New in Asian Journal of Ecotoxicology

Mechanistic Insight into Cytochrome P450‐catalyzed Activation of Tobacco‐specific N'‐Nitrosoanatabine and N'‐Nitrosoanabasine

Tobacco‐specific nitrosamines N'‐nitrosoanatabine (NAT) and N'‐nitrosoanabasine (NAB) are abundantly present in all tobacco products, cigarette smoke (see Figure 1), and even atmospheric particulate matters. The toxification routes of NAT and NAB metabolisms catalyzed by cytochrome P450 enzymes are important triggers for their carcinogenicity; however, the molecular details of respective toxification mechanisms are not well understood until now, which hampers a comprehensive risk assessment of NAT and NAB exposures. In the present investigation, density functional theory (DFT) calculations were performed to unravel the mechanistic details of P450‐catalyzed activations of these 2 contaminants. N'‐nitrosoanatabine and NAB α‐hydroxylations occurring at 2 C_α sites adjacent to nitrosamine group include the following elementary steps: 1) initial H_α atom abstraction (reaction barrier 12.7–21.6 kcal · mol⁻¹) by the highly reactive Fe^IV═O unit to form exothermic and unstable Cα radical intermediate, 2) energy‐free OH rebound from Fe^IV─OH to C_α radical to yield stable α‐hydroxyNAT/α‐hydroxyNAB intermediate, and 3) spontaneous intramolecular decomposition of α‐hydroxynitrosamine (barrier 14.1–20.1 kcal · mol⁻¹), generating reactive diazohydroxides as potential genotoxic metabolites to alkylate DNA. N'‐nitrosoanatabine and NAB have similar structural features, and correspondingly show comparable α‐hydroxylation routes. N'‐nitrosoanatabine 2‐hydroxylation shows higher H atom abstraction barrier but lower decomposition barrier than 6‐hydroxylation, suggesting that these 2 routes are kinetically feasible for NAT metabolism. In contrast, 2'‐hydroxylation is likely to dominate the metabolic fate of NAB. Moreover, the results further suggest that NAB may have slightly higher carcinogenic potential than NAT for the P450‐catalyzed metabolism. Overall, the calculation results can provide a comprehensive understanding of metabolic activations of NAT and NAB, and further enable the identification of mechanistic‐specific metabolites and biomarkers for assessing the health risk of both NAT and NAB exposures.

• Ni JX, Xu T, Wen JL, Xu XQ, Yu HY, Wei XX, Ma GC. 2019. Mechanistic insight into cytochrome P450‐catalyzed activation of tobacco‐specific N'‐nitrosoanatabine and N'‐nitrosoanabasine. Asian Journal of Ecotoxicology 14. Available from: http://www.stdlxb.cn/en/article/doi/10.7524/AJE.1673-5897.20190307003

中文翻译：

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新的亚洲生态毒理学杂志的

对细胞色素P450催化的烟草特异性N'-亚硝胺和N'-亚硝胺的活化的机理研究

烟草特有的亚硝胺N'-亚硝基天南碱（NAT）和N'-亚硝基天冬氨酸（NAB）广泛存在于所有烟草制品，卷烟烟雾（见图1），甚至大气颗粒物中。细胞色素P450酶催化的NAT和NAB代谢的毒化途径是其致癌性的重要诱因。然而，到目前为止，有关毒理作用机制的分子细节还不为人所知，这妨碍了对NAT和NAB暴露的全面风险评估。在本研究中，进行了密度泛函理论（DFT）计算，以揭示这两种污染物的P450催化活化机理细节。N'-亚硝基安他他汀和NABα-羟基化发生在2 C_α相邻亚硝胺组位点包括以下基本步骤：1）初始ħ _α原子抽象（反应阻挡12.7-21.6千卡·摩尔^-1）由高反应性的Fe ^IV = O单元以形成放热和不稳定Cα自由基中间体，2- ）自由能-选自Fe OH反弹^IV ─OH至C _α自由基，得到稳定的α -hydroxyNAT / α -hydroxyNAB中间，以及3）α-hydroxynitrosamine的自发分子内分解（阻挡14.1-20.1千卡·摩尔^-1），生成反应性重氮氢氧化物作为潜在的遗传毒性代谢产物使DNA烷基化。ñ'-亚硝基anatabine和NAB具有相似的结构特征，并相应显示出可比的α-羟基化途径。ñ'-亚硝基anatabine 2-羟基化显示出比6-羟基化更高的H原子提取屏障，但具有更低的分解屏障，表明这2条途径对于NAT代谢在动力学上是可行的。相比之下，2'-羟基化很可能主导着NAB的代谢命运。此外，结果进一步表明，NAP在P450催化的代谢中可能具有比NAT略高的致癌潜力。总体而言，计算结果可以全面了解NAT和NAB的代谢活化，并进一步鉴定机制特异性代谢物和生物标志物，以评估NAT和NAB暴露对健康的危害。

• 倪建勋，徐婷，温建霖，徐新强，于慧云，魏XX，马国强 2019.对细胞色素P450催化的烟草特有N'-亚硝基天南星和N'-亚硝基天南星的活化的机理研究。亚洲生态毒理学杂志14.可从以下网站获得：http://www.stdlxb.cn/en/article/doi/10.7524/AJE.1673-5897.20190307003