Metabolic activation of 1,2,4-trichlorobenzene and pentachlorobenzene by rat liver microsomes: a major role for quinone metabolites.

Microsomal metabolism of 1,2,4-[14C]trichlorobenzene (1,2,4-TrCB) and [14C]pentachlorobenzene (PeCB) was studied with special emphasis on the conversion-dependent covalent binding to protein and DNA. 1,2,4-TrCB was metabolized to 2,3,6- and 2,4,5-trichlorophenol, and to a lesser extent to 2,4,6- and 2,3,5-trichlorophenol, and trichlorohydroquinone. About 10% of all metabolites became covalently bound to protein in a rather nonselective way. For 1,2,4-TrCB and PeCB a strong correlation between secondary metabolism to hydroquinones and covalent binding was established. Protein binding was completely inhibited by the addition of ascorbic acid, indicating quinone metabolites as the sole reactive species formed. Both 1,2,4-TrCB and PeCB alkylated DNA, although to a much lesser extent than protein (0.5 and 0.3% of all metabolites, respectively). Nonquinone intermediates, presumably epoxides, were responsible for a minor portion of the observed DNA binding, since complete inhibition by ascorbic acid was not reached. The differential role of cytochrome P450 both in primary and in secondary metabolism was demonstrated by the use of microsomes from rats pretreated with different inducers. Dexamethasone (DEX) microsomes (cytochrome P450IIIA1) showed the highest activity toward these chlorinated benzenes (14 nmol/mg/5 min for 1,2,4-TrCB and 36 nmol/mg/10 min for PeCB, both with regard to the formation of phenols and to the formation of protein-bound metabolites. In addition, DEX microsomes preferentially formed 2,3,6-trichlorophenol, whereas other microsomal suspensions formed 2,4,5-trichlorophenol as the major isomer. The present study clearly demonstrates the high alkylating potency of secondary quinone metabolites derived from chlorinated benzenes and poses a need for reevaluation of the role of epoxides in the observed toxicity of these compounds.