728 PHYSIOLOGY OF DIGESTION AND SECRETION. 



first by Fischer among the products of acid hydrolysis of proteins, 

 has since been shown to occur in tryptic digestion. Like the gly- 

 cocoll and phenylalanin, it is produced with difficulty by trypsin 

 acting alone, but more readily if the tryptic action follows upon 

 previous peptic digestion, as is the case in the body. 



C . CH 3 



Tryptophan (skatolamino-acetic acid) : C C H 4 C . CH(NH 2 )COOH. This 



NH 



substance has long been recognized among the products of tryptic 

 digestion by the reddish-violet color (Tiedemann and Gmelin, 1826) 

 observed upon the addition of chlorin or bromin. Its chemical struc- 

 ture was determined by Hopkins and Cole (1901). According to 

 Ellinger,* tryptophan is an indol-amino-propionic acid of the formula 

 C . CHCOOHCH 2 NH 2 . When fed to dogs it causes the appear- 



NH 

 ance of kynurenic acid (Ci H 7 NO 3 ) in the urine. 



It is interesting as showing the existence of an indol grouping 

 in the protein molecule. 



II. THE DIAMINO-BODIES (HEXON BASES). 



Lysin (a-s-diaminocaproic acid): C 8 H 14 N 3 O 2 or CH 2 NH 2 (CH 2 ) 3 CHNH 2 - 



COOH. 

 Arginin (guanidin a-aminovalerianic acid) : C 6 H 14 N 4 O 2 or NHCNH 2 NH- 



CH 2 (CH 2 ) 2 CHNH 2 COOH. 



Histidin: C 6 H 9 N 3 O 2 (imidazolaminopropionic acid?). 



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 possible 

 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 peptic-tryptic digestion has shown that the hydrolysis 

 does not stop at the peptone stage; the protein molecule is split into 

 a number of simpler crystalline substances, the various ammo- 

 bodies. 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 end-products, and that in order to 

 serve its nutritive function these products or some of them must be 

 synthetically combined again during or after absorption. This view 

 is supported, moreover, by the discovery of the existence of the 

 enzyme erepsin (see below) in the intestinal mucosa. The action of 

 this latter enzyme is exerted especially upon the albumoses and pep- 

 tones, breaking them down into the amino-acids, so that apparently 

 whatever peptone or albumose may escape the final action of the 

 * Ellinger, "Zeitschrift f. physiol. Chemie," 43, 325, 1904. 



