Chemical Modification of the Thioether Bridges in Cytochrome c

1 Evidence is presented that an iodinating treatment of cytochrome c, in Tris buffer, pH 8.6, 8 M urea, leads to a chemical modification of the thioether bridges which link the heme to the polypeptide chain. It is suggested that the thioether sulfur atoms are oxidized to the sulfoxide stage. At low iodine levels (6–8 moles of iodine per mole of cytochrome c) no other amino acids are affected, as shown by amino acid analysis and use of 125I. 2 The main piece of evidence for the transformation is that treatment of this presumed cytochrome c 14, 17-disulfoxide leads to heme cleavage. 3 Both the oxidation and the cleavage reaction are dependent upon conformational factors. 4 A possible mechanism is proposed for the cleavage reaction which led to the prediction, borne out by experiment, that cleavage can also be effected by acetic anhydride.