Improved purification and sulfhydryl analysis of thiosulfate reductase

Thiosulfate reductase purified 900-fold from an extract of baker's yeast by a new procedure consisted of three charge-isomeric species, all with catalytic activity. Amino acid analysis of thiosulfate reductase and titrations with 5,5'-dithiobis(2-nitrobenzoate) and iodo[14C]acetate indicated only one cysteine residue per enzyme molecule. Alkylation of this cysteine with either iodoacetate or iodoacetamide did not inactivate the enzyme, indicating that sulfur-sulfur bond cleavage in the thiosulfate substrate does not require an enzymic sulfhydryl group as attacking nucleophile.