Binding and Metabolism of Brominated Flame Retardant β-1,2-Dibromo-4-(1,2-dibromoethyl)cyclohexane in Human Microsomal P450 Enzymes: Insights from Computational Studies

The emerging brominated flame retardant, 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) has recently attracted strong interest due to its extensive detection in environment and potential toxicological effects on humans. Previous in vitro experiments have shown that the technical mixture of TBECH and the pure beta-isomer (beta-TBECH) can be metabolized by cytochrome P450 enzymes (CYPs) into multiple metabolites, but the specific CYP isoforms involved in TBECH metabolism and the relevant metabolic regioselectivity remain unknown. Here we for the first time investigated the binding patterns and affinities of beta-TBECH in human CYPs 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1 and 3A4, through molecular dynamics (MD) simulations. The binding affinities of beta-TBECH in CYPs, which are estimated by the calculated binding free energies, follow the order of 2A6 > 2C9 > 2B6 > 2E1 > 3A4 approximately 2C19 approximately 1A2 > 2D6. Although all CYPs are important beta-TBECH receptors, only 2A6, 2C19, 2E1 and 3A4 are responsible for metabolizing beta-TBECH. Specially, 2A6 and 2E1 may selectively hydroxylate the C1 and C7 sites of beta-TBECH, while 2C19 and 3A4 show metabolic preference for C7- and C8-hydroxylations, respectively. The three hydroxylation routes proposed by the further density functional theory (DFT) calculations generate C1-, C7- and C8-hydroxylated metabolites, while the latter two may further undergo debromination to yield the respective ketone and aldehyde as additional metabolites. The results provide meaningful insight into the binding and metabolism of beta-TBECH by human CYPs, which is helpful for understanding the metabolic fate and toxicity mechanism of this chemical.