SOME PROPERTIES OF SULFITE REDUCTASE FROM YEAST

Abstract

The MVH- and NADPH-linked sulfite reducing activities were assayed in extracts obtained from a wild strain of Saccharomyces cerevisiae and various auxotrophic mutants derived from it. All the extracts possessing the NADPH-linked activity also showed the MVH linked activity, and all those lacking the latter also lacked the former. However, extracts from several mutants had the MVH-linked activity without having the other. NADPH-sulfite reductase was purified nearly 200-fold from extracts of the wild strain. Throughout the purification steps, the MVH-linked activity was also purified in association with the NADPH-linked activity, and the ratio between the two remained essentially constant. However, on exposure to heat, low ionic strengths and PCMB, only the NADPH-linked activity was lost, leaving the MVH-linked activity almost unaffected. Both activities were sensitive to cyanide. The sulfite reductase could also reduce nitrite and hydroxy lamine, and the nitrite- and hydroxylamine-reducing activity was sensitive to the treatments which inhibited the NADPH-sulfite reducing activity. Addition of nitrite or hydroxylamine competitively inhibits the NADPH linked sulfite reduction. The enzyme also showed diaphorase activity, reducing DPIP or MV. This activity was inhibited by the treatments to which the NADPH-linked sulfite reduction was sensitive, but it was not sensitive to cyanide. A tentative schematic model for yeast sulfite reductase is presented.