Mechanisms of Biotransformation of Methoxylated and Hydroxylated Polybrominated Diphenyl Ethers

Polybrominated diphenyl ethers (PBDEs) were the most commonly used non-chemically bound brominated flame retardants (BFRs). The PBDEs as well as their methoxylated- (MeO-) and hydroxylated- (OH-) analogs are distributed in environmental matrices worldwide and have been detected in several organisms. There is concern over the occurrence of the OH-BDEs because they have greater toxicities relative to the PBDEs, including neurotoxicity, immunotoxicity, disruption of energy metabolism, and reproductive and endocrine disruption. Relationships among PBDEs, OH-BDEs and MeO-BDEs are unclear. While MeO-BDEs and OH-BDEs are naturally occurring in the marine environment, it has been reported that OH-BDEs are formed by biotransformation of PBDEs. However, in these studies, the OH-BDEs occurred at trace levels, from <0.01-1% of exposed PBDEs levels. A recent study by Wan et al. (2009) suggested that instead of PBDEs, in vitro, naturally occurring MeO-BDEs are precursors of OH-BDEs. However, there is no evidence from in vivo studies that the MeO-BDEs are transformed to OH-BDEs. Further, the enzyme(s) that catalyze the transformation of MeO-BDEs to OH-BDEs were unknown. To further demonstrate in vivo, biotransformation relationships among these structurally related compounds dietary accumulation, maternal transfer, and tissue distribution of PBDEs, MeO-BDEs and OH-BDEs and their transformation products were investigated in sexually mature Japanese medaka (Oryzias latipes). Medaka were exposed to BDE-47, 6-OH-BDE-47, and 6-MeO-BDE-47 through their diet for 14 days. Concentrations of all compounds were quantified in livers and whole carcass (minus livers) of female medaka as well as in eggs. Significant concentrations of 6-OH-BDE-47 were quantified in medaka exposed to 6-MeO-BDE-47. Significant concentrations of 6-MeO-BDE-47 were also detected in fish exposed to 6-OH-BDE-47. However, 6-MeO-BDE-47 was not observed when microsomes from livers of medaka were exposed to 6-OH-BDE-47. OH-PBDEs and MeO-PBDEs were not detected in medaka exposed to BDE-47. Similar results were demonstrated in eggs from female medaka. Furthermore, as hypothesized, concentrations of BDE-47, 6-MeO-BDE-47 and 6-OH-BDE-47 in medaka eggs increased during exposure. Therefore, this study demonstrated in vivo biotransformation of 6-MeO-BDE-47 to 6-OH-BDE-47 as a primary pathway, while conversion from BDE-47 to 6-OH-BDE-47 did not occur. 　　The enzyme(s) that catalyze transformation of MeO-BDEs to OH-BDEs was characterized in liver of rainbow trout (Oncorhynchus mykiss). Significantly greater concentrations of 6-OH-BDE-47 were detected in microsomes than were observed in S9 fractions exposed to 6-MeO-BDE-47, which suggests that biotransformation of 6-MeO-BDE-47 to 6-OH-BDE-47 is localized in microsomes. The requirement for the co-factor NADPH further confirmed the catalysis by phase Ι enzymes in this biotransformation reaction. Non-significant transformation of 6-MeO-BDE-47 to 6-OH-BDE-47 in microsomes isolated from livers of rainbow trout exposed…