Mechanism of Tetravalent Manganese Reduction with Elemental Sulfur by Thiobacillus ferrooxidans

Washed intact cells of iron-grown T. ferrooxidans API9-3 could reduce manganese dioxide (MnO₂) enzymatically with elemental sulfur (S°) and the mechanism of Mn⁴⁺ reduction was studied. The optimum pH for Mn⁴⁺ reduction was 2.5. The amount of Mn⁴⁺ reduced by the strain was proportional to the amount of S° added to the reaction mixture. Anenzyme that directly catalyzes the reduction of Mn⁴⁺ to Mn²⁺ with S° was not found in the cell-free extract of this strain. T. ferrooxidans API9-3 was found to possess sulfur: ferric ion oxidoreductase (SFORase), which catalyzes the reduction of Fe³⁺ with S° to give Fe²⁺ and sulfite. The reduction of Mn⁴⁺ was inhibited by a specific inhibitor of SFORase or diamide, strongly suggesting that SFORase is involved in the Mn⁴⁺ reduction in this strain. The involvement of SFORase in Mn⁴⁺ reduction was further supported by the following results. Both the amounts of Fe²⁺ and sulflte produced by SFORase were markedly reduced by MnO₂. The amount of Mn⁴⁺ reduced increased 5.4-fold with 1 mm ferric ion and 10-fold with 2.5 mm cyanide or an inhibitor of iron oxidase, respectively. The sulfite and Fe²⁺ were chemically oxidized by MnO₂ instantly. From the results, it is concluded that the Mn⁴⁺ reduction with S° in this strain occurs through two steps. In the first step S° is enzymatically oxidized by SFORase to give Fe²⁺ and sulfite, and in the second step these two reduced compounds are chemically reduced by MnO₂ to give Mn²⁺.