Theoretical Study Of The Mechanisms Of PBDEs Mediated By The Cytochrome P450

Humans encounter a wide range of xenobiotics with potentially harmful consequences. These xenobiotic chemicals can be biotransformed, and the biotransformation occurs mostly in the liver. Among the various Phase I and Phase II transformation reactions, the oxidation of xenobiotics catalyzed by the cytochrome P450enzymes (CYP) plays an important role. Generally, the oxidative transformation serves to detoxify the body as they produce more water-soluble compounds and facilitate excretion of xenobiotic molecules. However, the metabolism can also transform xenobiotics into more reactive and toxic compounds. Thus, understanding the CYP metabolic mechanism and pathways is vital for predicting conversions of xenobiotics, and hence for the health risk assessment. Most previous studies rely heavily on in vivo or in vitro tests to probe the CYP metabolic reactions. Neither can the experimental methods fully explain the reaction mechanism since the primary oxidation products have never been directly identified due to their high reactivity toward covalent modification by proteins or by rapid reduction from soluble cofactors such as NADPH. Furthermore, experimental methods are generally laborious, time-consuming, costly and equipment dependent, and thus cannot meet the need of risk assessment for newly synthesized compounds prior to large-scale production and commercialization. Therefore, there is an urgent need to develop computational toxicology approaches to investigate the CYP metabolic reactions of xenobiotics.Polybrominated diphenyl ethers (PBDEs), a class of widely used additive flame retardants for which the environmental level increased exponentially, were supposed or observed to be biotransformed to hydroxylated PBDEs (HO-PBDEs) by CYP, and HO-PBDEs were observed to have the potential disrupting estrogen and thyroid hormone effects. By DFT computation with the B3LYP functional, we simulated the reaction of BDE-47catalyzed by the active species of CYP (Cpd I). The results reveal the addition of Cpd I to BDE-47is a rate-determining step. The addition to the ipso and non-substituted C atoms forms tetrahedral a-adducts that further transform into epoxides. Rearrangement of the a-adducts does not involve the proton shuttle and NIH mechanisms. Hydroxylation of the epoxides leads to HO-PBDEs and2,4-dibromophenol via cleavage of the ether bond. The addition to the Br-substituted C2and C4atoms has a higher barrier than the non-substituted C atoms, forming product complexes phenoxide and cyclohexadienone that subsequently undergo debromination/hydroxylation. The predicted products were verified by experimental results. As a first attempt to simulate the enzymatic transformation of a polycyclic compound, this study may enlighten a computational toxicological method to predict the biotransformation of xenobiotics catalyzed by CYP.Methoxylated polybrominated diphenyl ethers (MeO-PBDEs) have been found in the of a variety animals. The HO-PBBDs are also been identified as natural compounds which are considered forming by the biotransformation of MeO-PBDEs in some marine organisms. We simulated the reaction of6-MeO-BDE-47catalyzed by the Cpd â… . The geometry optimized results show that the reactant complex of P450and the6-MeO-BDE-47can form two distinct geometries. The phenyl moiety of substrate is either roughly parallel (face-on) or perpendicular (side-on) to the porphyrin ring of Cpd â… . The result show that, for the HS and LS states of face-on route and HS state of side-on route, the hydrogen atom abstraction step is then followed by a hydroxyl rebound process, which leads to C-O bond and formation the carbinol intermediates. However, in case of the LS state of side-on pathway, the geometry optimization leads to the radical partially evade recapture by the iron-bound hydroxyl radical, as do the above rebound processes, but instead of adding to the carbon positions on the periphery of P450heme. Therefore, the MeO-PBDEs are also might be the suicide inactivator of P450, at least for the6-MeO-BDE-47.The mechanism and kinetic isotope effect (KIE) of hydroxylation of1,2-dibromo-3-chloropropane (DBCP) by Cpd â… , the active species of the enzyme, have been studied here using computational toxicological method. Enzymatic and aqueous environment was simulated by the polarizable continuum model. The results show that there are some obvious differences between the hydroxylation of DBCP and alkanes. Firstly, compared to the two states reaction of the hydroxylation of alkanes, the hydroxylation of DBCP is a spin selective reaction. Secondly, the barrier of the rebound process is obviously higher than the hydroxylated reaction of alkanes. The calculated spin densities reveal the C-H activation steps are hydrogen atom transfer processes. The calculated KIE values are typical values of hydrogen atom abstraction process. Moreover, the KIE values are also sensitive to the temperature, which means that there is obvious quantum tunneling occurred in the hydroxylation of DBCP.