BDE-99 is Predominantly Metabolized by CYP2B6 in Human Liver Microsomes

Polybrominated diphenyl ethers (PBDEs) are persistent, bioaccumulative, and toxic environmental pollutants frequently detected in human samples. The purpose of this study was to investigate the oxidative metabolism of 2,2',4,4',5-pentabromodiphenyl ether (BDE-99), the major component of a widely used commercial PBDE mixture, by human liver microsomes and to determine the cytochrome P450 (CYP) enzymes involved. Using liquid chromatography/mass spectrometry (LC/MS), we had previously identified formation of four hydroxylated metabolites when BDE-99 was incubated with human liver microsomes. Using a more sensitive LC/MS instrument, we now report that BDE-99 was biotransformed by human liver microsomes to three major (5'-OH-BDE-99, 2,4,5-tribromophenol (2,4,5-TBP) and 4'-OH-BDE-101) and three minor (4-OH-BDE-90, 6'-OH-BDE-99 and 2-OH-BDE-123) oxidative products. Rates of formation of major and minor metabolites ranged between 25 and 45 and between 0.5 and 2.5 pmol/min/mg protein, respectively. In addition, one di-OH and four other mono-OH-penta-BDE metabolites of BDE-99 were detected but their structures were not elucidated because of the lack of authentic standards. Incubations with 4-OH-BDE-90, 5'-OH-BDE-99, 6'-OH-BDE-99 or 2-OH-BDE-123 in place of BDE-99  showed that 2,4,5-TBP, but not the di-OH metabolite of BDE-99, was produced from 5'-OH-BDE-99, 6'-OH-BDE-99 and 4'-OH-BDE-101. Using a panel of 15 recombinant CYP enzymes, CYP2B6 was found to be the only CYP enzyme to catalyze the formation of all the hydroxylated metabolites of BDE-99 formed in human liver microsomes. Formation of BDE-99 hydroxylated metabolites is of toxicological concern as recent in vitro studies shown that several hydroxy PBDEs including some of the human hepatic BDE-99 metabolites identified in the present study inhibit endogenous metabolism of thyroid and steroid hormones more potently than the parent PBDEs.