Thiosulfate Reductase from a Moderately Thermophilic Iron-oxidizing Bacterium, Strain TI-1 : Purification and Characterization

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When grown on Fe²⁺-medium (pH 1.8) containing the following five L-amino acids: aspartic acid, glutamic acid, serine, arginine, and histidine, a moderately thermophilic iron-oxidizing bacterium, strain TI-1, produced hydrogen sulfide (H₂S) outside of the cells and synthesized a novel thiosulfate reductase, which catalyzed the reduction of thiosulfate with NAD(P)H as an electron donor to give H₂S. The activity of this enzyme in this bacterium increased 2-fold when elemental sulfur was added to this medium. Thiosulfate reductase was in the cytosol of the strain and was purified to an electrophoretically homogeneous state. The apparent molecular weight of thiosulfate reductase was 230, 000 by gel filtration and 58, 000 by SDS-PAGE, indicating that the enzyme is a homotetramer. The enzyme was most active at pH 6.0 and 60°C. Thiosulfate, but not elemental sulfur, sultite, or tetrathionate, was specifically used as an electron accepter of this enzyme. Both NADH and NADPH were used as electron donors. However, NADH was approximately 10 fold superior as an electron donor to NADPH. Reduced glutathione and mammalian cytochrome c were not used as electron donors. The Michaelis constants of this enzyme for thiosulfate, NADH, and NADPH were 0.29, 0.125, and 5.0mM.