Unexpected strong and long-lasting hydroxyl radical production by pyridine disulfides with H2O2 and remarkable enhancement by physiological reducing agents.

2,2'-Dithiodipyridine (PySSPy) is a typical pyridine disulfide, which has been widely used in industry and biomedical research, and frequently identified as the photo-degradation product of ubiquitous metal pyrithiones. Although many disulfides can moderately react with physiological oxidants such as H2O2, it was rarely reported that the highly-reactive hydroxyl radicals (•OH) could be produced from the reaction of disulfides with H2O2. Here we unexpectedly found that PySSPy can readily react with H2O2 over a wide pH range to continuously produce •OH and lead to appreciable DNA damage. Compared to other well-known •OH-generating system, PySSPy/H2O2 produced •OH with a longer duration and higher •OH yield, although relatively slower. Interestingly, the addition of physiological thiol compounds such as glutathione and cysteine, as well as the classic reducing agent ascorbic acid, significantly accelerated and enhanced •OH production of PySSPy/H2O2, which was found to be mainly due to their reduction of PySSPy to 2-mercaptopyridine (PySH, the reduced monomer of PySSPy). Further studies found there is a correlation between product generation and •OH formation, suggesting that the unexpected PySSPy/H2O2 system may have multiple •OH-generating pathways: besides the unexpected sulfenic acid-dependent pathway similar to the PySH/H2O2 system, a direct nucleophilic reaction pathway between PySSPy and H2O2 to generate the reactive thiosulfinate and thiosulfonate, should make major contributions to •OH production. This represents the first report of a previously-unknown chemical property for •OH generation by the cell-permeable thiol oxidant PySSPy, which should be of great significance for future biomedical and environmental research on PySSPy and its ubiquitous precursors of metal pyrithiones.