CH. XXXIV.] COMPOSITION OF PANCREATIC JUICE 519 



magnesium. The alkalinity of the juice is due to phosphates and car- 

 bonates, especially of sodium. 



1. Action of Trypsin. Trypsin acts like pepsin, but with certain 

 differences, which are as follows : 



(a) It acts in an alkaline, pepsin in an acid medium. 



(&) It acts more rapidly than pepsin ; deutero-proteoses can be 

 detected as intermediate products in the formation of peptone ; the 

 primary proteoses have not been detected. 



(c) Alkali-meta-protein is formed in place of the acid-meta- 

 protein of gastric digestion. 



(d) It acts more powerfully on certain proteins (such as elastin) 

 which are difficult of digestion in gastric juice. It does not, however, 

 digest collagen. 



(e) Acting on solid proteins such as fibrin, it eats them away from 

 the surface to the interior ; there is no preliminary swelling as in 

 gastric digestion. 



(/) Trypsin acts further than pepsin, and rapidly splits up the 

 proteose and peptone which have left the stomach into simpler sub- 

 stances, the polypeptides. The polypeptides in their turn are resolved 

 into their constituent amino-acids, such as leucine, tyrosine, alanine, 

 aspartic acid, glutamic acid, arginine, tryptophane, and many others. 

 The constitution and properties of these cleavage products are 

 described on pp. 417 to 422. In addition to these there is a certain 

 amount of ammonia. The red colour which a tryptic digest strikes 

 with chlorine or bromine water is due to the presence of tryptophane 

 (indole-amino-propionic acid). 



When once the peptone stage is passed, the products of further- 

 cleavage no longer give the biuret reaction ; hence they are frequently 

 termed abiuretic. 



A variable fraction of the protein molecule is broken off with 

 comparative ease, so that certain free amino-acids appear in the 

 mixture, at a time when the remainder are still linked together as 

 polypeptides. But ultimately the whole molecule is resolved into 

 amino-acids, either entirely separated or in very short polypeptide 

 linkages. 



It will thus be seen that there are two important differences 

 between pepsin and trypsin ; one is a difference of degree, trypsin 

 being by far the more powerful and rapid catalyst; the second is a 

 difference of kind, pepsin not being able to cleave polypeptides into 

 amino-acids in the way trypsin can. The preliminary action of 

 pepsin, however, is beneficial, for trypsin cleavage occurs more readily 

 after pepsin has acted on a protein. 



2. Action of Amylopsin. The conversion of starch into maltose 

 is the most rapid of all the actions of the pancreatic juice. Its power 

 in this direction is much greater than that of saliva, and it will act 



