Quantitative analysis of cystine, methionine, lysine, and nine other amino acids by a single oxidation--4 hour hydrolysis method

The sulfur-containing amino acids cystine and methionine play important roles in animal, especially avian, nutrition. Because these sulfur-containing amino acids are destroyed to varying extents by 6N HCl hydrolysis, oxidation and hydrolysis of cystine to cysteic acid and methionine to methionine sulfone have been widely used for determination of cystine and methionine. Lysine is considered the next limiting amino acid after the sulfur amino acids in poultry nutrition; therefore, determination of the amino acid content of rations focuses first on these 3 amino acids. The objective of this investigation was to establish whether lysine and other amino acids could be accurately determined in proteinaceous materials which had undergone performic acid oxidation. To perform this evaluation, lysine was determined in a variety of protein-containing materials both with and without performic acid oxidation. Performic acid oxidation followed by 6N HCl hydrolysis at 145 degrees C for 4 h allows accurate measurement of 3 amino acids especially important to poultry nutrition, cystine, methionine, and lysine, in a single preoxidized hydrolysate; this method can be extended to another 9 protein amino acids.