Studies on the reaction of sulfite with proteins.

Structural investigations on proteins frequently require as a first step the cleavage of the disulfide bonds that link half-cystine residues. Oxidation with performic acid has been used for this purpose first by Sanger (2) and later by Hirs (3), before a detailed investigation of the sequences of the amino acid residues in insulin and ribonuclease, two proteins that are devoid of tryptophan. Special studies (3-5) of this reaction have succeeded in rendering the yield of cysteic acid and methionine sulfone virtually quantitative, and in minimizing undesirable side reactions with other amino acids, such as tyrosine. No conditions have been found, however, that will prevent the oxidative degradation of tryptophan. Since tryptophan would be expected to remain unaffected by reductive procedures capable of cleaving disulfide bonds, reductive methods have been investigated in several laboratories, including this one (1, 6-11). One of the procedures is based upon the well known reaction of sulfite with disulfide bonds in proteins noted in 1907 by Heffter (12) and studied later by Mirsky and Anson (13). The course of the reduction was described by Clarke (14) and by Lugg (15) (cf. also (16)), and the reaction has since been studied in detail by a number of investigators (17-20). Much of the work in peptide and protein chemistry in which sulfite has been used is reviewed by Cecil and McPhee (18). the structural examination of proteins will depend upon further studies of the properties of the resulting S-sulfoproteins.