A MAJOR ROLE FOR CYP2A6 IN NICOTINE C-OXIDATION BY HUMAN LIVER-MICROSOMES

Nicotine is primarily metabolized to cotinine by cytochromes P450 (CYP s). The degree of variation in the metabolism of nicotine to cotinine and the relative roles of the polymorphic enzymes CYP2A6 and CYP2D6 in this metabolism were investigated. The apparent K-m and V-max values (mean +/- S.D.) for cotinine formation in human liver microsomes (n = 31) were 64.9 +/- 32.7 mu M and 28.1 +/- 28.7 nmol/mg of protein/hr, r espectively. A 30-fold difference was seen among the individual V-max values, with four livers showing significantly higher rates of cotinin e formation. CYP2D6 is unimportant in nicotine metabolism because quin idine (a CYP2D6 inhibitor) had little effect on inhibition of cotinine formation; V-max values for dextromethorphan (CYP2D6 probe substrate) and nicotine (n = 9) did not correlate (r = .49, P = .18), and a cDNA CYP2D6 expression system failed to metabolize nicotine to cotinine. C YP2A6 appears to be the major P450 involved in human nicotine metaboli sm to cotinine. Coumarin, a specific and selective CYP2A6 substrate, c ompetitively inhibited cotinine formation by 85 +/- 11% (mean +/- S.D. ) in 31 human livers. The K-i value for this inhibition ranged from 1 to 5 mu M, and a CYP2A6 monoclonal antibody inhibited cotinine formati on by >75%. Immunochemically determined CYP2A6 correlated significantl y with nicotine-to-cotinine V-max values (r = .90, n = 30, P < .001) a nd to inhibition of nicotine metabolism by coumarin (r = .94, n = 30, P < .001). These data indicate that nicotine metabolism is highly vari able among individual livers and that this is due to variable expressi on of CYP2A6, not CYP2D6.