Spontaneous and oxidant-induced damage to cytosine is probably the main cause of CG to TA transition mutations in mammalian genomes. The reaction of hydroxyl radical (·OH) and one-electron oxidants with cytosine derivatives produces numerous oxidation products, which have been identified in large part by model studies with monomers and short oligonucleotides. Here, we developed an analytical method based on LC-MS/MS to detect 10 oxidized bases in DNA, including 5 oxidation products of cytosine. The utility of this method is demonstrated by the measurement of base damage in isolated calf thymus DNA exposed to ionizing radiation in aerated aqueous solutions (0-200 Gy) and to well-known Fenton-like reactions (Fe(2+) or Cu(+) with H(2)O(2) and ascorbate). The following cytosine modifications were quantified as modified 2'-deoxyribonucleosides upon exposure of DNA to ionizing radiation in aqueous aerated solution: 5-hydroxyhydantoin (Hyd-Ura) > 5-hydroxyuracil (5-OHUra) > 5-hydroxycytosine (5-OHCyt) > 5,6-dihydroxy-5,6-dihydrouracil (Ura-Gly) > 1-carbamoyl-4,5-dihydroxy-2-oxoimidazolidine (Imid-Cyt). The total yield of cytosine oxidation products was comparable to that of thymine oxidation products (5,6-dihydroxy-5,6-dihydrothymine (Thy-Gly), 5-hydroxy-5-methylhydantotin (Hyd-Thy), 5-(hydroxymethyl)uracil (5-HmUra), and 5-formyluracil (5-ForUra)) as well as the yield of 8-oxo-7,8-dihydroguanine (8-oxoGua). The major oxidation product of cytosine in DNA was Hyd-Ura. In contrast, the formation of Imid-Cyt was a minor pathway of DNA damage, although it is the major product arising from irradiation of the monomers, cytosine, and 2'-deoxycytidine. The reaction of Fenton-like reagents with DNA gave a different distribution of cytosine derived products compared to ionizing radiation, which likely reflects the reaction of metal ions with intermediate peroxyl radicals or hydroperoxides. The analysis of the main cytosine oxidation products will help elucidate the complex mechanism of oxidative degradation of cytosine in DNA and probe the consequences of these reactions in biology and medicine.

Nitrogen-centered radicals are major species generated by the addition of hydroxyl radicals and the one-electron oxidation of adenine derivatives. Aminyl radicals are also generated in the decomposition of adenine chloramines upon reaction of hypochlorite. Here, we report the photochemistry of modified 2'-deoxyadenosine (dAdo) containing photoactive hydrazone substituents as a model to investigate the chemistry of dAdo N(6)-aminyl radicals. Derivatives of dAdo containing a phenylhydrazone moiety at N6 displayed UV absorption between 300 and 400 nm. Upon UV photolysis in the presence of a H-donor, that is, glutathione, two major products were formed, dAdo and benzaldehyde, indicating efficient homolytic cleavage to dAdo N(6)-aminyl radicals and benzylidene iminyl radicals. dAdo N(6)-phenylhydrazone was photolyzed in the presence of a molar excess of nonmodified dAdo to mimic the reactions taking place in DNA, and the major photoproducts were identified by high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance. The formation of 2-(benzylideneamino)-2'-deoxyadenosine as well as a more extensive oxidation product may be explained by the recombination of initial dAdo N(6)-aminyl and benzylidene iminyl radicals. The formation of 2'-deoxyinosine may be explained by hydrolytic deamination of dAdo N(6)-aminyl radicals. Interestingly, a dimeric product containing two dAdo moieties was identified in the photolysis mixture. The present studies demonstrate the ability of dAdo N(6)-aminyl radicals to undergo H-abstraction to give dAdo, deamination to give 2'-deoxyinosine, and addition to the adenine moiety to give dimers.