Toxicity of 3,4,5,3',4',5'-hexabrominated biphenyl and 3,4,3',4'-tetrabrominated biphenyl

Immature male rats were given a single equimolar dose (21.3 mumol/kg body wt) of 3,4,5,3',4',5'-hexabromobiphenyl (HBB) or 3,4,3',4'-tetrabromobiphenyl (TBB) and terminated at various times up to 14 days after treatment. Hepatic microsomal aryl hydrocarbon hydroxylase (AHH) activity for the TBB treatment group was maximal at Day 2 and then steadily decreased, whereas this activity was induced in 1 day and remained high for the HBB treatment group. Tissue concentrations of HBB appeared to be unchanged over time whereas tissue concentrations of TBB decreased in a biphasic manner. Rates of in vitro metabolism of TBB with hepatic microsomes from TBB-treated animals showed a similar time-course relationship to AHH induction. HBB caused moderate to severe hepatic changes while TBB-treated rats had only mild hepatic changes. The relative binding of TBB by the hepatic receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was about 10 times that of HBB. The results suggest that even though the receptor-binding affinities imply that TBB should be more toxic than HBB, it is less toxic than HBB because it is metabolized. Studies with the chlorinated analogs of TBB and HBB suggested that PCB behave similarly. These results also suggest that receptor binding and AHH induction do not accurately reflect toxicity for polyhalogenated aromatic hydrocarbons which are metabolized, presumably because continued occupation of the receptor and persistent induction of some enzyme activity are required for toxicity.