Reduction of Cupric Ions with Elemental Sulfur by Thiobacillus ferrooxidans

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In anaerobic or aerobic conditions in the presence of 5 mM sodium cyanide, an inhibitor of iron oxidase, cupric ion (Cu2+) was reduced enzymatically with elemental sulfur (S0) by washed intact cells of Thiobacillus ferrooxidans AP19-3 to give cuprous ion (Cu+). The rate of Cu2+ reduction was proportional to the concentrations of S0 and Cu2+ added to the reaction mixture. The pH optimum for the cupric ion-reducing system was 5.0, and the activity was completely destroyed by 10-min incubation of cells at 70°C. The activity of Cu2+ reduction with S0 by this strain was strongly inhibited by inhibitors of hydrogen sulfide: ferric ion oxidoreductase (SFORase), such as α,α′-dipyridyl, 4,5-dihydroxy-m-benzene disulfonic acid disodium salts, and diazine dicarboxylic acid bis-(N, N-dimethylamide). A SFORase purified from this strain, which catalyzes oxidation of both hydrogen sulfide and S0 with Fe3+ or Mo6+ as an electron acceptor in the presence of glutathione, catalyzed a reduction of Cu2+ by S0, and the Michaelis constant of SFORase for Cu2+ was 7.2 mM, indicating that a SFORase catalyzes the reduction of not only Fe3+ and Mo6+ but also Cu2+.

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