Metabolic oxidation of carcinogenic arylamines by p450 monooxygenases: theoretical support for the one-electron transfer mechanism.

N-oxidation by cytochrome p450 enzymes is an initial step in the metabolic activation of aromatic amine compounds. Once metabolized, these compounds are converted to DNA-reactive species which can exhibit potent mutagenic and/or carcinogenic activity. The precise mechanism of p450 enzyme oxidation is not completely understood, although various theories, involving either one-electron transfer, two-electron transfer or addition-rearrangement, have been debated. In previous studies, selection of the most probable mechanism has been based on consideration of Hückel theory charge distribution calculations and experimentally-derived enzyme metabolism data. This approach can now be improved by incorporating contemporary, ab initio quantum chemical methods to accurately determine the chemical properties of p450 aromatic amine substrates. In this work, we have re-examined the feasibility of three proposed p450 oxidation mechanisms by comparing the experimental oxidation yields and rates of 1-naphthylamine (1-NA), 2-naphthylamine (2-NA) and 2-aminofluorene (2-AF). This data has then been analyzed with respect to ab initio-calculated charge distributions and energies of reactants, oxidation products and proposed intermediates. Our analysis of theoretical and experimental data indicates that the one-electron model is more consistent with oxidation rate data for 1-NA, 2-NA, and 2-AF. We therefore conclude, in contrast to earlier studies, that the one-electron mechanism is more likely to be the pathway for p450-catalyzed aromatic amine oxidation.