Rapid Determination of Tryptophan in Intact Proteins by Derivative Spectrophotometry

Abstract

A method for rapid determination of total tryptophan in intact proteins was developed. Sample is homogenized with 0.1 M sodium hydroxide, and the homogenate, if it contains suspended material, is centrifuged. Tryptophan is directly quantified in sample extract on the basis of its fourth-derivative UV absorption spectrum. Protein hydrolysis and/or extract purification is not required because the fourth-derivative transformation of the conventional analytical band around 285.5 nm virtually eliminates interferences arising from both tyrosine and other UV-absorbing components. When pure proteins were analyzed by the method, the values of tryptophan residues found coincided well with literature data based on sequence analysis. The applicability of the method was also tested on several food and feed ingredient samples selected to include a range of protein content and a variety of protein sources. Owing to its simplicity and reliability, the method is particularly recommended for everyday analysis of a large number of samples.