Hecht S S, Hochalter J B, Villalta P W, Murphy S E
University of Minnesota Cancer Center, Minneapolis, MN 55455, USA.
Proc Natl Acad Sci U S A. 2000 Nov 7;97(23):12493-7. doi: 10.1073/pnas.220207697.
Smokers or people undergoing nicotine replacement therapy excrete approximately 10% of the nicotine dose as 4-oxo-4-(3-pyridyl)butanoic acid (keto acid) and 4-hydroxy-4-(3-pyridyl)butanoic acid (hydroxy acid). Previously, these acids were thought to arise by secondary metabolism of the major nicotine metabolite cotinine, but our data did not support this mechanism. Therefore, we hypothesized that nicotine is metabolized by 2'-hydroxylation, which would ultimately yield keto acid and hydroxy acid as urinary metabolites. This pathway had not been established previously in mammalian systems and is potentially significant because the product of nicotine 2'-hydroxylation, 4-(methylamino)-1-(3-pyridyl)-1-butanone (aminoketone), can be converted to the potent tobacco-specific lung carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. Incubation of nicotine with cytochrome P450 2A6 and cofactors did indeed produce aminoketone, which was identified as its N-benzoyl derivative by GC-MS. The rate was 11% of that of cotinine production. Incubation of human liver microsomes with nicotine gave keto acid by using aminoketone as an intermediate; keto acid was not formed from cotinine. In 10 human liver samples, rates of formation of keto acid were 5.7% of those of cotinine and production of these metabolites correlated. These results provide definitive evidence for mammalian 2'-hydroxylation of nicotine and elucidate a pathway by which endogenous formation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone could occur in humans.
吸烟者或接受尼古丁替代疗法的人会将约10%的尼古丁剂量以4-氧代-4-(3-吡啶基)丁酸（酮酸）和4-羟基-4-(3-吡啶基)丁酸（羟基酸）的形式排泄出来。此前，人们认为这些酸是主要尼古丁代谢物可替宁的次生代谢产物，但我们的数据并不支持这一机制。因此，我们推测尼古丁通过2'-羟基化进行代谢，最终会产生酮酸和羟基酸作为尿液代谢物。这条途径此前在哺乳动物系统中尚未得到证实，且可能具有重要意义，因为尼古丁2'-羟基化的产物4-(甲氨基)-1-(3-吡啶基)-1-丁酮（氨基酮）可转化为强效的烟草特异性肺致癌物4-(甲基亚硝基氨基)-1-(3-吡啶基)-1-丁酮。将尼古丁与细胞色素P450 2A6及辅因子一起孵育确实产生了氨基酮，通过气相色谱-质谱联用仪将其鉴定为N-苯甲酰基衍生物。其生成速率为可替宁生成速率的11%。用尼古丁与人肝微粒体一起孵育，以氨基酮作为中间体产生了酮酸；可替宁并未生成酮酸。在10份人肝样本中，酮酸的生成速率为可替宁生成速率的5.7%，且这些代谢物的生成具有相关性。这些结果为尼古丁在哺乳动物体内的2'-羟基化提供了确凿证据，并阐明了人体内可能发生内源性生成4-(甲基亚硝基氨基)-1-(3-吡啶基)-1-丁酮的途径。
