Abstract

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.