Biotransformation of the Flame Retardant 1,2-Dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) in Vitro by Human Liver Microsomes

• Published in: Environmental science & technology 2017-09, Vol.51 (18), p.10511-10518
• Main Authors: Nguyen, Khanh-Hoang, Abou-Elwafa Abdallah, Mohamed, Moehring, Thomas, Harrad, Stuart
• Format: Article
• Language: English
• Subjects: Acetic acid; Bioaccumulation; Biomarkers; Biotransformation; Chromatography; Cyclohexane; Cyclohexanes - metabolism; Enzyme kinetics; Flame retardants; Flame Retardants - metabolism; Humans; Hydroxylation; Incubation; Kinetics; Liquid chromatography; Liver; Mass spectrometry; Mass spectroscopy; Metabolic rate; Metabolites; Microsomes; Microsomes, Liver; Oxidation; Toxicology
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.7b02834

The technical mixture of 1,2-dibromo-4-(1,2-dibromoethyl)­cyclohexane (TBECH or DBE-DBCH) and the pure β-TBECH isomer were subjected to in vitro biotransformation by human liver microsomes (HLM). After 60 min of incubation, 5 potential metabolites of TBECH were identified in microsomal assays of both the TBECH mixture and β-TBECH using ultraperformance liquid chromatography–Q-Exactive Orbitrap mass spectrometry. These include mono- and dihydroxylated TBECH and mono- and dihydroxylated TriBECH as well as an α-oxidation metabolite bromo-(1,2-dibromocyclohexyl)-acetic acid. Our results indicate potential hepatic biotransformation of TBECH via cyctochrome P450-catalyzed hydroxylation, debromination, and α-oxidation. Kinetic studies revealed that the formation of monohydroxy-TBECH, dihydroxy-TBECH, and monohydroxy-TriBECH were best fitted to a Michaelis–Menten enzyme kinetic model. Respective estimated V max values (maximum metabolic rate) for these metabolites were 11.8 ± 4, 0.6 ± 0.1, and 10.1 ± 0.8 pmol min–1 mg protein–1 in TBECH mixture and 4992 ± 1340, 14.1 ± 4.9, and 66.1 ± 7.3 pmol min–1 mg protein–1 in β-TBECH. This indicates monohydroxy-TBECH as the major metabolite of TBECH by in vitro HLM-based assay. The estimated in vitro intrinsic clearance (Clint) of TBECH mixture was slower (P < 0.05) than that of pure β-TBECH. While the formation of monohydroxy-TBECH may reduce the bioaccumulation potential and provide a useful biomarker for monitoring TBECH exposure, further studies are required to fully understand the levels and toxicological implications of the identified metabolites.