Elsevier

Biochemical Pharmacology

Volume 31, Issue 11, 1 June 1982, Pages 2083-2092
Biochemical Pharmacology

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

https://doi.org/10.1016/0006-2952(82)90425-7Get rights and content

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 Ks (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 Ks for the low-affinity site by 3-fold to ca. 1.6 mM. The maximum extents of binding (ΔAmax or ΔAmax/nmole cytochrome P-450) of vinylidene chloride were decreased or not affected by β-naphthoflavone induction, while ΔAmax but not ΔAmax/ 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:O2 (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 Vmax 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 H2O2. 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.

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    This research was supported by grants from the Medical Research Council, the Atomic Energy Board and the National Cancer Association.

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