DIGESTION AND ABSORPTION IN THE INTESTINES. 801 



3. Tryptophan (a-amino-/8-indolpropionic acid): 



C 8 H 6 NCH 2 CHNH 2 COOH, or 

 CH 



i 



. CH 2 CHNH 2 COOH. 



H 



4. Histidin (a-amino-/3-imidazolpropionic acid): 



C 3 H 2 N 2 CH 2 CHNH 2 COOH, or 

 HC NH 





N 

 H 2 CHNH 2 COOH. 



III. THE PYRROL SERIES. 

 1. Prolin (a-pyrollidin carboxylic acid) : 

 H 2 C - CH 2 



H 2 C C 



HCOOH 



2. Oxyprolin (oxypyrollidin carboxylic acid) : 

 HOHC - CH 2 



H 2 C CHCOOH. 



The Significance of Tryptic Digestion. It was formerly 

 supposed that the object of peptic and tryptic digestion is to con- 

 vert the insoluble and non-dialyzable proteins into the simpler, 

 more soluble, and more diffusible peptones and proteoses. In this 

 way absorption of protein material was explained. This view, 

 however, is not sufficient. On the one hand, it has not been pos- 

 sible to prove conclusively that peptones or proteoses are found in 

 the blood; on the other hand, a better knowledge of the processes of 

 tryptic or of tryptic-ereptic digestion has shown that the hydrolysis 

 does not stop at the peptone stage; the protein molecule is hydro- 

 lyzed still further with the production of simpler molecules. At 

 present different views exist as to the extent of this latter process. 

 Some believe that the protein molecule is entirely broken down into 

 its so-called building-stones, that is, the various amino-acids, de- 

 scribed on the preceding page. This view is supported by the dis- 

 covery of the existence of the enzyme erepsin (see below) in the 

 51 



