Reaction of NO with Nucleic Acid Bases and its Biological Implication

Chronic inflammation is a risk factor for many human cancers, and nitric oxide (NO) produced in inflamed tissues has been proposed to cause DNA damage via nitrosation or oxidation of base moieties. Thus, NO-induced DNA damage could be relevant to carcinogenesis associated with chronic inflammation. We have explored, therefore, DNA damage caused by NO (or slightly acidic HNO2). Before our study, only oxidative deamination was established as a major pathway to convert dGuo to dXao, dAdo to dIno and dCyd to dUrd. In our study, another major pathway initiated by the attack to the exo-amino groups has been demonstrated. For dGuo, 2'-deoxyoxanosine (dOxo) production through the dGuodiazoate (intermediate) formation has been determined: The dOxo yield is 1/3 of that for dXao, and the glycosylic bond is as stable as that of dGuo. DNA polymerases recognize dOxo as both dGuo and dAdo, indicative of G:C to A:T conversion. Also it has been found that both dOxo and the intermediate show high reactivity with amino groups, and that a stable diazoate with the similar high reactivity is produced from dCyd. So we have investigated DNA-protein crosslinks (DPCs) induced by dOxo. When a DNA duplex containing dOxo at the sitespecific position was incubated with DNA-binding proteins such as histone, high mobility group (HMG) protein, and DNA glycosylases, DPCs were formed between dOxo and protein. A HeLa cell extract also gave rise to two major DPCs when incubated with DNA-containing dOxo. These results reveal a dual aspect of Oxa as causal damage of DPC formation and as a suicide substrate of DNA repair enzymes, both of which could pose a threat to the genetic and structural integrity of DNA, hence potentially leading to carcinogenesis.