86 THE ACTION OF ENZYMES 



violet colour with a-naphthol. With thymol the colour 

 is carmine-red. 



(6) Reaction of Hopkins and Cole. Prepare a solution 

 of glyoxylic acid by adding sodium amalgam to a strong 

 solution of oxalic acid ; filter after the gas bubbles 

 disappear. To a solution of a protein add a little of 

 the above ; shake, and then add strong sulphuric acid. 

 A bluish violet colour is produced, which is due to the 

 presence of tryptophane, one of the dissociation products 

 of most proteins, except gelatine and the protamines. 

 Glacial acetic acid which has been exposed to sunlight 

 for some time often contains a small amount of glyoxylic 

 acid ; when such acid is added to a solution of a protein, 

 and strong sulphuric acid then poured in, a violet colour 

 is seen at the junction of the two liquids. 



4. " Salting out " of Proteins. Most proteins are 

 colloidal bodies, and can be precipitated from their solu- 

 tions by adding solutions of various inorganic crystalline 

 salts. Some of them are easily salted out in this way, 

 while others are thrown out of solution with difficulty. 

 By using salts of different nature in varying strengths 

 of solution, mixtures of the proteins may be more or 

 less completely separated into their constituents. The 

 method of salting out has been of great service in 

 obtaining pure unaltered proteins for chemical and 

 physiological study. The substances most frequently 

 used for salting out proteins are : 



Sodium chloride Potassium acetate 



sulphate 



acetate Ammonium sulphate 



nitrate 



Zinc sulphate 

 Magnesium sulphate 



