Theoretical Study On Molecular Mechanism Of Typical Environmental Endocrine Disruptors Metabolized By Cytochrome P450

Traditional experimental methods are difficult to capture some of the fleeting transition states and reactive intermediates,such as the active species of P450s called Compound Ⅰ(Cpd Ⅰ),and the quantum chemical calculation can provide a very useful tool for us to study the mechanism of enzymatic metabolism.In this work,we have applied density functional theory(DFT)methods and selected Cpd I as the enzyme active site of P450s to investigate the metabolic mechanism of several typical environmental endocrine-disrupting chemicals by P450s.Moreover,we tried to explore some "black box" problems of the mechanism from molecular level in order to fill the gaps in mechanism research and provide some reference for the prediction of metabolic mechanisms of the analogues.The main conclusions are as follows(1)We have investigated the metabolic pathway of typical phenolic endocrine-disrupting chemical bisphenol A(BPA)using the catalytic active site of the enzyme by including key surrounding important amino acids of Cpd I,the P450s metabolize BPA to form ortho-OH-BPA and an ipso-position quinol intermediate by H-abstraction/OH-rebound reaction mechanism,the quinol can spontaneously decompose into the carbocation and hydroquinone,this carbocation can further evolve into the highly estrogenic metabolites.In addition,the H-abstraction/OH-rebound reaction mechanism has been verified as a general reaction mode for many other phenolic endocrine-disrupting chemicals.We have also analyzed the types of decomposition reactions of various ipso-position quinol intermediates,and made corresponding summaries to provide some prediction information for the types of ipso-substitution/addition reaction of the analogue.(2)Triphenyl phosphate(TPHP)is a very typical organophosphate flame retardant.Quantum chemical calculations have shown that the dominant reaction channel is the O-addition of Cpd I onto the aromatic ring of TPHP,followed by a hydrogen-shuttle mechanism leading to the ortho-hydroxy-TPHP(M1)as the main monohydroxylated metabolite.Further,subsequent H-abstraction/OH-rebound reaction acting on Ml yields two meta-position isomeric di-hydroxy-TPHPs and an ipso-position quinol intermediate,then the quinol can spontaneously decompose into a diester anion and a quinone after deprotonation,such diester anion can readily evolve into diphenyl phosphate(DPHP)by fast protonation in a nonenzymatic environment.(3)Aldrin and dieldrin are two typical organochlorine insecticides,and the aldrin can be metabolized to dieldrin through epoxidation by P450s,however,the metabolic mechanism of dieldrin by P450s has been controversial.Through research we propose that the two-state reactivity(TSR)mechanism of Cpd I can suitably explain the process that P450s metathesize dieldrin to form different oxidative products:The reaction starts from the activation of C-H bond,on the doublet low-spin state of Cpd I,it goes through H-abstraction/radical-rebound process directly to form a hydroxylated metabolite without any rearrangement reaction;On the quartet high-spin state of Cpd I,the radical of dieldrin from the H-abstraction step will undergo a rearrangement process to form a new radical which further goes through a radical-rebound process to form another hydroxylated metabolite,and then,it will undergo dechlorination and deprotonation processes spontaneously to form the intramolecularly bridged pentachloro ketone,which is in accord with experiments.