Photolysis products of 2,4,5,2',4',5'-hexabromobiphenyl: hepatic microsomal enzyme induction and toxicity in Sprague-Dawley rats.

The irradiation of 2,4,5,2',4',5'-hexabromobiphenyl (2,4,5-HBB) by ultraviolet light created a mixture of lower brominated polybrominated biphenyl (PBB) congeners. Three photoproducts, 2,4,5,3',4'-pentabromobiphenyl (-PBB), 2,4,5,2',5'-PBB, and 3,4,3',4'-tetrabromobiphenyl (3,4-TBB), as well as 2,4,5-HBB and the photolyzed 2,4,5-HBB mixture, were administered to rats as a single ip injection (90 mg/kg, except 3,4-TBB, which was given at 2 mg/kg) 2 weeks before sacrifice. All treatments except 3,4-TBB induced NADPH-cytochrome P-450 reductase and aminopyrine-N-demethylase activities while all treatments except 2,4,5-HBB induced ethoxyresorufin-O-deethylase and UDP-glucuronosyltransferase activities. Thymus to body weight and spleen to body weight ratios were unchanged compared to controls for all treatments whereas an increase in the liver weights was observed for all treatment groups. Histologic examination revealed that the photolyzed 2,4,5-HBB mixture caused moderate to severe hepatocyte enlargement. Results of tissue analysis for the pure PBB congeners indicated that 2,4,5,2',5'-PBB and 3,4-TBB were metabolized in vivo and this was confirmed by in vitro metabolism studies. The results revealed that the photolyzed 2,4,5-HBB mixture caused a mixed-type induction of hepatic drug-metabolizing enzymes. This is most likely due to the effect of 2,4,5-HBB and toxic congeners formed during the irradiation of 2,4,5-HBB. 2,4,5,3',4'-PBB, which is toxic and apparently not metabolized, is believed to be the major congener contributing to the increased toxicity of the photolyzed 2,4,5-HBB mixture since 3,4-TBB was metabolized and appeared not to be as potent as inducer of aryl hydrocarbon hydroxylase activity.