title: Kinetic isotope effects in hydroxylation reactions effected by cytochrome P450 compounds I implicate multiple electrophilic oxidants for P450-catalyzed oxidations.
abstract: Kinetic isotope effects were measured for oxidations of (S,S)-2-(p-trifluoromethylphenyl)cyclopropylmethane containing zero, two, and three deuterium atoms on the methyl group by Compounds I from the cytochrome P450 enzymes CYP119 and CYP2B4 at 22 degrees C. The oxidations displayed saturation kinetics, which permitted solution of both binding constants (K(bind)) and first-order oxidation rate constants (k(ox)) for both enzymes with the three substrates. The binding constant for CYP2B4 Compound I was about 1 order of magnitude greater than that for CYP119 Compound I, but the oxidation rate constants were similar for the two. In oxidations of 1-d(0), k(ox) = 10.4 s(-1) for CYP119 Compound I, and k(ox) = 12.4 s(-1) for CYP2B4 Compound I. Primary kinetic isotope effects (P) and secondary kinetic isotope effects (S) were obtained from the results with the three isotopomers. The primary KIEs were large, P = 9.8 and P = 8.9 for CYP119 and CYP2B4 Compounds I, respectively, and the secondary KIEs were small and normal, S = 1.07 and S = 1.05, respectively. Large intermolecular KIEs for 1-d(0) and 1-d(3) of k(H)/k(D) = 11.2 and 9.8 found for the two Compounds I contrast with small intermolecular KIEs obtained previously for the same substrate in P450-catalyzed oxidations; these differences suggest that a second electrophilic oxidant, presumably iron-complexed hydrogen peroxide, is important in cytochrome P450 oxidations under turnover conditions.
authors.affiliation: Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, USA.
authors.forename: Xin
authors.initials: X
authors.lastname: Sheng
authors.forename: Haoming
authors.initials: H
authors.lastname: Zhang
authors.forename: Paul F
authors.initials: PF
authors.lastname: Hollenberg
authors.forename: Martin
authors.initials: M
authors.lastname: Newcomb
mesh_terms: D055162:Biocatalysis; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006900:Hydroxylation; D007554:Isotopes; D007700:Kinetics; D010084:Oxidation-Reduction; D013056:Spectrophotometry, Ultraviolet; D013379:Substrate Specificity
chemical_list: D007554:Isotopes; D003577:Cytochrome P-450 Enzyme System