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Many physiological functions of hydrogen sulfide (H2S) have been reported in mammalian cells over the last 20 years. These physiological effects have been ascertained through in vitro treatment of cells with Na2S or NaHS, both of which are precursors of H2S. Since H2S exists as HS− in a neutral solution, a disulfide compound such as cystine could react with HS− in culture medium as well as in the cell.This study demonstrated that after the addition of Na2S solution into culture medium, HS− was transiently generated and disappeared immediately through the reaction between HS− and cystine to form cysteine persulfides and polysulfides in the culture medium (bound sulfur mixture: BS-Mix). Furthermore, we found that the addition of Na2S solution resulted in an increase of intracellular cysteine persulfide levels in SH-SY5Y cells. This alteration in intracellular persulfide was also observed in cystine-free medium.Considering this reaction of HS− as a precursor of BS-Mix, we highlighted the cytoprotective effect of Na2S on human neuroblastoma SH-SY5Y cells against methylglyoxal (MG)-induced toxicity. BS-Mix produced with Na2S in cystine-containing medium provided SH-SY5Y cells significant protective effect against MG-induced toxicity. However, the protective effect was attenuated in cystine-free medium. Moreover, we observed that Na2S or BS-Mix activated the Keap1/Nrf2 system and increased glutathione (GSH) levels in the cell. In addition, the activation of Nrf2 is significantly attenuated in cystine-free medium.These results suggested that Na2S protects SH-SY5Y cells from MG cytotoxicity through the activation of Nrf2, mediated by cysteine persulfides and polysulfides that were generated by Na2S addition.

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