Concerning the Source of "Labile" Sulfur in Clostridial Ferredoxin

Ferredoxin was isolated and purified from Clostridium pasteurianum grown in the presence of [35S]sulfate as the sole sulfur source. Apoferredoxin prepared from this [35S]ferredoxin (uniformly labeled) retained 46–48% of the specific radioactivity. Ferredoxin was reconstituted from this apoferredoxin with unlabeled sulfide and found to retain 52% of the specific activity of the native radioactive [35S]ferredoxin. These results indicate that 8 atoms of 35S were removed from [35S]ferredoxin as sulfide in the preparation of apoferredoxin and the same amount of sulfide was supplied by the unlabeled sulfide in the reconstituted ferredoxin. Apoferredoxin-I and apoferredoxin-II prepared from ferredoxin previously reconstituted with Na235S retained only 1.3% and 1.4% of the 35S. These observations further support previous conclusions that clostridial ferredoxin contains two distinct classes of sulfur: the acid-labile sulfide and the sulfur in the cysteine residues of the polypeptide. Previous experiments concerning the origin of the acid-labile sulfide in clostridial ferredoxin are summarized and discussed.