THE ACTION OF AMYLASE ON STARCH 105 



centration ; in this way the actual amount of enzyme present 

 in the solution may be roughly determined. This method 

 is very useful for quickly following the rate at which an 

 enzyme is developed under different conditions, e.g., at the 

 different stages of growth of the roots or leaves of plants, or 

 during the progress of a technical process, e.g., the withering of 

 tea leaves. 



2. A small portion of the precipitate is warmed with strong 

 caustic soda ; the presence of ammonia can be recognised by 

 the smell and by introducing red litmus paper into the upper 

 portion of the test-tube. 



3. A small portion of the precipitate is dissolved in strong 

 caustic soda and a few drops of very dilute copper sulphate 

 solution added ; a violet colour is produced. This is known 

 as the biuret reaction, as it is given by biuret, a substance 

 produced by heating urea. 



4. To the aqueous solution of a portion of the precipitate 

 a few drops of Millon's reagent are added ; a white precipitate 

 is obtained. Millon's reagent is a solution of mercuric nitrate 

 containing free nitric acid. It is prepared by dissolving one 

 part of mercury in two parts of strong nitric acid and diluting 

 the solution with twice its bulk of water ; after standing some 

 time the supernatant liquid is decanted from the precipitate. 



5. A portion of the precipitate is warmed in a small 

 porcelain dish with a little concentrated nitric acid, and the 

 excess of acid gently evaporated ; on addition of a drop or 

 two of strong ammonia a bright orange colour is obtained. 

 This is known as the Xanthoproteic reaction. 



All the above reactions, with the exception of the colour 

 reaction with guaiacum resin and hydrogen peroxide, are 

 characteristic of albumin and its derivatives. Enzymes, 

 therefore, can be broadly described chemically as complex 

 nitrogenous substances akin to albumin. Many attempts 

 have been made to obtain enzymes in the pure state, but 

 with little success. Like all complicated nitrogenous bodies of 



