Murphy S E, Isaac I S, Ding X, McIntee E J
Department of Biochemistry Molecular Biology & Biophysics University of Minnesota Cancer Center Minneapolis, Minnesota, USA.
Drug Metab Dispos. 2000 Nov;28(11):1263-6.
N'-nitrosonornicotine (NNN) induces tumors in the rat nasal cavity and esophagus and is believed to be a causative agent for esophageal cancer in tobacco users. To exert its carcinogenic potential, NNN must be metabolically activated by alpha-hydroxylation at either the 2'- or 5'-carbon. We previously reported that the human cytochrome P450 (P450), 2A6, efficiently and specifically catalyzed NNN 5'-hydroxylation. P450 2A3, which is expressed in the rat nasal cavity and to a small extent in the esophagus, is closely related to P450 2A6. P450 2A3, like 2A6, is a good catalyst of NNN alpha-hydroxylation (K(m) 7 microM; 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'- to 5'-hydroxylation was 1:3. These data, both with P450 2A6 and 2A3, were obtained using racemic NNN. P450 2A3 catalyzed metabolism of (S)-NNN occurred exclusively at the 5'-position. The predominant pathway of (R)-NNN metabolism 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 recent study of (R)- and (S)-NNN metabolism by cultured rat esophagus. In that study, (S)-NNN was metabolized predominantly by 2'-hydroxylation and (R)-NNN equally by 2'- and 5'-hydroxylation. Taken together, these data provide strong evidence that P450 2A3 is not the rat esophageal P450 that catalyzes the metabolic activation of NNN. P450 2A3 may be an important catalyst of NNN activation in rat nasal mucosa.
N'-亚硝基降烟碱（NNN）可诱发大鼠鼻腔和食管肿瘤，被认为是烟草使用者患食管癌的致病因素。为发挥其致癌潜能，NNN必须通过在2'-或5'-碳位的α-羟基化进行代谢激活。我们之前报道过，人类细胞色素P450（P450）2A6能高效且特异性地催化NNN的5'-羟基化。P450 2A3在大鼠鼻腔中表达，在食管中少量表达，与P450 2A6密切相关。P450 2A3与2A6一样，是NNNα-羟基化的良好催化剂（米氏常数7微摩尔；最大反应速度17纳摩尔/分钟/纳摩尔）。然而，与P450 2A6不同的是，2A3催化NNN的5'-和2'-羟基化反应。2'-与5'-羟基化的比例为1:3。这些关于P450 2A6和2A3的数据均使用外消旋NNN获得。P450 2A3催化（S）-NNN的代谢仅发生在5'-位。（R）-NNN代谢的主要途径是2'-羟基化，其发生程度比5'-羟基化高3倍。这些数据与最近一项关于培养的大鼠食管对（R）-和（S）-NNN代谢的研究结果相反。在该研究中，（S）-NNN主要通过2'-羟基化代谢，（R）-NNN通过2'-和5'-羟基化的代谢程度相同。综上所述，这些数据提供了有力证据，表明P450 2A3不是催化NNN代谢激活的大鼠食管P450。P450 2A3可能是大鼠鼻黏膜中NNN激活的重要催化剂。
