98 NUTRITION OF FARM ANIMALS 



simple amino acids out of which the protein molecule is built 

 up. Even the most prolonged action of trypsin, however, 

 leaves a considerable residue in which no free amino acids are 

 found but which on hydrolysis with strong mineral acids yields 

 them in abundance. 



Conjugated proteins seem to be acted upon by trypsin in the 

 same manner as by pepsin but much more energetically. 



138. Erepsin. The presence of a proteolytic enzym in 

 the intestinal juice and in the epithelial cells of the small intes- 

 tine has already been noted (135) . This enzym does not act on 

 unaltered proteins, with the exception of casein, but it hy- 

 drolyzes proteoses and peptones energetically, yielding crystal- 

 line cleavage products. It is of special interest to note that, 

 according to Cohnheim, 1 erepsin is capable of effecting the 

 cleavage of that part of the protein molecule which is not at- 

 tacked by pepsin and trypsin and that in artificial digestion 

 experiments almost complete conversion into comparatively 

 simple crystalline products may be obtained in a relatively short 

 time. 



139. Extent of protein cleavage in natural digestion. 

 When it was first shown by Kiihne and Chittenden that the 

 action of trypsin upon proteins yielded among other products 

 such simple crystalline substances as leucin and tyrosin, com- 

 paratively little importance was attached to the fact from the 

 physiological standpoint. While the fact was interesting as 

 throwing light upon the chemical structure of the proteins, it 

 was believed that in natural digestion the soluble proteoses and 

 peptones were promptly removed from the digestive tract by 

 resorption and that at most but a small proportion of the feed 

 protein underwent any profound cleavage. This belief was 

 the stronger because it was believed that only proteins or their 

 slightly altered derivatives, the proteoses and peptones, could 

 supply the demands of the organism for proteins. Whatever 

 protein was broken down into crystalline products was looked 

 upon as wasted. With the progress of investigation, however, 

 it has become increasingly clear that the processes of hy- 

 drolytic cleavage go further and play a much more important 

 part than was formerly supposed. While it is admitted that 

 peptones, or even soluble proteins, may be resorbed, the weight 



3 Ztschr. Physiol. Chem. 49 (1906), 64; 51 (1907), 415. 



