Hydrolysis of Proteins with p-Toluenesulfonic Acid

A new procedure for analyses for tryptophan in proteins and glycoproteins is described. The method utilizes ptoluenesulfonic acid as the catalyst for hydrolysis and the amino acid analyzer for the quantitative estimation of tryptophan. The hydrolysis of proteins is carried out in 3 N p-toluenesulfonic acid containing 0.2 % 3-(Z-aminoethyl)indole in evacuated sealed tubes at 110” for 22, 48, and 72 hours. The values of tryptophan obtained were close to the expected integral values and the recoveries of all other amino acids were comparable to those observed after hydrolysis with 6 N HCl. Since the hydrolysate can be placed on the ion exchange column, without the prior removal of the solvent which is required when 6 N HCl is used for the hydrolysis, the present procedure is more convenient than the use of 6 N HCl. This procedure, however, is not designed for the analyses of proteins that are grossly contaminated with carbohydrates such as feeds. The limit of carbohydrate content that the present procedure can tolerate is 2.0 mg when 2 to 4 mg of protein are being hydrolyzed with 1.0 ml of the acid.