92 THE ACTION OF ENZYMES 



salted out of their solutions, but are precipitated by 

 phosphotungstic acid and other alkaloidal reagents, 

 and give a pure red biuret reaction. They are acid 

 subtances, very soluble in water, and contain no sulphur. 

 Only very small amounts of ammonia and amino-acids, 

 if any, are produced even by prolonged peptic digestion 

 of proteins, but in many instances dissociation products 

 intermediate between peptones and amino-acids are 

 formed in considerable amounts : these are probably 

 peptides, bodies which on hydrolysis with acids give 

 leucine and other amino-acids. 



Ex. 51. Take three large test-tubes, label them A, B, and 

 C respectively. 

 Half fill 



A with a dilute solution of hydrochloric acid (i c.c. of 



strong acid in 150 of water). 

 B with dilute solution of hydrochloric acid, to which a 



few drops of pepsin solution has been added. 

 C with dilute solution of pepsin only. 



Into each tube add a small piece of fibrin which has been 

 washed and allowed to soak for half an hour in distilled water. 



Place all three tubes in a water bath kept at 40 C. for twenty 

 to thirty minutes. Examine them at intervals and note that in 

 A and C, in which the ferment and acid act singly upon the 

 fibrin, the latter is undissolved : in B, where the ferment and 

 acid act together, the fibrin is digested and dissolved. 

 With the solution in B try the following experiments : 



(a) To a small portion of the liquid add a few drops of 



litmus and then neutralize with very dilute caustic 

 soda : acid-albumin is precipitated. 



(b) To another small quantity of the liquid add a drop of 



strong nitric acid : a white precipitate of a protease or 

 albumose appears, which dissolves with yellow colora- 

 tion on heating but reappears when the liquid is 

 cooled. 



