Tyrosine ninhydrin reaction

Zahnley and Davis 456) found that colorimetric determination of tryptophan in intact proteins by the ninhydrin reaction gives high apparent tryptophan contents for proteins having high tyrosine/tryp-tophan ratios. An equation was proposed to correct for this interference by tyrosine. To apply the correction, the tyrosine content must be known, but it can be determined readily by other methods. Application of such a correction to chicken ovoinhibitor, which contains 17 tyrosine residues and one tryptophan residue per molecule, gave results that agree with the tryptophan content determined by other methods such as fluorescence, oxidation with NBS and magnetic circular dichroism. 

More recent studies by Schmir and Cohen (1961) have led to a better understanding of the details and to improved yields in the tyrosine cleavage Phloretylglycine was oxidized with three equivalents of NBS in various buffer mixtures and the extent of cleavage determined both by ninhydrin assay for glycine and by the intensity of the dienone peak at 260 m/u in the ultraviolet. From the results (Table VIII) it is evident that cleavage is favored by increasing the acidity of the reaction mixture. 

Bovine vasopressin does not react with ninhydrin. It gives a reaction for tyrosine (a-nitroso-j3-naphthol), cystine (iodoplatinate), and arginine (Sakaguchi) (11),... 

---