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UBC Theses and Dissertations

In vitro hepatic metabolism of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and 2,2',4,4',5-pentabromodiphenyl ether (BDE-99) Erratico, Claudio


Polybrominated diphenyl ethers (PBDEs) are flame retardants that were added to many consumer products and have emerged as persistent and bioaccumulative environmental contaminants. Penta-BDE was a commercial PBDE mixture that was used extensively in North America. 2,2',4,4'-Tetrabromodiphenyl ether (BDE-47) and 2,2',4,4',5-pentabromodiphenyl ether (BDE-99) are the dominant congeners in the Penta-BDE mixture and occur at similar levels in the mixture and in air, dust, and sediments. In contrast, the concentration of BDE-99 is 10-fold lower than that of BDE-47 in most wildlife and human samples, which could be due to more extensive metabolism of BDE-99 than BDE-47 by hepatic cytochrome P450 (CYP) enzymes. To investigate this hypothesis, a liquid chromatography-mass spectrometry based assay was developed and validated to characterize the biotransformation of BDE-99 by liver microsomes. Rat liver microsomes were obtained from animals treated with dexamethasone, phenobarbital, 3-methylcholanthrene or corn oil. Up to six hydroxylated metabolites of BDE-99 were formed by different rat liver microsomal preparations. The major metabolite, 4-hydroxy-2,2',3,4',5-pentabromodiphenyl ether (4-OH-BDE-90), was formed at 2.7 pmol/min/mg protein by liver microsomes obtained from corn oil treated rats. CYP3A1, CYP1A1, and CYP2A2 were the most active rat recombinant CYP enzymes. Incubating BDE-99 with human liver microsomes resulted in the formation of 10 hydroxylated metabolites. The major metabolites were 2,4,5-tribromophenol (2,4,5-TBP), 5ʹ-hydroxy-2,2ʹ,4,4ʹ,5-pentabromodiphenyl ether (5'-OH-BDE-99) and 4'-hydroxy-2,2',4,5,5'-pentabromodiphenyl ether (4'-OH-BDE-101) and their rates of formation ranged between 25 and 45 pmol/min/mg protein. Incubating BDE-47 with human liver microsomes resulted in the formation of 9 hydroxylated metabolites. The major metabolites were 5-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (5-OH-BDE-47) and 6-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (6-OH-BDE-47) and their rates of formation were 23 and 27 pmol/min/mg protein, respectively. CYP2B6 was the major human CYP enzyme responsible for the formation of all the hydroxylated metabolites of BDE-47 and BDE-99. In conclusion, BDE-99 underwent more extensive oxidative metabolism by human than rat liver microsomes and was biotransformed into a different set of hydroxylated metabolites by human than rat liver microsomes. Metabolism of BDE-47 and BDE-99 by human liver microsomes proceeds at similar pace, which suggests that oxidative metabolism does not explain the difference in BDE-47 and BDE-99 blood concentrations in humans.

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