© 1994 Oxford University Press
research-article |
Hepatic Uptake and Metabolism of 2,3,7,8-Tetrachlorodibenzo-p-dioxin and 2,3,7,8-Tetrachlorodibenzofuran1,2
Division of Environmental Toxicology and Chemistry, Center for Environmental Research, SUNYCollege at Buffalo Buffalo, New York 14222
*Toxicology Research Center, Departments of Pharmacology and Therapeutics, University at Buffalo Buffalo, New York 14214
Toxicology Research Center, Department of Biophysical Sciences, University at Buffalo Buffalo, New York 14214
Received February 17, 1993; accepted November 5, 1993
The pharmacokinetics of TCDD and related compounds is congener, dose, and species specific, with urinary and biliary excretion being dependent on the metabolism of these compounds. Isolated hepatocytes and liver slices in suspension culture and hepatic microsomes were used as in vitro models to assess the hepatic uptake and metabolism of [3H]- and [14C]- TCDD and [3H]TCDF (0.01–1.0 µM) in control and induced (5 µg TCDD/kg, 3 days earlier) male Sprague-Dawley rats. TCDD pretreatment, with an increase in cytochromes P450 1A1 and 1A2 (CYP1 Al, CYP1A2), produced an increase in the hepatic uptake of TCDD, while no increase in the hepatic uptake of TCDF was observed. The results are consistent with CYP1A2 serving as a hepatic binding protein for TCDD but not for TCDF. The rates of metabolism of TCDD and TCDF were directly proportional to their concentrations, indicating that the reaction follows first order kinetics at concentrations from 0.01 to 1.0 µM. Very limited metabolism of TCDD and TCDF was observed in control rat liver (0.45 and 3.2 pmol/hr/g hepatocyte wet wt at 0.1 µm, respectively). TCDD induced its own rate of metabolism about two- to fivefold at 1.0 µM but no induction was observed at 0.01 and 0.1 µM. In contrast, TCDD markedly induced the rate of TCDF metabolism at all substrate concentrations. While the results support the role of rat CYP1A1 in TCDF metabolism, the data suggest that CYP1 Al or CYP1A2 may not metabolize TCDD. These results also support the hypothesis that the more rapid metabolism and excretion of TCDF accounts for the relative resistance of the rat to the acute toxicity of TCDF. Comparative studies in rat and human liver microsomes found that TCDF metabolism exhibited first order kinetics in both species. Furthermore, the rate of TCDF metabo-lism in human liver microsomes was similar to that of control rat liver microsomes. Together the results suggest that TCDF will be far more persistent in rats, and possibly humans, following exposure at low doses which do not significantly induce cytochrome P450 1A1 and/or 1A2.