ACTION OF THE ENZYMES. 147 



fore disappear, not because they are synthesized into more 

 complex compounds, but because they are broken up into 

 simpler ones. A recognition of this fact led to the discov- 

 ery of protease (erepsin), to a discussion of the identifica- 

 tion and properties of which we shall now turn. 



The proof that the change from peptone to the simple 

 crystalline compounds mentioned above occurs under the 

 influence of a ferment contained in the wall of the small 

 intestine can be shown very well by introducing into each of 

 two tubes a solution of peptones and several pieces of well- 

 washed intestinal mucous membrane from a freshly killed dog 

 or cat. One of the two tubes is then boiled> after which both 

 are set aside in an incubator at 39 C. for a number of hours. 

 While in the boiled tube the biuret reaction persists even if we 

 wait for days or weeks, it is found to become fainter and 

 fainter in the unboiled tube, until in the course of perhaps an 

 hour or two depending upon the amount of peptone and in- 

 testinal mucous membrane originally present the biuret 

 reaction disappears entirely. Hand in hand with this dis- 

 appearance of the biuret reaction, goes an increase in the 

 amount of crystalline precipitate which may be obtained 

 upon the addition of suitable reagents, and to a discussion of 

 which we shall return immediately. 



It is only necessary to add tnat this ferment, which the 

 above simple experiment has indicated exists in the mucous 

 membrane of the intestine, can be extracted from it. Simple 

 extraction with an alkaline physiological salt solution of the 

 intestinal mucous membrane, after it has been scraped from 

 the submucosa and thoroughly triturated in a mortar with 

 sand, suffices to yield a very active solution of the enzyme, 

 though, of course, not a very pure one. The same difficulties 

 which were found to exist in the preparation of the other 

 ferments in a pure state exist here also. By fractional 

 precipitation with ammonium sulphate much purer specimens 

 of the ferment may be obtained, but for the details of this 

 process the reader must consult the original, Y 



