Abstract

Nicotine is primarily metabolized to cotinine by cytochromes P450 (CYPs). 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 apparentK_m and V _max values (mean ± S.D.) for cotinine formation in human liver microsomes (n = 31) were 64.9 ± 32.7 μM and 28.1 ± 28.7 nmol/mg of protein/hr, respectively. A 30-fold difference was seen among the individual V _max values, with four livers showing significantly higher rates of cotinine formation. CYP2D6 is unimportant in nicotine metabolism because quinidine (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. CYP2A6 appears to be the major P450 involved in human nicotine metabolism to cotinine. Coumarin, a specific and selective CYP2A6 substrate, competitively 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 μM, and a CYP2A6 monoclonal antibody inhibited cotinine formation by >75%. Immunochemically determined CYP2A6 correlated significantly with nicotine-to-cotinine V _max values (r = .90, n = 30, P < .001) and to inhibition of nicotine metabolism by coumarin (r = .94, n = 30, P < .001). These data indicate that nicotine metabolism is highly variable among individual livers and that this is due to variable expression of CYP2A6, not CYP2D6.