Quantitation of Cysteine Residues and Disulfide Bonds by Electrophoresis

Abstract

Amino acid analysis quantifies the molar ratios of amino acids per mole of protein. This generally gives a nonintegral result, yet clearly there are integral numbers of the amino acids in each protein. A method was developed by Creighton (1) to count integral numbers of amino acid residues, and it is particularly useful for the determination of cysteine residues. Sulfhydryl and disulfide groups are of great structural, functional, and biological importance in protein molecules. For example, the Cys sulfhydryl is essential for the catalytic activity of some enzymes (e.g., thiol proteases) and the interconversion of Cys SH to Cystine S—S is directly involved in the activity of protein disulfide isomerase (2). The conformation of many proteins is stabilized by the presence of disulfide bonds (3), and the formation of disulfide bonds is an important posttranslational modification of secretory proteins (4).