Purification and characterization of sulfur reductase from a moderately thermophilic bacterial strain, TI-1, that oxidizes Iron

A moderately thermophilic bacterium, strain TI-1, produces H₂S outside of the cells when grown at 45°C on Fe²⁺-medium (pH 1.8) containing elemental sulfur and L-glutamic acid. A newly identified sulfur reductase was present in the cytosol of this strain and was purified to an electrophoretically homogeneous state from strain TI-1. The apparent molecular weight of sulfur reductase was 86,000 by gel filtration and 48,000 by SDS-PAGE, so the enzyme was a homodimer. The enzyme was most active at pH 9.0 and 60 to 70°C, and it catalyzed the reduction of 1 mol of elemental sulfur with 1 mol of NADH to give 1 mol of H₂S and 1 mol of NAD⁺. Elemental sulfur was a specific electron acceptor of this enzyme. Thiosulfate, sulfite, and tetrathionate were not electron acceptors, but inhibited sulfur reductase activity. NADPH was not used as an electron donor.