Hydroxyl Radical-Induced Oxidation of a Phenolic C-Linked 2′-Deoxyguanosine Adduct Yields a Reactive Catechol

Phenolic toxins stimulate oxidative stress and generate C-linked adducts at the C8-site of 2′-deoxyguanosine (dG). We previously reported that the C-linked adduct 8-(4″-hydroxyphenyl)-dG (<i>p</i>-PhOH-dG) undergoes oxidation in the presence of Na₂IrCl₆ or horseradish peroxidase (HRP)/H₂O₂ to generate polymeric adducts through phenoxyl radical production [Weishar (2008) Org. Lett. 10, 1839−1842]. We now report on reaction of <i>p</i>-PhOH-dG with two radical-generating systems, Cu^II/H₂O₂ or Fe^II-EDTA/H₂O₂, which were utilized to study the fate of the C-linked adduct in the presence of hydroxyl radical (HO^•). The radical-generating systems facilitate (i) hydroxylation of the phenolic ring to afford the catechol adduct 8-(3″,4″-dihydroxyphenyl)-dG (3″,4″-DHPh-dG) and (ii) H-atom abstraction from the sugar moiety to generate the deglycosylated base <i>p</i>-PhOH-G. The ratios of 3″,4″-DHPh-dG to <i>p</i>-PhOH-G were ∼1 for Cu^II/H₂O₂ and ∼0.13 for Fe^II-EDTA/H₂O₂. The formation of 3″,4″-DHPh-dG was found to have important consequences in terms of reactivity. The catechol adduct has a lower oxidation potential than <i>p</i>-PhOH-dG and is sensitive to aqueous basic media, undergoing decomposition to generate a dicarboxylic acid derivative. In the presence of excess <i>N</i>-acetylcysteine (NAC), oxidation of 3″,4″-DHPh-dG produced mono-NAC and di-NAC conjugates. Our results imply that secondary oxidative pathways of phenolic-dG lesions are likely to contribute to toxicity.