Actual substrate for elemental sulfur oxidation by sulfur:ferric ion oxidoreductase purified from Thiobacillus ferrooxidans

Initial step of elemental sulfur (S⁰) oxidation by a purified sulfur:ferric ion oxidoreductase from Thiobacillus ferrooxidans was investigated. When S⁰ and reduced glutathione (GSH), whichwas absolutely required for S⁰ oxidation by sulfur:ferric ion oxidoreductase, were incubated in a buffer solution (pH 6.5), hydrogen sulfide (H ₂S) and GSSG were chemically produced at the rate of 0.021 and 0.082 μmol/ml per h, respectively. If sulfur:ferric ion oxidoreductase was addedto the incubation mixture, H₂S production immediately stopped and sulfite production opened, suggesting that H₂S is an actual substrate of sulfu:ferric ion oxidoreductase. Amongthe reduced sulfur compounds tested, S⁰, H₂S and FeS were utilized as an electron donorof sulfur:ferric ion oxidoreductase and a mechanism of initial steps of S⁰ oxidation was proposed. It was also found that when S⁰ was oxidized by sulfur:ferric ion oxidoreductase in the presence of GSH, contact of sulfur:ferric ion oxidoreductase with solid element sulfur was unnecessary.