Purification and characterization of a sulfite : cytochrome c oxidoreductase from Thiobacillus acidophilus

Cell-free extracts of Thiobacillus acidophilus prepared at neutral pH showe d oxidation of sulfite to sulfate with ferricyanide as electron acceptor. H orse heart cytochrome c could be used as alternative electron acceptor; how ever, the observed activity was only 0.1% of that found for ferricyanide. T he enzyme responsible for the oxidation of sulfite was purified to homogene ity. The purified enzyme was a monomer of 42 kDa and contained one haem c p er monomer. Electron paramagnetic resonance (EPR) spectroscopical analysis of the sulfite:cytochrome c oxidoreductase showed the presence of molybdenu m (V), only after reduction of the enzyme with sulfite. The pH optimum for the enzymatic reaction was 7.5 and the temperature optimum 40 degrees C. En zymatic activity was strongly reduced in the presence of the anions: chlori de, phosphate and nitrate. Zn contrast to other enzymes involved in sulfur metabolism and previously isolated from T. acidophilus, sulfite:cytochrome c oxidoreductase activity is not stimulated by the presence of sulfate ions . (C) 2000 Elsevier Science B.V. All rights reserved.