroacetaldehyde were not elevated by either pretreatment. Incubation of vinylidene chloride, NADPH-generating system, EDTA and hepatic microsomes from untreated and β-naphthoflavone- or phenobarbital-treated rats did not result in any significant alterations in the levels of microsomal cytochrome P-450 and heme or in the covalent binding of the mono- or dichloroacetyl moieties to microsomal or buffer constituents, but it did result in significant production of H 2O 2. It is concluded that multiple forms of cytochrome P-450 bind and metabolize vinylidene chloride. However, the form of the enzyme elevated by phenobarbital plays, at most, a minor role in these processes, while the form induced by β-naphthoflavone is not involved in either process. The effect of metabolism of vinylidene chloride by cytochrome P-450 on the relationship between the metabolism and toxicity of vinylidene chloride in vivo and its mutagenicity in vitro is considered.