Studies on yeast sulfite reductase II. Partial purification and properties of genetically incomplete sulfite reductases

Enzymes catalyzing the reduction of sulfite by reduced methyl viologen (MVH) were partially purified from four mutants of Saccharomyces cerevisiae, strains 6, 11, 20 and 21, which are genetically blocked in the sulfite reduction step in the sulfate assimilation pathway. Unlike NADPH-sulfite reductase from the wild-type strain, the enzymes from the mutants showed no activities coupled to NADPH oxidation. Sedimentation coefficients of these mutant enzymes ranged from 5.1 S to 6.6 S, values which are much smaller than the value of 14.8 S determined for yeast NADPH-sulfite reductase. All the mutant enzymes contained a chromophore or chromophores absorbing at 386 and 587 mμ. In contrast to the wild-type enzyme possessing both FMN and FAD, the enzymes from strains 6, 11 and 20 contained only FMN, and that from strain 21 lacked both flavins. Iron and acid-labile sulfide were detected in these mutant enzymes as well as in the wild-type enzyme.