Department of Nuclear Medicine and Radiobiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, Québec J1H 5N4, Canada.
Acc Chem Res. 2010 Apr 20;43(4):564-71. doi: 10.1021/ar9002637.
Indirect evidence strongly suggests that oxidation reactions of cytosine and its minor derivative 5-methylcytosine play a major role in mutagenesis and cancer. Therefore, there is an emerging necessity to identify the final oxidation products of these reactions, to search for their formation in cellular DNA, and to assess their mutagenic features. In this Account, we report and discuss the main *OH and one-electron-mediated oxidation reactions, two of the most potent sources of DNA damage, of cytosine and 5-methylcytosine nucleosides that have been recently characterized. The addition of *OH to the 5,6-unsaturated double bond of cytosine and 5-methylcytosine generates final degradation products that resemble those observed for uracil and thymine. The main product from the oxidation of cytosine, cytosine glycol, has been shown to undergo dehydration at a much faster rate as a free nucleoside than when inserted into double-stranded DNA. On the other hand, the predominant *OH addition at C5 of cytosine or 5-methylcytosine leads to the formation of 5-hydroxy-5,6-dihydro radicals that give rise to novel products with an imidazolidine structure. The mechanism of the formation of imidazolidine products is accounted for by rearrangement reactions that in the presence of molecular oxygen likely involve an intermediate pyrimidine endoperoxide. The reactions of the radical cations of cytosine and 5-methylcytosine are governed by competitive hydration, mainly at C6 of the pyrimidine ring, and deprotonation from the exocyclic amino and methyl group, leading in most cases to products similar to those generated by *OH. 5-Hydroxypyrimidines, the dehydration products of cytosine and uracil glycols, have a low oxidation potential, and their one-electron oxidation results in a cascade of decomposition reactions involving the formation of isodialuric acid, dialuric acid, 5-hydroxyhydantoin, and its hydroxyketone isomer. In biology, GC --> AT transitions are the most common mutations in the genome of aerobic organisms, including the lacI gene in bacteria, lacI transgenes in rodents, and the HPRT gene in rodents and humans, so a more complete understanding of cytosine oxidation is an essential research goal. The data and insights presented here shed new light on oxidation reactions of cytosine and 5-methylcytosine and should facilitate their validation in cellular DNA.
间接证据强烈表明，胞嘧啶及其微量衍生物 5-甲基胞嘧啶的氧化反应在突变和癌症中起着重要作用。因此，需要识别这些反应的最终氧化产物，寻找其在细胞 DNA 中的形成，并评估其诱变特征。在本报告中，我们报告并讨论了最近表征的胞嘧啶和 5-甲基胞嘧啶核苷的两种最有效的 DNA 损伤源——羟基自由基和单电子介导的氧化反应。羟基自由基加成到胞嘧啶和 5-甲基胞嘧啶的 5,6-不饱和双键上，生成与尿嘧啶和胸腺嘧啶相似的最终降解产物。已证明胞嘧啶氧化的主要产物胞嘧啶二醇作为游离核苷比插入双链 DNA 时更快地进行脱水。另一方面，胞嘧啶或 5-甲基胞嘧啶的 C5 上的主要羟基自由基加成导致形成 5-羟基-5,6-二氢自由基，从而产生具有咪唑烷结构的新型产物。咪唑烷产物的形成机制可归因于重排反应，在分子氧的存在下，该反应可能涉及中间嘧啶内过氧化物。胞嘧啶和 5-甲基胞嘧啶自由基阳离子的反应受竞争水合的控制，主要在嘧啶环的 C6 上，并从环外氨基和甲基基团脱质子化，导致大多数情况下生成与羟基自由基类似的产物。5-羟嘧啶，胞嘧啶和尿嘧啶二醇的脱水产物，具有低氧化电位，其单电子氧化导致一系列分解反应，包括异二脲酸、二脲酸、5-羟基尿嘧啶和其羟基酮异构体的形成。在生物学中，GC→AT 转换是需氧生物基因组中最常见的突变，包括细菌中的 lacI 基因、啮齿动物中的 lacI 转基因和啮齿动物和人类中的 HPRT 基因，因此更全面地了解胞嘧啶氧化是一个重要的研究目标。这里呈现的数据和见解为胞嘧啶和 5-甲基胞嘧啶的氧化反应提供了新的认识，并应有助于在细胞 DNA 中验证它们。
