Isolation from Aspergillus nidulans of a Protein Catalyzing the Reduction of Sulfite by Reduced Viologen Dyes*

Abstract

A protein catalyzing the reduction of sulfite and hydroxylamine by reduced viologen dyes was purified from Aspergillus nidulans to an ultra-centrifugally homogeneous state. No physiological electron donors, including NADH and NADPH, could be utilized by this protein for sulfite reduction. The purified protein had a sedimentation coefficient of 4.23 S and showed a spectrum having peaks at 384 and 585 mμ and a shoulder at 453 mμi. When hydrogen and Desulfovibno hydrogenase were used to generate reduced methyl viologen (MVH), this protein reduced sulfite quantitatively to sulfide according to the equation: SO₃⁻⁻+3H₂→S⁻⁻+3H₂O. The sulfite-reducing activity was inhibited by cyanide, arsenite, and sulfide. A mutant strain of A. nidulans incapable of assimilating sulfate and sulfite did not contain the MVH-sulfite reductase activity.