Increased retinoic acid metabolism following 3,3',4,4',5,5'-hexabromobiphenyl injection.

Young male Wistar rats received single i.p. injections of 3,3',4,4',5,5'-hexabromobiphenyl. In rats dosed with 40 mg/kg, food consumption and growth as well as liver retinol and retinyl palmitate concentrations decreased, while serum retinol and liver weight increased within 28 days following the injection. In rats receiving a 20-mg/kg dose, food consumption, growth, liver weight, and serum retinol were not affect, although liver retinol and retinyl palmitate concentrations declined to 23 and 21% of their respective control values. Vitamin A metabolism was studied in liver microsomes prepared from rats sacrificed 7 days after the 20-mg/kg injection. The rate of retinoic acid hydroxylation via the cytochrome P-450 system to 4-hydroxyretinoic acid plus the subsequent oxidation to 4-ketoretinoic acid was significantly elevated. Retinoic acid conjugation by UDP-glucuronyl transferase was also significantly increased. These changes corresponded with increased activities of cytochrome P-450-dependent aryl hydrocarbon hydroxylase and UDP-glucuronyltransferase conjugation of p-nitrophenol. These results provide a direct link between enzyme induction due to xenobiotics and specific steps in the vitamin A metabolic pathway.