Figure 1. The two proposed sulfide oxidation models.

Abbreviations: SQR, sulfide: quinine oxidoreductase; ST, sulfur transferase; SDO, sulfur dioxygenase; GS⁻, glutathione; GSS⁻, glutathione persulfide. Model A: The sulfide can be converted to GSS⁻ by SQR, and the sulfane sulfur of GSS⁻ is oxidized to sulfite using O₂ by SDO in the mitochondrial-matrix; finally, the generated sulfite is converted by ST catalysis to thiosulfate, which is less toxic to the organism. Model B: Thiosulfate biosynthesis occurs in the first step of sulfide oxidation catalyzed by SQR with sulfite as the acceptor of the sulfane sulfur, then thiosulfate can act as the ST substrate to produce GSS⁻ and regenerate the sulfite; finally, the sulfane sulfur of GSS⁻ is oxidized to sulfite using O₂ by SDO. The newly generated sulfite could then enter the cycle again.