DIGESTION AND ABSORPTION IN THE INTESTINES. 707 



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

 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 (skatolamido-acetic acid) : C 6 H 4 C . CH(NH 2 )COOH. This 



YH 



substance has long been recognized among the products of tryptid 

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

 observed upon the addition of chlorin or bromin. Its chemical 

 structure was determined by Hopkins and Cole (1901). According 

 to Ellinger,* tryptophan is an indol compound of the formula 

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



C 6 H 4 ()CH 



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 proteid molecule. 



II. THE DIAMINO-BODIES (HEXON BASES). 



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



COOH. 

 Arginin (guanidin a-amidovalerianic acid): C 6 Hi 4 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 . 

 These three substances occur among the products of pancreatic digestion. 



They may be separated from the monamino-bodies by the fact that 



they are precipitated in acid solutions by phosphotungstic acid 



while the monamino-acids are not. 



The Significance of Tryptic Digestion. It was formerly 

 supposed that the object of peptic and tryptic digestion is to 

 convert the insoluble and non-dialyzable proteids into the simpler, 

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

 way absorption of proteid 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 proteid molecule is split into 

 a number of simpler, crystalline substances, the various amino- 

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

 process. Some believe that the proteid 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 synthet- 

 ically 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 lat- 

 ter enzyme is exerted especially upon the albumoses and peptones, 

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



