Oxidative damage to guanine in DNA caused by reactive oxygen species

Oxidative damage to DNA, a factor in cancer, mutation, and aging, is attributed to reactive oxygen species (ROS). The less well characterized ROS, organic peroxyl radicals and peracid are present during lipid peroxidation and also produced by peroxidases from organic hydroperoxides. Peracetic acid is also formed in mitochondria. Guanine (Gua) is the nucleobases most susceptible to oxidation due to its lowest electron potential. The study described here focuses on Gua oxidation by epoxidizing reagents including peroxyl radicals and organic peracids. Dimethyldioxirane (DMDO), peracetic acid and m-chloroperbenzoic acid selectively oxidizes the guanine moiety of dGuo, dGMP and dGTP to 5-carboxamido-5-formamido-2-iminohydantoin (2-Ih). Structures were established on mass spectrometry and NMR studies. Labeling studies support a mechanism involving initial epoxidation of the guanine 4-5 bond and contraction of the pyrimidine ring by a 1,2-migration of the guanine carbonyl C6 to form a transient dehydrodeoxyspiroiminodihydantoin followed by hydrolytic ring opening of the imidazolone ring. The 2-Ih is shown to be a major transformation in the oxidation of the single-stranded DNA 5-mer d(TTGTT) and the 5-base pair duplex d[( TTGTT)˙(AACAA)]. 2-Ih has not previously been reported as an oxidative lesion in DNA. Consistent with the proposed mechanism, no 8-oxoguanine was detected as a product of the oxidations of the oligonucleotides or monomeric species mediated by the monooxygen donors. The 2-Ih base thus appears to be a pathway-specific lesion and holds promise as a potential biomarker.;N9-(beta-D-2-deoxyribofuranosyl)-N 2,3-ethenoguanine is a highly mutagenic DNA adduct arising from exposure to known occupational and environmental carcinogens and lipid peroxidation products in vivo. Chemical synthesis has proven to be challenging because of the reported lability of the glycosidic bond under conditions generally applicable to chemical synthesis. Enzymatic and chemical glycosylations of N2,3-ethenoguanine were attempted as approaches to obtain this nucleoside under mild conditions. Both glycosylations led to nucleosides with ribosylation at positions corresponding to N7- and N2 of the Gua framework. A minor product of the enzymatic ribosylation has tentatively been assigned as the alpha-anomer of the desired N3 riboside, and rigorous confirmation of this structure would demonstrate an unusual stereochemistry for the trans ribosylation.