Quantitation of Tryptophan in Proteins

Abstract

Accurate measurement of the amount of tryptophan in a sample is problematic, as it is completely destroyed under normal conditions employed for the complete hydrolysis of proteins. Strong acid is ordinarily the method of choice, and constant boiling 6 M hydrochloric acid is most frequently used. The reaction is usually carried out in evacuated sealed tubes or under nitrogen at 110°C for 18-96 h. Under these conditions, peptide bonds are quantitatively hydrolyzed (although relatively long periods are required for the complete hydrolysis of valine, leucine, and isoleucine). As well as complete destruction of tryptophan, small losses of serine and threonine occur, for which corrections are made. The advantages of amino acid analysis include the measurement of absolute amounts of protein, provided that the sample is not contaminated by other proteins. However, it may be a disadvantage if an automated amino acid analyzer is not readily available.