Pentachlorophenol enhances 9-hydroxybenzo [a] pyrene-induced hepatic DNA adduct formation in vivo and inhibits microsomal epoxide hydrolase and glutathione S-transferase activities in vitro: likely inhibition of epoxide detoxication by pentachlorophenol.

We recently reported that co-administration to female mice of tamoxifen or 4-hydroxytamoxifen (4-OH-tamoxifen) with pentachlorophenol (PCP), but not with 2,6-dichloro-4-nitrophenol (DNCP) results in strong intensification of a specific subgroup, termed group I, of tamoxifen-DNA adducts in female mouse liver. As both PCP and DCNP are sulfotransferase inhibitors, we concluded that the intensification of tamoxifen group I adducts is probably not due to inhibition of sulfation by these phenols of a tamoxifen metabolite. Since epoxide derivatives of 4-OH-tamoxifen are potential candidates involved in tamoxifen-induced DNA damage, the hypothesis was developed and tested that PCP inhibits epoxide detoxication. As 4-OH-tamoxifen metabolites were unavailable to us, we employed indirect approaches to test this hypothesis. In the first set of experiments we determined whether PCP would augment DNA adduct formation from the benzo[a]pyrene metabolite, 9-hydroxybenzo[a]pyrene (9-OH-BP), as 9-OH-BP-4,5-epoxide is known to be involved in the metabolic activation of this compound. Female mice were given a single i.p. dose of 9-OH-BP (50 mumol/kg) either alone or in combination with PCP (75 mumol/kg), and hepatic DNA adducts were measured 24 h later by nuclease P1-enhanced bisphosphate 32P-postlabeling. Co-administration of PCP with 9-OH-BP resulted in a statistically significant 1.5- to 1.7-fold increase in 9-OH-BP adduct levels versus 9-OH-BP controls. In order to determine whether PCP inhibits the enzymatic detoxication of epoxides in vitro, in a second set of experiments, the effects of PCP on liver microsomal epoxide hydrolase (mEH) and purified equine liver glutathione S-transferase (GST) activities were studied using, respectively, styrene-7,8-oxide and 1-chloro-2,4-dinitrobenzene (CDNB) as substrates. Incubation of mouse liver microsomes with PCP (10-100 microM) strongly inhibited (by 21-97%) mEH activity in a dose-dependent manner, the IC50 being 35 microM. DCNP was ineffective as a mEH inactivator. PCP also inhibited purified equine liver GST activity, with an IC50 of 23.5 microM. Taken together, the results of this study strongly support the hypothesis that PCP inhibited enzymatic detoxication of epoxides in vivo and in vitro. By this mechanism PCP would lead to enhancement of DNA damage caused by 9-OH-BP, and possibly other drugs and their metabolites, which undergo epoxidation prior to DNA binding.