Effects of chemical and enzymic probes on microsomal covalent binding of bromobenzene and derivatives. Evidence for quinones as reactive metabolites

Abstract

1. The chemical reactivity of bromobenzene metabolite(s) responsible for its protein covalent binding was investigated by determining the effects of many chemical and enzymic probes on the metabolism and covalent binding of [3,5-³H]bromobenzene with rat liver microsomes in vitro.

2. Classical cytochrome P-450 enzyme inhibitors decreased both metabolism and binding in parallel, whereas scavenging agents for reactive oxygen species and free radicals exhibited little or no effect. Sulphur nucleophiles were extremely efficient in decreasing binding with little or no effect on metabolism. Reducing agents such as ascorbate and diaphorase decreased binding slightly more than metabolism.

3. UDP-Glucuronic acid inhibited neither metabolism nor binding, but all three mono-bromophenols decreased binding more than metabolism. Trichloropropene oxide was unique in decreasing metabolism more than binding.

4. The effects of ascorbate, glutathione, bisulphite and butylated hydroxytoluene (BHT) on metabolism and binding of five ortho-substituted bromobenzene derivatives (o–BrC₆H₄X; X = OCH₃, CH₃, Br, CF₃, and CN) were similar to their effects on the metabolism and binding of bromobenzene.

5. Collectively these results support a major role for quinones as the reactive metabolites responsible for the majority of the protein covalent binding of bromobenzene and its ortho-substituted derivatives in microsomal systems in vitro.