Tyrosine and Tryptophane Determinations in One-tenth Gram of Protein

In the calorimetric methods for the determination of tyrosine and tryptophane, described by Folin and Ciocalteu (1) in 1927, 1 or 2 gm. of protein material are taken for the preliminary hydrolysis. It has seemed desirable to try to adapt these methods to the use of smaller amounts of material (0.1 gm.) in order to broaden the applicability of the methods. While this work was in progress, there appeared a lengthy paper, by Hanke (2), on colorimetric tyrosine determinations, and it seems suitable first to consider that paper before taking up the constructive work. Hanke’s method for tyrosine was published in 1922 and, during the intervening 7 years, Hanke has repeatedly used that method as a basis for criticism of other methods, particularly those published from this laboratory. He now finds that one essential step in his method, namely the removal of histidine with silver oxide, also removes tyrosine and, on the basis of that error alone, the tyrosine content of casein moves from 4.5 per cent (his earlier figures) to about 5.5 per cent. As the silver oxide treatment was a necessary prerequisite step in his method, he now definitely abandons that meth0d.l It is to be noted further that there is now no continuity between his tyrosine work of the past 7 years and the main part of the work reported in his present paper. What he now presents is just a modification of the Folin-Ciocalteu method, and on that