Purification and Some Properties of a Tetrathionate Decomposing Enzyme from Thiobacillus thiooxidans

A tetrathionate-decomposing enzyme that catalyzes the decomposition of tetrathionate into thiosulfate and sulfate was purified to homogeneity from tetrathionate-grown Thiobacillus thiooxidans. The enzyme had an apparent molecular weight of 104,000, and was composed of two identical subunits (MW = 58,000) as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme had an isoelectric point at 9.6 and was most active at pH 3.0-3.5 and 40°C. Enzyme activity was increased approximately 100-fold in the presence of 400 mm sulfate ion. The Michaelis constant of this enzyme for tetrathionate in the presence of 20, 50, and 200 mm of sulfate ion was 2.4 mm. Mercuric and ferric ions completely inhibited the enzyme activity at 1 mm. Though cupric ion up to 0.01 mm markedly stimulated the activity in the presence of 20 mm sulfate ion, a higher concentration (1 mm) rather strongly inhibited the activity. Ethylenediaminetetraacetic acid (EDTA) strongly inhibited the activity, but this inhibiton was completely restored by cupric ion.