DIGESTION AND ABSORPTION IN THE INTESTINES. 793 



to act and the conditions, favorable or unfavorable, under which it 

 acts. The end-products usually obtained most easily are tyrosin, 

 leucin, aspartic acid, glutaminic acid, tryptophan, lysin, arginin, 

 histidin. The first two of these substances have been known for a 

 long time and may be obtained easily in crystalline form from 

 pancreatic digestions. If the trypsin is allowed to exert its com- 

 plete action upon the protein, the end-products are closely similar 

 to those obtained by boiling protein with acids. The hydrolysis 

 caused by the acids and by the trypsin seems to be nearly identical, 

 although that caused by the acids is probably more complete and 

 perhaps is attended by secondary reactions. The numerous prod- 

 ucts obtained by this complete hydrolysis consist chiefly of amino- 

 acids that is, organic acids containing one or more amino-groups 

 (NH 2 ) in direct union with carbon. Most of them are monamino- 

 acids, that is, contain one NH 2 group, and this group is united with 

 the carbon occupying the alpha position. The nitrogen of the 

 protein molecule appears in the split products chiefly as amino- 

 acids, but in small part as ammonia. 



A list of the known amino-acids obtained from proteolytic 

 cleavage of the protein molecule is appended. For a more com- 

 plete description of their properties and chemical relationships 

 reference must be made to text-books on physiological chemistry. 

 With regard to the nomenclature used, it will be borne in mind 

 that the various carbon atoms in the straight chain of the fatty 

 acids are designated by Greek letters that indicate their position 

 in relation to the carboxyl (COOH) group, as may be illustrated 

 by the formula for caproic acid : 



CH 3 CH 2 CH 2 CH 2 CH 2 COOH 



e 5 7 ft a 



I. AMINO-ACIDS OF THE FATTY ACID (ALIPHATIC) SERIES. 



1. Glycocoll or glycin (ammo-acetic acid): 



CH 2 NH 2 COOH. 



2. Alanin (a-aminopropionic acid) : 



CH 3 CHNH 2 COOH. 



3. Serin (a-amino-/3-oxypropionic acid) : 



CH 2 OHCHNH 2 COOH. 



4. Cystein (a-amino-/3-thiopropionic acid) : 



CH 2 SHCHNH 2 COOH. 



5. Valin (a-amino-isopropionic acid) : 



CH 3 



\ 



CHCHNH 2 COOH. 



