148 PHYSIOLOGY OF ALIMENTATION. 



We have now to discuss the character of the crystalline 

 products into which protease splits peptones. If a protease 

 solution obtained from dog's intestine, for example- is added 

 to a solution of peptones (in KUHNE'S sense), and the whole 

 is kept in an incubator at body temperature, it is found that 

 only a slight biuret reaction is obtained on the third or 

 fourth day from the reaction mixture which on the first day 

 gave an intense reaction. If, now, the digestion experiment 

 is continued a few days longer, the biuret reaction disappears 

 entirely, indicating that the peptones have been changed 

 into something else. This ''something else" has been shown 

 to consist of leucin, tyrosin, lysin, histidin, arginin, and 

 ammonia, all of them substances therefore identical, quali- 

 tatively at least, with those obtained when alkali-proteinase 

 (trypsin) is allowed to act on a protein. The question may 

 therefore very justly be raised, Are we not perhaps really 

 dealing with the action of alkali-proteinase in an extract of 

 the small intestine? This question is to be answered in the 

 negative, and for the following reasons. Alkali-proteinase, 

 it is well known, acts upon a large number of the so-called 

 "native" proteins for example, fibrin, white of egg, serum 

 albumin, etc. If proper precautions are taken to obtain a 

 protease solution free from alkali-proteinase this property 

 of acting on native proteins is lacking. Fibrin, for example, 

 which is acted upon so rapidly by alkali-proteinase that it 

 can scarcely be used in quantitative studies with this fer- 

 ment, may remain in a protease solution for days, even if the 

 fibrin has not been boiled, without showing any evidences 

 of having been attacked. 



One of the ordinary "native" proteins is, however, acted 

 upon by protease, and that is the casein of milk. This 

 fact is of physiological importance, as the presence of protease 

 in the intestine renders one of the foods of infants capable 

 of digestion even when the ordinary proteolytic ferments 

 (acid- and alkali-proteinase) are lacking. Protease will act 

 also upon certain of the proteoses, but its activity manifests 



