Ferredoxin-Linked Sulfite Reductase From Clostridium-Pasteurianum

A soluble sulfite reductase (EC. 1.8.1.2) system is present in cell-free extracts of Clostridium pasteurianum that reduces sulfite to sulfide in the presence of molecular hydrogen. The natural electron donor for this reductase has been found to be ferredoxin, which can be completely replaced by methyl viologen or partially by benzyl viologen. The physiological electron donors NAD, NADP, FMN, and FAD were not active in the transfer of electrons from hydrogen for sulfite reduction. The pH optimum of the sulfite reductase was found to be 7.0 using phosphate buffer, and the extracts reduced nitrite and hydroxyl-amine in addition to sulfite. The stoichiometry of the reaction in terms of hydrogen uptake to sulfide formation was 2.7, which is very close to the theoretical ratio of 3. The above properties of the system indicate that it is an assimilatory rather than a dissimilatory sulfite reductase, as outlined in the Discussion.