Oxidation Reactions of Cytosine DNA Components by Hydroxyl Radical and One-Electron Oxidants in Aerated Aqueous Solutions

摘要

Inndirect evidence strongly suggests that oxidation reac-ntions of cytosine and its minor derivative 5-methylcy-ntosine play a major role in mutagenesis and cancer.nTherefore, there is an emerging necessity to identify thenfinal oxidation products of these reactions, to search forntheir formation in cellular DNA, and to assess theirnmutagenic features. In this Account, we report and dis-ncuss the main OH and one-electron-mediated oxida-ntion reactions, two of the most potent sources of DNAndamage, of cytosine and 5-methylcytosine nucleosidesnthat have been recently characterized. The addition ofnOH to the 5,6-unsaturated double bond of cytosine andn5-methylcytosine generates final degradation products that resemble those observed for uracil and thymine. The main productnfrom the oxidation of cytosine, cytosine glycol, has been shown to undergo dehydration at a much faster rate as a free nucleo-nside than when inserted into double-stranded DNA. On the other hand, the predominant OH addition at C5 of cytosine or 5-me-nthylcytosine leads to the formation of 5-hydroxy-5,6-dihydro radicals that give rise to novel products with an imidazolidine structure.nThe mechanism of the formation of imidazolidine products is accounted for by rearrangement reactions that in the presence ofnmolecular oxygen likely involve an intermediate pyrimidine endoperoxide. The reactions of the radical cations of cytosine and 5-me-nthylcytosine are governed by competitive hydration, mainly at C6 of the pyrimidine ring, and deprotonation from the exocyclicnamino and methyl group, leading in most cases to products similar to those generated by OH. 5-Hydroxypyrimidines, the dehy-ndration products of cytosine and uracil glycols, have a low oxidation potential, and their one-electron oxidation results in a cas-ncade of decomposition reactions involving the formation of isodialuric acid, dialuric acid, 5-hydroxyhydantoin, and its hydroxyketonenisomer. In biology, GC f AT transitions are the most common mutations in the genome of aerobic organisms, including the lacIngene in bacteria, lacI transgenes in rodents, and the HPRT gene in rodents and humans, so a more complete understanding ofncytosine oxidation is an essential research goal. The data and insights presented here shed new light on oxidation reactions ofncytosine and 5-methylcytosine and should facilitate their validation in cellular DNA.