THIOSULFATE REDUCTASE FROM A MODERATELY THERMOPHILIC IRON-OXIDIZING BACTERIUM, STRAIN TI-1 - PURIFICATION AND CHARACTERIZATION

When grown on Fe2+-medium (pH 1.8) containing the following five L-ami no acids: aspartic acid, glutamic acid, serine, arginine, and histidin e, a moderately thermophilic iron-oxidizing bacterium, strain Tl-1, pr oduced hydrogen sulfide (H2S) outside of the cells and synthesized a n ovel thiosulfate reductase, which catalyzed the reduction of thiosulfa te 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 a dded to this medium, Thiosulfate reductase was in the cytosol of the s train and was purified to an electrophoretically homogeneous state, Th e apparent molecular weight of thiosulfate reductase was 230,000 by ge l filtration and 58,000 by SDS-PAGE, indicating that the enzyme is a h omotetramer. The enzyme was most active at pH 6.0 and 60 degrees C, Th iosulfate, but not elemental sulfur, sulfite, or tetrathionate, was sp ecifically 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 5.0 mM.