Jalas John R, Ding Xinxin, Murphy Sharon E
Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA.
Drug Metab Dispos. 2003 Oct;31(10):1199-202. doi: 10.1124/dmd.31.10.1199.
The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and its carbonyl-reduction product, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), are potent lung carcinogens in rats and are presumed human lung carcinogens. NNK and NNAL are bioactivated to DNA-binding intermediates via hydroxylation of the carbon atoms adjacent to the nitroso moiety (i.e., alpha-hydroxylation) by cytochrome p450s (p450s). Therefore, it is important to delineate which p450s are efficient catalysts of this metabolic transformation. In this study, the kinetic parameters for NNK and NNAL metabolism were determined for two extrahepatic p450s that are expressed in the lung: rat p450 2A3 and human p450 2A13. p450s 2A3 and 2A13 exhibited Vmax values for NNK 4-hydroxylation of 10.8 +/- 0.4 and 13.8 +/- 0.8 pmol min-1 pmol P450-1, respectively; the corresponding Km values were 4.6 +/- 0.5 and 3.6 +/- 0.7 microM. The respective Vmax values for p450 2A3- and 2A13-mediated N-methyl hydroxylation of NNK were 8.2 +/- 0.3 and 4.6 +/- 0.2 pmol min-1 pmol p450-1. These data indicate that p450s 2A3 and 2A13 are both efficient catalysts of the metabolic activation of NNK and are, along with mouse p450 2A5, the best catalysts of this reaction currently known. Both enzymes also catalyzed the alpha-hydroxylation and N-oxidation of NNAL, and its oxidation to NNK. In general, Vmax/Km values for NNAL metabolism were 1 to 2 orders of magnitude lower than those for NNK metabolism, and p450 2A3 was a slightly better catalyst of NNAL metabolism than was p450 2A13. Given the exquisite sensitivity of the rat lung to NNK-induced carcinogenesis, the efficient bioactivation of NNK by rat p450 2A3, and the similar catalytic efficiency of p450s 2A3 and 2A13, p450 2A13 may be an important contributor to NNK bioactivation in the human lung.
烟草特异性亚硝胺4-（甲基亚硝胺基）-1-（3-吡啶基）-1-丁酮（NNK）及其羰基还原产物4-（甲基亚硝胺基）-1-（3-吡啶基）-1-丁醇（NNAL）是大鼠体内强效的肺癌致癌物，并且被认为是人类肺癌致癌物。NNK和NNAL通过细胞色素P450（P450s）对与亚硝基部分相邻的碳原子进行羟基化作用（即α-羟基化）而被生物活化为与DNA结合的中间体。因此，明确哪些P450s是这种代谢转化的有效催化剂很重要。在本研究中，测定了在肺中表达的两种肝外P450s（大鼠P450 2A3和人类P450 2A13）对NNK和NNAL代谢的动力学参数。P450 2A3和2A13对NNK 进行4-羟基化反应的Vmax值分别为10.8±0.4和13.8±0.8 pmol min-1 pmol P450-1；相应的Km值分别为4.6±0.5和3.6±0.7 μM。P450 2A3和2A13介导的NNK的N-甲基羟基化反应的各自Vmax值分别为8.2±0.3和4.6±0.2 pmol min-1 pmol P450-1。这些数据表明，P450 2A3和2A13都是NNK代谢活化的有效催化剂，并且与小鼠P450 2A5一样，是目前已知的该反应的最佳催化剂。这两种酶还催化了NNAL的α-羟基化和N-氧化反应，以及其氧化为NNK的反应。一般来说，NNAL代谢的Vmax/Km值比NNK代谢的Vmax/Km值低1至2个数量级，并且P450 2A3作为NNAL代谢的催化剂比P450 2A13略好。鉴于大鼠肺对NNK诱导的致癌作用极为敏感，大鼠P450 2A3对NNK的有效生物活化作用，以及P450 2A3和2A13相似的催化效率，P450 2A13可能是人类肺中NNK生物活化作用的重要促成因素。
