Stereoselectivity and isotope effects associated with cytochrome P450- catalyzed oxidation of (S)-nicotine: The possibility of initial hydrogen atom abstraction in the formation of the δ1',5'-nicotinium ion

The stereochemical course of cytochromes P450 [P4501A1, P4502B1, P4502B4, and P450101 (P450cam)] catalyzed α-carbon oxidations of the cis-(Z)- and trans-(E)-5'-d₁ diastereomers of (S)nicotine has been examined. All enzyme preparations led to the stereoselective abstraction of the 5'-hydrogen atom trans to the pyridine ring with P450101 and human liver microsomal preparations displaying the highest (90%) and P450281 the lowest (67%) degree of stereoselectivity. No isotope effect was detected for any of the enzyme- catalyzed reactions, although the existence of an intrinsic isotope effect was inferred by the observation of an intramolecular isotope effect of 2-2.6 observed for the N-demethylation of (S)-N'-dideuteromethylnornicotine. Evidence for P450101-catalyzed N'-oxidation was sought but could not be found at higher than trace levels. These results, together with those obtained by computational methods, are interpreted in terms of an α-carbon oxidative pathway involving hydrogen atom abstraction rather than single electron transfer as the initiating event in the P450-catalyzed oxidation of (S)- nicotine to its Δ(1',5')-iminium ion metabolite.