Diastereospecific kinetics of nicotine N-oxidation in rat liver microsomes

Abstract

1. In kinetic studies, both Eadie-Hofstee plots for cis- and trans-nicotine-1′-N-oxide formation from nicotine in rat liver microsomes were linear. For the formation of cis- and trans-nicotine-1′-N-oxide, the apparent K_m were 0·240 ± 0·069 and 1·524 ± 0·951 mM respectively. Corresponding V_max were 1·52 ± 0·48 and 1·19 ± 0·74 nmol/mg/min respectively. 2. The formation of cis-nicotine-1′-N-oxide was greater than the formation of trans-nicotine-1′-N-oxide in rat liver microsomes and the intrinsic clearance of cis-nicotine-1′-N-oxide formation was 8·1-fold greater than that of trans-nicotine-1′-N-oxide formation. 3. The formation of both cis- and trans-nicotine-1′-N-oxide in rat liver microsomes was inhibited by the addition of 1-(1-naphthyl)-2-thiourea or by heat-treatment of microsomes. 2-Diethylaminoethyl-2, 2-diphenylvalerate (SKF525A) and carbon monoxide did not affect these activities even at high concentrations. 4. Formations of cis- and trans-nicotine-1′-N-oxide correlated significantly with each other (r = 0·862, p < 0·01). These results suggested that the same flavin-containing monooxygenase (FMO) isoform is responsible for the formation of cis- and trans-nicotine-1′-N-oxide in rat liver.