TY - JOUR T1 - 2,3,7,8-Tetrachlorodibenzo-<em>p</em>-Dioxin Induction of Aryl Hydrocarbon Hydroxylase in Female Rat Liver. Evidence for <em>De Novo</em> Synthesis of Cytochrome P-448 JF - Molecular Pharmacology JO - Mol Pharmacol SP - 890 LP - 899 VL - 14 IS - 5 AU - KIRK T. KITCHIN AU - JAMES S. WOODS Y1 - 1978/09/01 UR - http://molpharm.aspetjournals.org/content/14/5/890.abstract N2 - 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 δ-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. 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. ER -