In living tissues under inflammatory conditions, reactive oxygen and nitrogen species are known to cause the oxidation and nitration of cellular DNA. However, the mechanisms of action are poorly understood. We explored the oxidation and nitration reactions in DNA associated with the combination of nitrogen dioxide radicals with 8-oxo-7,8-dihydroguanine (8-oxoGua) and guanine radicals by kinetic laser spectroscopy and mass-spectrometry methods. The oxidation/nitration processes were triggered by photonionization of 2-aminopurine (2AP) by a two-photon absorption mechanism induced by intense 308 nm excimer laser pulses. The 2AP residues were site-specifically positioned in 2’-deoxyribooligonucleotides-containing single 8-oxoGua separated from the 2AP by a TC dinucleotide step. The primary photoionization products, the 2AP radicals, rapidly oxidize either 8-oxoGua or G within the same oligonucleotide strand by a one-electron transfer mechanism. The photoionization of the 2AP residue also generates hydrated electrons that are trapped by nitrate anions thus forming nitrogen dioxide radicals. The bimolecular combination reaction of nitrogen dioxide radicals with the 8-oxoGua and G radicals occurs with similar rate constants in both single- and double-stranded DNA. In the case of 8-oxoGua, the major end-products of this bimolecular radical-radical addition are spiroiminodihydantoin lesions, the products of 8-oxoGua oxidation. Oxygen-18 isotope labeling experiments reveal that the O-atom in the spiroiminodihydantoin lesion originates from water molecules, not from nitrogen dioxide radicals. In contrast, the combination of nitrogen dioxide and guanine neutral radicals generated under the same conditions results in the formation of the nitro products, 5-guanidino-4-nitroimidazole and 8-nitroguanine adducts. The mechanistic aspects of the oxidation/nitration processes and their biological implications are discussed. Supported by NIH Grant 5 R01 ES11589.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]