Primary Role of Cytochrome P450 2B6 in the Oxidative Metabolism of 2,2′,4,4′,6-Pentabromodiphenyl Ether (BDE-100) to Hydroxylated BDEs

Human exposure to polybrominated diphenyl ethers (PBDEs) through various routes poses deleterious health effects. PBDEs are biotransformed into hydroxylated metabolites (OH-BDEs) via cytochrome P450s (P450s), which may add to their neurotoxic effects. This study characterizes the <i>in vitro</i> metabolism of 2,2′,4,4′,6-pentabromodiphenyl ether (BDE-100), one of the most abundant PBDE congeners found in humans, by recombinant human P450s and pooled human liver microsomes (HLMs). Ten recombinant P450s were individually incubated with BDE-100 to monitor P450-specific metabolism. P450 2B6 was found to be the predominant enzyme responsible for nearly all formation of six mono-OH-pentaBDE and two di-OH-pentaBDE metabolites. Four metabolites were identified as 3-hydroxy-2,2′,4,4′,6-pentabromodiphenyl ether (3-OH-BDE-100), 5′-hydroxy-2,2′,4,4′,6-pentabromodiphenyl ether (5′-OH-BDE-100), 6′-hydroxy-2,2′,4,4′,6-pentabromodiphenyl ether (6′-OH-BDE-100), and 4′-hydroxy-2,2′,4,5′,6-pentabromodiphenyl ether (4′-OH-BDE-103) through use of reference standards. The two remaining mono-OH-pentaBDE metabolites were hypothesized using mass spectral fragmentation characteristics of derivatized OH-BDEs, which allowed prediction of an <i>ortho-</i>OH-pentaBDE and a <i>para</i>-OH-pentaBDE positional isomer. Additional information based on theoretical boiling point calculations using COnductor-like Screening MOdel for Realistic Solvents (COSMO-RS) and experimental chromatographic retention times were used to identify the hypothesized metabolites as 2′-hydroxy-2,3′,4,4′,6-pentabromodiphenyl ether (2′-OH-BDE-119) and 4-hydroxy-2,2′,4′,5,6-pentabromodiphenyl ether (4-OH-BDE-91), respectively. Kinetic studies of BDE-100 metabolism using P450 2B6 and HLMs revealed <i>K</i>_m values ranging from 4.9 to 7.0 μM and 6–10 μM, respectively, suggesting a high affinity toward the formation of OH-BDEs. Compared to the metabolism of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) and 2,2′,4,4′,5-pentabromodiphenyl ether (BDE-99) reported in previous studies, BDE-100 appears to be more slowly metabolized by P450s due to the presence of a third <i>ortho</i>-substituted bromine atom