Specificity of cytochrome P450 2A3-catalyzed alpha-hydroxylation of N '-nitrosonornicotine enantiomers

N'-nitrosonornicotine (NNN) induces tumors in the rat nasal cavity and esop hagus and is believed to be a causative agent for esophageal cancer in toba cco users. To exert its carcinogenic potential, NNN must be metabolically a ctivated by alpha-hydroxylation at either the 2'- or 5'-carbon. We previous ly reported that the human cytochrome P450 (P450), 2A6, efficiently and spe cifically catalyzed NNN 5'-hydroxylation. P450 2A3, which is expressed in t he rat nasal cavity and to a small extent in the esophagus, is closely rela ted to P450 2A6. P450 2A3, like 2A6, is a good catalyst of NNN alpha-hydrox ylation (K-m 7 mu M; V-max 17 nmol/min/nmol). However, in contrast to P450 2A6, 2A3 catalyzed both 5'- and 2'-hydroxylation of NNN. The ratio of 2'- t o 5'-hydroxylation was 1:3. These data, both with P450 2A6 and 2A3, were ob tained using racemic NNN. P450 2A3 catalyzed metabolism of (S)-NNN occurred exclusively at the 5'-position. The predominant pathway of (R)-NNN metabol ism was 2'-hydroxylation, and occurred to a 3-fold greater extent than did 5'-hydroxylation. These data are in contrast to those obtained from a recen t study of (R)- and (S)-NNN metabolism by cultured rat esophagus. In that s tudy, (S)-NNN was metabolized predominantly by 2'-hydroxylation and (R)-NNN equally by 2'- and 5'-hydroxylation. Taken together, these data provide st rong evidence that P450 2A3 is not the rat esophageal P450 that catalyzes t he metabolic activation of NNN. P450 2A3 may be an important catalyst of NN N activation in rat nasal mucosa.