Mechanistic Investigation On Transformation Of Perfluorooctane Sulfonate Precursors Catalyzed By The Active Species Of P450Enzymes

Perfluorooctane Sulfonate (PFOS) is among the late-model persistent organic pollutants, which calls for considerable concern and investigation on its environmental toxicological effects and exposure risks to humans. PFOS gets itself exposed to humans both directly and indirectly, with biotransformation of its precursors being the major indirect pathway. Thus it is quite significant on evaluation of total PFOS exposure and risk by investigating the transformation of PFOS precursors. Cytochrome P450enzymes are kown as the ubiquitous and versatile heme enzymes found in biological tissues, and are responsible for the metabolism of xenobiotics. As shown in microsomal and human P450enzymes incubations, PFOS precursors are mainly metabolized by P450, with PFOSA being the major intermediate. Still, the precursors isomers possess different transformation profiles. Nevertheless, the detailed mechanism remains unclarified. Restricted by detection methods, it is impossible for experiments to dissect into the intermediates, letting alone to predict the possible toxicity caused thereby. Thus based on Density Functional Theory (DFT), the P450catalyzed metabolism of PFOS precursors was investigated herein theoretically with the Cpd I model for the P450reactive center. The main parts of the study are as follows:1. We calculated the metabolic reaction of a typical PFOS precursor, N-Ethyl substituted PFOSA (N-EtPFOSA) by P450. The results show that N-EtPFOSA goes through P450catalyzed N-dealkylation to give PFOSA, a process comprising Hydrogen Atom Transfer (HAT) and decomposition of ethanol amine. The HAT process initiates at the high spin quartet state of Cpd I, with the hydrogen atom at position Cα to N abstracted, giving rise to an alkane radical. The radical is then combined with barrierless oxygen rebound to form the ethanol amine intermediate, which then decomposes via proton transfer, with the assistance of a water molecule, leading to the formation of PFOSA and aldehyde products. Calculated reaction barriers of△E<20kcal/mol indicate the feasibility of the N-dealkylation reaction in physiological environment, and HAT process is the rate-determining step. Meanwhile, conclusions can be made from the calculation that branched N-EtPFOSA ismoers are N-dealkylated more easier than linear ones, which is consistent with experiment observations.2. The hydroxylation of PFOSA by P450was studied currently in parallel. It was found that the hydroxylation process is achieved via N-oxidation, and the reaction proceeds on the low doublet state of Cpd I. Reaction barriers of the N-oxidation are higher than those of N-dealkylation, suggesting a slower process of PFOS formation, thus this process is the rate-limiting step in the overall metabolism of PFOS precursors. The results give explaination for the observation of accumulated PFOSA concentrations in experiment.The calculated PFOSA intermediate concurs well with experiment resluts. The resultant aldehyde in the N-dealkylation reaction exerts potential risks on human health. This study proves that the computational methods can efficiently be applied to uncover the metabolic fate of PFOS precursors catalyzed by P450, therefore is promising to evaluate the origin and risk of total PFOS exposure to mammals.