Effect of monooxygenase inducers and inhibitors on the hepatic metabolism of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the rat and hamster

The rat and hamster exhibit about a 100-fold difference in sensitivity to the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), with the hamster representing the most resistent species examined to date. The present study compared the induction and inhibition of hepatic TCDD metabolism in these species using suspensions of isolated hepatocytes. Purified ¹⁴C-TCDD or ³H-TCDD (2.2 μM) was incubated for 2-6 hr with hepatocytes isolated from untreated, TCDD-pretreated (5 μg/kg, ip), 3-methylcholanthrene-pretreated (3-MC, 50 mg/kg, ip, x 3 days), isosafrole-pretreated (ISO, 150 mg/kg, ip, x 3 days), or phenobarbital-pretreated (PB, 80 mg/kg, ip, x 3 days) rats and hamsters. Untreated rat and hamster hepatocytes metabolized TCDD at a similar rate (0.20 and 0.18 pmol/hr/mg, respectively). In both species, TCDD and 3-MC pretreatments elevated the rate of TCDD metabolism by 5-6-fold, while PB pretreatment had no effect. Isosafrole modestly increased (1.8-2.5-fold) TCDD metabolism in each species. Analysis by high performance liquid chromatography indicated similarities in the TCDD-metabolite profiles formed by hepatocytes from both species, with two major metabolite peaks detected following induction by TCDD and 3-MC. The in vitro metabolism of TCDD in hepatocytes from TCDD-pretreated rats and hamsters was inhibited by 7,8-benzoflavone (100 μM), but not by metyrapone (100 μM). The effect of these cytochrome P-450 inducers and inhibitors on the metabolism of ³H-benzo(a)pyrene (BaP) in rat hepatocytes was similar to their effect on TCDD metabolism. However, marked differences were observed in their effects on the metabolism of BaP and TCDD in hamster hepatocytes. The results indicate that TCDD is metabolized by a form of cytochrome P-448 in both the rat and hamster; however, species differences appear to exist in the substrate specificity of this enzyme.