The Stability of Cysteine and Cystine during Acid Hydrolysis of Proteins and Peptides

Abstract When solutions of cystine and cysteine are treated with dithiothreitol and then with sodium tetrathionate, quantitative yields of S-sulfocysteine can be obtained. This reaction has been applied to protein hydrolysates and the resulting S-sulfocysteine has been determined by ion exchange chromatography on an amino acid analyzer. With ribonuclease and reduced glutathione this procedure has given accurate values for the cystine or cysteine content of the protein or peptide. When tryptophan is also present, the addition of a small amount of sodium tetrathionate to the 6 n HCl used for the hydrolysis has usually proved to be desirable; the function of the tetrathionate appears to be to react with the indole nucleus. With this modification, the procedure has given precise results for the cysteine plus cystine contents of streptococcal proteinase, lysozyme, and wool. The results indicate that, in terms of their primary structures, cysteine and cystine are not destroyed during acid hydrolysis under these conditions; treatment with dithiothreitol and tetrathionate serves to convert any altered forms of the amino acids in the hydrolysates to S-sulfocysteine and to permit quantitative recovery in the form of this derivative. This method for determining the half-cystine content of proteins provides an alternative to the determinations of the amino acids as cysteic acid or S-carboxymethylcysteine. The treatment does not affect the recoveries of other amino acids in the hydrolysate, with the possible exception of tyrosine.