Nicotinamide adenine dinucleotide (NAD)-dependent oxidation of nicotine- δ1′(5′)-iminium ion to cotinine by rabbit liver microsomes

Abstract

Nicotine is metabolized to cotinine via the action of two sequential enzymatic reactions. Cytochrome P-450 oxidizes the 5′ position to yield the intermediate nicotine- Δ 1′(5′)-iminium ion (1). Further oxidation of this compound to cotinine by cytosolic aldehyde oxidase has been described (2,3). The oxidation of tertiary amine xenoblotics to amides through an iminium ion intermediate is well documented as a general pathway of detoxication of this class of compounds (4). Since iminium ions are electrophiles with potentially deleterious effects, an understanding of their metabolism and disposition is important (5). During its metabolism, nicotine undergoes covalent binding to microsomal protein (6), while the iminium ion binds to sulfhydryl groups on macromolecules in the absence of enzymes (7). In this report, we demonstrate the existence of a second pathway of cotinine formation from nicotine- Δ 1′(5′)-iminium ion (Fig. 1). This reaction is catalyzed by the microsomal fraction and requires NAD. The enzyme involved is distinguished from aldehyde oxidase by its subcellular location, oxidant specificity, and sensitivity to different inhibitors. This pathway uses water as the source of the oxygen in the production of cotinine.