Metabolic Interrelations of Sulfur in Proteins, Thiosulfate, and Cystine

Abstract Examination of the reactions of protein disulfide sulfur led to the implication of three mechanisms in the removal of sulfur from cystinyl residues. (a) Persulfide formation in alkali with subsequent loss of the reactive persulfide sulfur; (b) organic thiocyanate formation by direct attack of CN-, followed by elimination of SCN- by OH- or by appropriately located intraprotein nucleophiles; and (c) sulfite reduction of disulfide bonds with formation of S-sulfocysteinyl residues, followed by elimination of thiosulfate. Spectral as well as kinetic evidence was obtained for persulfide formation from a variety of disulfide compounds. The ease of persulfide formation correlated with the ability of base to abstract a proton from the carbon atom β to the disulfide bond. Evidence was obtained indicating that sulfur eliminated from cystine or cystinyl residues in vivo occupies a metabolic branch point. Incorporation into cystine of the labeled outer sulfur atom of thiosulfate, the corresponding labeled sulfur in the form of tetrathionate, and labeled elemental sulfur occurred in vivo. Unlabeled sulfide failed to suppress the level of radioactivity incorporated and the inner atom of thiosulfate failed to be incorporated. Evaluation of the time course of incorporation as well as the chemical reactivities of the various sulfur species led to the proposal of protein-coated elemental sulfur as a retained intermediate in mammalian sulfur metabolism.