Investigation of DNA-protein cross-link formation between lysozyme and oxanine by mass spectrometry.

Reactive nitrogen species are implicated in inflammatory diseases and cancers. Oxanine (Oxa) is a DNA lesion product originating from the guanine base through exposure to nitric oxide, nitrous acid, or N-nitrosoindoles. Oxanine was found to mediate formation of DNA-protein cross-links (DPCs) in the cell extract. We have previously characterized two DNA-protein cross-links from the reaction between Oxa and glutathione: namely, the thioester and the amide. In this study, lysozyme was used to study site-specific modification on protein by Oxa moieties in DNA. With the aid of nanoLC coupled with nanospray ionization tandem mass spectrometry, addition of Oxa was found at Lys13, Lys97, Lys116, Ser85, and Ser86 of lysozyme when it was treated with 2'-deoxyoxanosine (dOxo). Furthermore, incubation of lysozyme with Oxa-containing calf thymus DNA, produced by treating DNA with nitrous acid, led to lysozyme modification at Lys116, Ser85, and Ser86. Interestingly, none of the cysteine residues was modified by dOxo, in contrast with our previous findings that dOxo reacted with oxidized glutathione disulfide, forming the thioester. This might be due to the half-life of the dOxo-derived thioester being 2.2 days at the pH of incubation. Furthermore, the sites of modifications on lysozyme are in good agreement with the solvent accessibility of the residues. Since repair of Oxa-derived DPCs has not been extensively investigated, these results suggest that these stable DPCs might represent important forms of cellular damage caused by reactive nitrogen species involved in inflammationrelated diseases.