Abstract

Because of the prevalence of cigarette smoking in the general population and because studies suggest that a large percentage of nicotine is metabolized to cotinine in humans, it is important to study the enzymes responsible for nicotine metabolism. The cytochromes P-450 have long been implicated in the first step in the conversion of nicotine to nicotine delta 1'(5')-iminium ion. We demonstrate here that rat liver P-450IIB1 is able to convert nicotine to cotinine in the presence of cytosol with a Km of 5-7 microM. A constitutive form of P-450 is also implicated in nicotine metabolism, while purified P-450IA1 and P-450IIC6 show no detectable activity. The lack of P-450IA1 activity substantiates work by others who also failed to observe an increase in the efficiency of nicotine metabolism to cotinine by microsomes from rats that had been pretreated with benzanthracene. This result is in contrast to work with purified rabbit liver enzymes, in which P-450IA1 exhibited low but measurable activity. Our results support the notion that nicotine metabolism to cotinine by P-450 enzymes is highly species dependent. Thus, it is unwise in some cases to extrapolate results obtained by animal model study to the possible role of specific forms of the P-450 enzymes in nicotine metabolism in humans.