PROTEOLYTIC ENZYMES 



93 



(c) To a third portion of the liquid add a drop of a i per 

 cent, solution of copper sulphate and a few drops of 

 strong caustic soda solution : a pink colour shows 

 the presence of peptone (the biuret reaction). 



Ex. 52. Repeat the above Ex., using pieces of the coagu- 

 lated protein of a hard-boiled egg instead of fibrin. 



Ex. 53. To 10 c.c. of milk in a test-tube add a few drops of 

 pepsin solution and a few drops of dilute hydrochloric acid 

 (see Ex. 51). Keep the whole for two hours in a water bath 

 at 40 C. Note that the milk is at first curdled ; after a time 

 the curd is gradually digested and dissolved, the liquid becoming 

 yellowish and bitter. 



Filter some of the liquid and test it for peptones (the biuret 

 test). 



The enzyme pepsin, which can be easily extracted from 

 gastric juice, acts only in an acid solution. 



Proteins which escape or are not changed by it, as 

 well as those of simple constitution produced by its 

 action, pass along the intestinal tract and become sub- 

 jected to the action of the pancreatic juice poured out by 

 the pancreas into the duodenum. This secretion con- 

 tains several different enzymes, one of them being trypsin, 

 a proteolytic ferment, which breaks down proteins best 

 in a faintly alkaline solution. It not only forms soluble 

 proteoses and peptones from proteins but carries on the 

 digestion or splitting of all these compounds further, the 

 peptones being decomposed into several kinds of amino- 

 acids (the chief of which are glycocoll, alanine, phenyla- 

 lanine, leucine, aspartic, and glutamic acids and tyrosine), 

 a characteristic substance named tryptophane and small 

 quantities of ammonia : none of the latter products are 

 of protein nature, and consequently do not give the 

 biuret reaction. 



