Purification and some properties of sulfur reductase from the iron-oxidizing bacterium Thiobacillus ferrooxidans NASF-1

Thiobacillus ferrooxidans strain NASF-1 grown aerobically in an Fe2+ (3%)-m edium produces hydrogen sulfide (H2S) from elemental sulfur under anaerobic conditions with argon gas at pH 7.5. Sulfur reductase, which catalyzes the reduction of elemental sulfur (S-0) with NAD(P)H as an electron donor to p roduce hydrogen sulfide (H2S) under anaerobic conditions, was purified 69-f old after 35-65% ammonium sulfate precipitation and Q-Sepharose FF, Phenyl- Toyopearl 650 ML, and Blue Sepharose FF column chromatography, with a speci fic activity of 57.6 U (mg protein)(-1). The purified enzyme was quite labi le under aerobic conditions, but comparatively stable in the presence of so dium hydrosulfite and under anaerobic conditions, especially under hydrogen gas conditions. The purified enzyme showed both sulfur reductase and hydro genase activities. Both activities had an optimum pH of 9.0. Sulfur reducta se has an apparent molecular weight of 120,000 Da, and is composed of three different subunits (M-r 54,000 Da (alpha), 35,000 Da (beta), and 35,000 Da (gamma)), as estimated by sodium dodecyl sulfate-polyacrylamide gel electr ophoresis. This is the first report on the purification of sulfur reductase from a mesophilic and obligate chemolithotrophic iron-oxidizing bacterium.