PROTEINS. 9 1 



present in the protein molecule it is evident that protein substances 

 respond to Millon's reaction because of the presence of this tyro- 

 sine complex. The test is not a very satisfactory one for use in 

 solutions containing inorganic salts in large amount, since the mer- 

 cury of the Millon's reagent 1 is thus precipitated and the reagent 

 rendered inert. This reagent is therefore never used for the detec- 

 tion of protein material in the urine. 



2. Xanthoproteic Reaction. To 2-3 c.c. of egg albumin solu- 

 tion in a test-tube add concentrated nitric acid. A white precipi- 

 tate forms, which upon heating turns yellow and finally dissolves, 

 imparting to the solution a yellow color. Cool the solution and 

 carefully add ammonium hydroxide, potassium hydroxide or sod- 

 ium hydroxide in excess. Note that the yellow color deepens into 

 an orange. This reaction is due to the presence in the protein 

 molecule of the phenyl group, with which the nitric acid forms 

 certain nitro modifications. The particular complexes of the pro- 

 tein molecule which are of especial importance in this connection are 

 those of tyrosine, phenylalanine and tryptophane. The test is not a 

 satisfactory one for use in urinary examination because of the 

 color of the end-reaction. 



3. Adamkiewicz Reaction. Thoroughly mix i volume of con- 

 centrated sulphuric acid and 2 volumes of acetic acid in a test-tube, 

 add a few drops of egg albumin solution and heat gently. A 

 reddish-violet color is produced. Gelatin does not respond to this 

 test. This reaction shows the presence of the tryptophane group 

 (see next experiment). The test depends upon the presence of 

 glyoxylic acid, CHO COOH + H 2 O or CH ( OH ) 2 COOH, 

 in the reagents. This is shown by the failure to secure a 

 positive reaction when acetic acid free from glyoxylic acid is used. 



Rosenheim has recently advanced the view that the reaction may 

 be due to the presence of oxidizing agents such as nitrous acid 

 and ferric salts in the sulphuric acid. 



4. Hopkins-Cole Reaction. Place 1-2 c.c. of egg albumin solu- 

 tion and 3 c.c. of glyoxylic acid, CHO COOH + H 2 O or 

 CH(OH) 2 COOH, solution (Hopkins-Cole reagent 2 ) in a test-tube 



1 Millon's reagent consists of mercury dissolved in nitric acid containing some 

 nitrous acid. It is prepared by digesting one part (by weight) of mercury 

 with two parts (by weight) of HNO 3 (sp. gr. 1.42) and diluting the resulting 

 solution with two volumes of water. 



2 Hopkins-Cole reagent is prepared as follows: To one liter of a saturated 

 solution of oxalic acid add 60 grams of sodium amalgam and allow the mixture 

 to stand until the evolution of gas ceases. Filter and dilute with 2-3 volumes 

 of water. 



