THE CHEMISTRY OF THE DIGESTIVE JUICES 359 



does not digest proteins. The same is true of extracts of perfectly 

 fresh pancreas, but if the pancreas is allowed to stand for a time, the 

 extracts contain active trypsin, perhaps because some decomposition 

 product has activated the trypsinogen. Some writers, however, 

 state that when contamination of the gland with intestinal contents 

 or contact with the mucosa has been avoided in its removal from 

 the body, such extracts will remain inactive for months, although 

 the trypsinogen can at once be activated to trypsin by the addition 

 of enterokinase. 



Trypsin, to a certain extent, corresponds with pepsin in its action 

 on proteins. But it acts energetically in an alkaline as well as in a 

 not too acid medium (a very slight amount of digestion may go on in 

 distilled water) ; and its action, unlike that of pepsin at least in 

 digestions of moderate duration does not stop at the peptone stage, 

 but goes on rapidly to the production of the amino-acids, the basic 

 substances arginin, lysin, and histidin, known as the hexone bases, 

 and most of the other decomposition products obtained by boiling 

 proteins with dilute acids. The most important of these products, 

 so far as they have been isolated and identified, are enumerated in 

 the table on p. 360 (see also pp. 1-3). 



As to the chemical nomenclature of these bodies, the student should 

 refer to his textbook of organic chemistry. It need only be remarked 

 here, by way of illustration, that when, e.g., leucin is designated as 

 a-amino-isobutylacetic acid, this indicates that it can be derived from 

 the fatty acid isobutylacetic acid by the substitution of an amino group, 

 NH 2 , for a hydrogen atom in the a-carbon group (see p. 556) of the 

 fatty acid i.e., the group next the carboxyl (COOH) group. Thus, 



~H 3 NcH.CH 2 .CH 2 .COOH , 3\ C H.CH.CH.COOH. 

 CH 3 / * ^ <-H 3 / ' | 



Isobutylacetic acid. Leucin. NH 2 



When norleucin, an amino-acid found especially in the proteins of 

 nervous tissue, is termed a-amino-caproic acid, it is indicated that NH 2 

 replaces one H in the aCH 2 group of caproic acid (CH 3 .CH 2 .CH 2 .CH 2 . 

 CH 2 .COOH). The long chemical name of isoleucin (a compound also 

 derived from the proteins of nervous tissue and from some plant proteins} 

 indicates that in propionic acid, 



CH 3 .CH a .COOH, 

 

 NH 2 is introduced into the a group, yielding 



CHg.CH.COOH, 



ft I 

 NH 2 



or amino-propionic acid (alanin); while in the j8 group one H is replaced 

 by a methyl (CH 3 ) group, and another H by an ethyl (G>H 5 ) giouy, 

 yielding finally isonuclein : 



H* 3 \CH.CH.COOH. 

 NH 2 



