Purification and characterization of an inducible dissimilatory type sulfite reductase from Clostridium pasteurianum

Abstract

An inducible sulfite reductase was purified from Clostridium pasteurianum. The pH optimum of the enzyme is 7.5 in phosphate buffer. The molecular weight of the reductase was determined to be 83,600 from sodium dodecyl sulfate gel electrophoresis with a proposed molecular structure: α₂β₂. Its absorption spectrum showed a maximum at 275 nm, a broad shoulder at 370 nm and a very small absorption maximum at 585 nm. No siroheme chromophore was isolated from this reductase. The enzyme could reduced the following substrates in preferential order: NH₂OH> SeO ^2-₃ >NO ^2-₂ at rates 50% or less of its preferred substrate SO ^2-₃ . The proposed dissimilatory intermediates, S₃O ^2-₆ or S₂O ^2-₃ , were not utilized by this reductase while KCN inhibited its activity. Varying the substrate concentration [SO ^2-₃ ] from 1 to 2.5 μmol affected the stoichiometry of the enzyme reaction by alteration of the ratio of H₂ uptake to S^2- formed from 2.5:1 to 3.1:1. The inducible sulfite reductase was found to be linked to ferredoxin which could be completely replaced by methyl viologen or partially by benzyl viologen. Some of the above-mentioned enzyme properties and physiological considerations indicated that it was a dissimilatory type sulfite reductase.