THIOSULFATE OXIDATION BY SULFUR-GROWN THIOBACILLUS THIOOXIDANS CELLS, CELL-FREE-EXTRACTS, AND THIOSULFATE-OXIDIZING ENZYME

The oxidation of thiosulfate by Thiobacillus thiooxidans grown on sulfur was studied in cells, cell-free extracts, and a thiosulfate-oxidizing enzyme system. Thiosulfate was oxidized to tetrathionate by cells treated with N-ethylmaleimide with a pH optimum at 2.3. The cell-free extracts also oxidized thiosulfate with the same pH optimum and O-2 consumption. Unheated cells, on the other hand, oxidized thiosulfate to sulfite in the presence of 2-n-heptyl-4-hydroxyquinoline N-oxide, an inhibitor of sulfite oxidation. The cells treated with N-ethylmaleimide showed two K-m values for thiosulfate while the cell-free system showed only one K-m. The K-m value for thiosulfate generally increased with the increasing pH. A soluble thiosulfate-oxidizing enzyme system was extracted from the cells at pH 2.5 in the presence of 1 M ammonium sulfate by passage through a French pressure cell. The system contained a native cytochrome c that was reduced by thiosulfate at pH 2.5 and a thiosulfate-ferricyanide oxidoreductase activity with a pH optimum around 2.0. The acidic extract also contained a component that reduced horse heart cytochrome c at a neutral pH. The reduction at an acidic pH required sulfite.