Biotransformation of BDE-47 to Potentially Toxic Metabolites Is Predominantly Mediated by Human CYP2B6

Abstract

Background: Previous studies have indicated that cytochrome P450s (CYPs) are involved in the metabolism of polybrominated diphenyl ether (PBDE) flame retardants in humans, resulting in the formation of hydroxylated PBDEs (OH-PBDEs) that are potentially more toxic than the parent PBDEs. However, the specific enzymes responsible for the formation of OH-PBDEs are unknown.Objectives: The purposes of this study were to characterize the in vitro metabolism of 2,2´,4,4´-tetrabromodiphenyl ether (BDE-47) by human liver microsomes (HLM) and recombinant human CYPs, and to identify the CYP(s) that are active in the oxidative metabolism of BDE-47.Methods: Recombinant human CYPs (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, and 3A4) were incubated with BDE-47 (20 µM), and the metabolites were measured and characterized using gas chromatography with tandem mass spectrometry (GC-MS/MS). For kinetic studies, CYP2B6 and pooled human liver microsomes (HLMs) were incubated with BDE-47 (0–60 µM).Results: CYP2B6 was the predominant CYP capable of forming six OH-BDEs, including 3-OH-BDE-47, 5-OH-BDE-47, 6-OH-BDE-47, 4-OH-BDE-42, 4´-OH-BDE-49, and a metabolite tentatively identified as 2´-OH-BDE-66. On the basis of full-scan GC-MS analysis, we hypothesized the formation of two other metabolites: di-OH-tetra-BDE and di-OH-tetrabrominated dioxin. In kinetic studies of BDE-47 metabolism by CYP2B6 and pooled HLMs, we found Km values ranging from 3.8 to 6.4 µM and 7.0 to 11.4 µM, respectively, indicating the high affinity toward the formation of OH-BDEs.Conclusion: Our findings support a predominant role of CYP2B6 in the metabolism of BDE-47 to potentially toxic metabolites, including a hypothesized di-OH-tetrabrominated dioxin metabolite. These results will assist future epidemiological studies investigating the potential of PBDEs and their metabolites to produce neurobehavioral/neurodevelopmental disorders.

A mechanistic view of polybrominated diphenyl ether (PBDE) developmental neurotoxicity

“…Formation of hydroxylated metabolites of a number of PBDEs (e.g. BDE-47, BDE-99, and BDE-153) has been reported, and CYP2B6 is emerging as the main metabolic enzyme in this regard (Erratico et al 2012; 2013; Feo et al 2013). For example, Fig.…”

“…2 shows that metabolic products of BDE-47 formed by CYP2B6 include BDE-47 hydroxylated at the 3, 5 or 6 position, as well as different hydroxylated congeners (e.g. 4-OH-BDE-42, 4′-OH-BDE-49) (Erratico et al 2013; Feo et al 2013). …”

Persisting effects of a PBDE metabolite, 6-OH-BDE-47, on larval and juvenile zebrafish swimming behavior

“…Potential sources of 6-OH-BDE-47 from oxidative metabolism of anthropogenic PBDEs, and from inter-conversion with marine natural products (Source: Erratico et al, 2013; Feo et al, 2013; Wan et al, 2010, 2009). …”

“…In mammals, PBDEs are oxidatively metabolized by cytochrome p450 enzymes to form OH-BDEs (Erratico et al, 2013; Feo et al, 2013). Interestingly, there are also some natural sources of brominated phenolic compounds (Figure 1), including methoxylated-BDEs (Me-O-BDE) and OH-BDEs that originate from marine algae and sponges (Kelly et al, 2008; Wan et al, 2009).…”

Halogen−π Interactions in the Cytochrome P450 Active Site: Structural Insights into Human CYP2B6 Substrate Selectivity

“…The current observation is not just limited to halogenated drugs on market that are preferred substrates of CYP2B6, but also extends to environmental contaminants. These include the most abundant polybrominated diphenyl ether congeners in humans (BDE-47), 41 the polychlorinated biphenyls (PCB-153), 42 and the commonly used halogenated pesticide, Chlorpyrifos, 43 each of which is metabolized with high affinity and selectivity by CYP2B6 (Supporting figure 4). A fundamental question has been how halogenated environmental contaminants make specific contacts with the active site of CYP2B6 that lead to tight binding and selective oxidation by this enzyme as opposed to other human hepatic P450 enzymes with binding pockets of comparable size and shape.…”