AVAILABLE LYSINE IN PROTEIN, ASSAY USING O-PHTHALALDEHYDE N-ACETYL-L-CYSTEINE SPECTROPHOTOMETRIC METHOD

An assay was based on reaction of free epsilon-amino groups in proteins with the omicron-phthalaldehyde/N-acetyl-L-cysteine reagent to form isoindoles, which absorb at 335 nm. The procedure was suitable for proteins or mixtures of proteins with available lysine contents of more than 5 moles lysine/mole protein and required absence of free amino acids and peptides. This method was simpler and more convenient than other methods, since it did not require hydrolysis, amino acid analysis, long heating periods or solvent extraction.