Effect of high tyrosine content on the determination of tryptophan in protein by the acidic ninhydrin method. Application to chicken ovoinhibitor

Colorimetric determination of tryptophan in intact proteins by the acidic ninhydrin method of Gaitonde & Dovey (1970) gives high apparent tryptophan contents for proteins having high tyrosine/tryptophan ratios. Correction for this interference by tyrosine can be achieved by plotting the ratio of observed to expected tryptophan content as a function of tyrosine/tryptophan ratio for proteins of known composition. The equation of the line is: [Formula: see text] Application of this correction to chicken ovoinhibitor, which contains 17 tyrosine residues per molecule, gave results that agree with tryptophan content determined by other methods.