Molecular Pharmacology, Vol 14, 890-899, Copyright © 1978 by the American Society for Pharmacology and Experimental Therapeutics

2,3,7,8-Tetrachlorodibenzo-p-Dioxin Induction of Aryl Hydrocarbon Hydroxylase in Female Rat Liver. Evidence for De Novo Synthesis of Cytochrome P-448

¹ Laboratory of Environmental Toxicology, National Institute of Environmental Health Sciences, National Institutes of Health, P. O. Box 12233, Research Triangle Park, N.C. 27709

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on heme synthesis and the synthesis of cytochrome P-448, as reflected in aryl hydrocarbon hydroxylase activity, were determined in female rat liver. TCDD treatment did not increase the activity of -aminolevulinic acid synthetase, the rate limiting enzyme in heme biosynthesis. In contrast, a 23-fold increase in the level of cytochrome P-448-mediated aryl hydrocarbon hydroxylase was observed 24 hours after TCDD treatment. Actinomycin D and cycloheximide, but not CoCl₂, completely prevented the induction of new cytochrome P-448 and increased aryl hydrocarbon hydroxylase activity by TCDD. In vitro reconstitution of cytochrome P-450 and increases in enzymatically active aryl hydrocarbon hydroxylase levels were achieved by adding hemin to whole liver homogenates from rats given TCDD (a hemoprotein inducer) and CoCl₂ (a heme depleter). These results suggest that TCDD induces de novo protein synthesis of apocytochrome P-448, which combines with heme to yield new cytochrome P-448 and a concomitant increase in aryl hydrocarbon hydroxylase activity. Therefore it is concluded that TCDD selectively induces the formation of cytochrome P-448 leading to increased aryl hydrocarbon hydroxylase activity, and that protein synthesis, rather than heme synthesis, is the rate-limiting event in this process.

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ACKNOWLEDGMENTS We thank Drs. Beth Gladen and Joe Haseman for statistical analysis of the data and Mrs. Geraldine Carver and Mrs. Patsy Daniels for protein determinations.