Fluorescent probes for hydrogen sulfide (H₂S) and sulfane sulfur and their applications to biological studies

Hydrogen sulfide (H₂S), a toxic gas with the smell of rotten eggs, plays key roles in many physiological processes, including relaxation of vascular smooth muscles, mediation of neurotransmission, inhibition of insulin signaling, and regulation of inflammation. The most commonly used methods or detecting H₂S are the methylene blue method and the electrode method, but these methods require destructive sampling, e.g., homogenization of biological samples. On the other hand, the fluorescence detection method has been widely used in biological studies to study the physiological roles of H₂S, because this technology provides real-time, easy-to-use, nondestructive detection in live cells or tissues. Many selective fluorescent probes for H₂S have been reported. Sulfane sulfur compounds contain divalent sulfur atoms bonded to other sulfur atom(s), as in persulfides (RSSH) and polysulfides (RSS_nSR). They are currently attracting increasing interest because one of the mechanisms of activity regulation of proteins by H₂S is sulfhydration of cysteine residues (RSH→RSSH). Since H₂S and sulfane sulfur are redox partners, they are very likely to coexist in biological systems, and from a reactivity point-of-view, sulfane sulfur seems likely to be much more effective than H₂S in S-sulfhydration. Therefore, sulfane sulfur may be involved in mediating at least some of the biological activities of H₂S. In this review, we summarize recent work on fluorescent probes selective for H₂S and/or sulfane sulfur, and we briefly review their applications to biological studies.