Role of Nitrite on Nitration of 2′-Deoxyguanosine by Nitryl Chloride

Biological and dietary antioxidants protect against DNA nitration induced by reaction of hypochlorous acid with nitrite

Nitryl chloride, formed by reaction of hypochlorous acid with nitrite, might contribute to nitrative damage of biomolecules in addition to peroxynitrite. Damage of DNA by these reactive nitrogen oxide species is implicated in carcinogenesis associated with chronic infections and inflammation. Nitrated DNA adducts, such as 8-nitroguanine and 8-nitroxanthine, are not stable in DNA since they undergo spontaneous depurination, leading to apurinic site formation. In this report, we investigate the protective effect of biological and dietary antioxidants in inhibiting DNA nitration induced by nitryl chloride. The effect of inhibition was evaluated by decrease of 8-nitroxanthine and 8-nitroguanine formation. Among the 21 compounds examined, dihydrolipoic acid is the most effective in preventing DNA nitration, followed by N-acetyl-L-cysteine and folic acid. For sulfur-containing compounds, the more highly reduced compounds are stronger inhibitors of DNA nitration. The major product of N-acetyl-L-cysteine reaction with nitryl chloride is characterized as the (R)-2-acetylamino-3-sulfopropionic acid, a physiologically irreversible product, suggesting that nitryl chloride is a strong oxidizing agent.

Hemoprotein-mediated reduction of nitrated DNA bases in the presence of reducing agents

DNA damages by reactive nitrogen oxide species may contribute to the multistage carcinogenesis processes associated with chronic infections and inflammation. The nitrated DNA adducts 8-nitroguanine (8NG) and 8-nitroxanthine (8NX) have been shown to derive from these reactive nitrogen oxide species, but they are not stable in DNA since they undergo spontaneous depurination. We herein report that hemin and hemoproteins, including hemoglobin and cytochrome c, mediate reduction of 8NG and 8NX to their corresponding amino analogues in the presence of reducing agents under physiologically relevant conditions. This reaction is believed to involve the reduced heme moiety produced from the reduction of oxidized hemoglobin or cytochrome c by reducing agents. The combination of hemoglobin and dihydrolipoic acid generated the reduced products in high yields. Ascorbate, quercetin, and glutathione are also capable of reducing these nitrated DNA adducts. The hemoglobin macromolecule reduces 8NG and 8NX formed in nitryl chloride-treated calf thymus DNA, as evidenced by the formation of the amino adducts using reversed-phase HPLC with photodiode array detection. Hemin is more efficient than equal molar of heme on hemoglobin in reducing 8NG-containing DNA, indicating the role of protein in impeding the reaction. Furthermore, we also show that the reduction product 8-aminoguanine is persistent on DNA. These findings suggest that reduction of nitrated DNA by the heme/antioxidant system might represent a possible in vivo pathway to modify DNA nitration.