Heme catalyzed synthesis of thiotaurine from hypotaurine and hydrogen sulfide: a possible novel biochemical pathway

Sulfane sulfur is a form of highly reactive sulfur species (RSS) with six valence electrons, no charge, also known as zero-valent sulfur, and the unique ability to attach reversibly to other sulfur atoms such as protein thiols. Noteworthy, biological effects initially attributed to hydrogen sulfide actually are caused by sulfane sulfur compounds. The transsulfuration pathway is the main source of sulfane sulfur-containing compounds. The enzymes involved in the H2S/sulfane sulfur production include PLP-dependent cystathionineγ-lyase (CSE) and cystathionine β-synthase (CBS) as well as cysteine aminotransferase (CAT) in conjunction with PLP-independent mercaptopyruvate sulfurtransferase (MST). However, the possibility that sulfane sulfur species are signaling agents greatly increases the possible number of generating systems beyond transsulfuration path. At this regard, H2S oxidation pathways can generate a series of reactive sulfur species, including sulfane sulfur species Recently, thiotaurine, a possible novel sulfane sulfur metabolite, has been investigated as an inflammation modulating agent as a result of its ability to release H2S. Thiotaurine can be produced by different pathways, such as the spontaneous transsulfuration between thiocysteine and hypotaurine as well as in vivo from cystine. Moreover, thiotaurine can be generated by the transfer of sulfur from mercaptopyruvate to hypotaurine catalyzed by a sulfurtransferase. In this work, we observed for the first time the generation of thiotaurine from hypotaurine and hydrogen sulfide by horseradish peroxidase. The interaction of heme proteins with hydrogen sulfide is gaining attention as an important element in the sulfide-mediated protection against oxidative stress and in regulation of red/ox signaling. Interestingly, heme-dependent sulfide oxidation pathways reveal to be important sources of reactive sulfur species, including persulfides, polysulfides and thiosulfonates, such as thiotaurine