Theoretical Studies On The Metabolic Hydroxylation Mechanism Of Diverse N-nitrosamines Mediated By Cytochrome P450

N-Nitroso Compounds (NNCs) is a kind of important environmental carcinogenswhich can cause variety of tumors. Carcinogenic and tumor targets are different dueto the different structures. The NNCs are relative stable before they are metabolizedby cytochrome P450(CYP450). The NNCs metabolite products show significantcarcinogenic activity after metabolized by P450in human body. Researches show that,the first metabolite step is α-hydroxylation catalyzed by CYP450, and then aα-hydroxyalkylamine generated. Then the α-hydroxyalkylamine decompositionproducts can active with DNA, DNA bases alkylated and broken up, and finally leadto tumor formation. The second step has been testified by many experiments, whilethe α-hydroxylation maybe the reason why NNCs have diverse tumor targets. Thisthesis is mainly involved in the study of the mechanisms of the NNCs C-Hhydroxylation catalyzed by cytochrome P450by using quantum mechanical methods.This research can also provide important theoretical basis for the NNCscarcinogenicity.1. The metabolic mechanism of N-Nitrosodimethylamine (NDMA) C-Hhydroxylation catalyzed by two different active centers, Cpd I and Cpd0, was studiedby using density functional theory (DFT). Results showed that, Cpd I was the maincatalyst, compared with Cpd0. Combined high spin (HS) and low spin (LS) stateswere involved in the hydroxylation reaction. The reaction had two steps. Initially,NDMA and Cpd I formed HS and LS reactant complexes, stabilized through weakH-bonding between the Fe-coordinated oxygen and H atom of the α-methyl group ofNDMA. Then H atom transferred from C-H to the Fe=O moiety of Cpd I to formedα-nitrosamino radical intermediated complexesAfter the H atom transferred to theFe=O, the reaction went into two ways. The HS intermediate, α-nitrosamino radicalneeded to rebound to the hydroxide radical of Fe-OH to form alcohol product, whilethe LS intermediate didn’t went through the rebounding step.. H-abstraction was therate-determining step for both HS and LS pathways, with the reaction barrier17.56/17.46kcal/mol (HS/LS). The similar LS and HS reaction barriers showed thatthe initial H-abstraction from NDMA was governed by a Two-State Reactivity (TSR).The research offered a reasonable model for the NNCs α-hydroxylation reactionmechanism. 2. The metabolic mechanism of N-Nitrosoethylmethylamine C-H hydroxylationcatalyzed by two different active center Cpd I and Cpd0of cytrochrome P450wasstudied. Results showed that, Cpd I was the main catalyst for theN-Nitrosoethylmethylamine C-H hydroxylation reaction, with the H atom abstractionwas the rate-determining step. The cis-form and trans-form α-H hydroxylationmechanism were compared. Results showed that the long-chain of the cis-form haslower H transfer barrier. The mechanism was also governed by a TSR. The workmade it clear that Cpd I had priority in catalyzing NNCs C-H hydroxylation process.3. The metabolic mechanism of N-Nitrosomethylbutylamine C-H hydroxylationcatalyzed by Cpd I was studied. Results showed that, the reaction had two stepprocesses. First was the C-H activation, H atom transferred from α-methyl group tothe Fe=O to form PorFe-OH and α-nitrosamino radical intermediate. Then theα-nitrosamino radical intermediate rebounded to the hydroxide radical, yielding thealcohol product. The reaction was governed by a TSR, and H-abstraction step was therate-determining step. Compared with the cis-form C-H activation, the trans-formMENA C-H hydroxylation has lower energy barrier, and the reaction is easier to react.In this research, the C-H hydroxylation mechanism was studied. A mechanismmodel of NNCs hydroxylation was established. The research confirmed that Cpd Iwas the main catalyst in the NNCs α-H hydroxylation mechanism. Preliminaryconclusions that the long-chain of cis-formed NNCs was easier to be meditated byP450. This research was of great significance for the next relative research.