Hepatic metabolism of 7,12-dimethylbenz(a)anthracene in male, female, and ovariectomized Sprague-Dawley rats

Abstract

Dimethylbenz(a)anthracene (DMBA) is a potent inducer of mammary tumors in intact female Sprague-Dawley rats, but not in males or ovariectomized females (OVX). Qualitative and quantitative aspects of hepatic metabolism of DMBA were examined in these three groups of rats, using the nonrecirculating perfused liver, to determine whether the production of proximate carcinogenic metabolites of DMBA by the liver differed among these groups in the same manner as does sensitivity to tumor induction. DMBA was infused into the liver at a constant rate for 60 min. Rates of appearance of DMBA and its metabolites were measured in perfusate and bile during the infusion period and the first 60 min thereafter. The maximum rate of appearance of total metabolites in the perfusate, seen at the end of the infusion period, was highest in the intact female [2.6 +/- 0.3 nmol/(g x min)], slightly lower in the OVX [2.3 +/- 0.2 nmol/(g x min)] and significantly lower in the male [1.0 +/- 0.1 nmol/(g x min)]. The rates of appearance of metabolites in the bile showed the same order as those seen in the perfusate. The major metabolites extracted from the perfusate in all three groups were dihydrodiols, hydroxymethyl metabolites, and several unidentified metabolites. The 3,4-dihydrodiol, a proximate carcinogenic metabolite, appeared in the perfusate at higher rates in the intact female and OVX than in the male. Hydrolysis of bile samples showed that glucuronidation was a major pathway in the excretion of DMBA metabolites in bile. High performance liquid chromatographic analysis indicated that hydrolysis of DMBA glucuronides yielded the 7- and 12-hydroxymethyl metabolites and an unidentified metabolite designated X. The major hydrolysis product in the male was 12-hydroxymethyl while X was found to be the major product in the intact female and OVX. Under the conditions of this study, there were differences in the metabolic activation of DMBA by male and female rat liver. Ovariectomy, followed by DMBA perfusion 7 days later, did not result in significant changes in DMBA metabolism relative to the intact female, except for a decreased rate of excretion of metabolites in bile.