CONCERNING 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. Copyright © 1969, Wiley Blackwell. All rights reserved