Abstract
A convenient determination of the combined content of cystine and cysteine in proteins depends on the oxidation of the amino acids to cysteic acid. This chapter discusses the determination of cystine as cysteic acid. A variety of strong oxidizing agents accomplish this reaction. The most suitable reagents are hydrogen peroxide and various peracids, particularly performic acid. The protein is subjected to acid hydrolysis and cysteic acid is determined by chromatography. This method provides a highly reliable analysis for methionine that is converted to the sulfone. The methods used are performic acid reagent, oxidation, hydrolysis, chromatography, methionine sulfone, and calculations. The quantity of protein required for a single determination will depend on the method used for the chromatography of cysteic acid. The cystine content may be expressed on a weight basis. It is convenient to express the cysteic acid and methionine sulfone values relative to a stable amino acid, such as alanine or leucine, determined in the same chromatogram.
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