The metabolism of the carcinogen dibenz[a,j]acridine in isolated rat hepatocytes and in vivo in rats

Abstract

1. ³H-Dibenz[a,j]acridine (DBAJAC) metabolism occurred readily in vitro in incubations with hepatocytes from phenobarbital-pretreated, 3-methylcholanthrene-pretreated and untreated rats, with the formation of water-soluble conjugates and unconjugated metabolites.

2. For incubations of ³H-DBAJAC with hepatocytes the major organic solvent-soluble metabolites found with and without β-glucuronidase/arylsulphatase hydrolysis were the phenols, 3-hydroxy-DBAJAC, and 4-hydroxy-DBAJAC, and the proposed proximate carcinogen, trans-3,4-dihydroxy-3,4-dihydro-DBAJAC. The latter comprised 34—66% of the total organic solvent-soluble metabolites.

3. In contrast to results previously reported for rat hepatic microsomes, the K-region 5,6-oxide, and its dihydrodiol were minor metabolites detected after hepatocyte incubations.

4. Faecal excretion accounted for the bulk of radioactivity after i.p. doses of ³H-DBAJAC (0.5 mg/kg), and i.v. doses (0.5 mg/kg) were rapidly excreted into the 6 h bile. The organic solvent-soluble fraction obtained after enzymic hydrolysis of bile (~25% of excreted radioactivity) was subjected to h.p.l.c. It contained polar secondary oxidation products and virtually no 3,4-dihydrodiol.

5. Experiments conducted with greater hepatocyte densities (10⁷ cells/ml) and longer incubation times showed increased extents of metabolism, DNA and protein binding of radioactivity which paralleled the extent of metabolism. Very considerable metabolism of the 3,4-dihydrodiol occurred by the end of the incubation period.