252 DIGESTION 



B. THE PROTEOLYTIC ENZYME, TRYPSIN, 



is distinguished from pepsin mainly by the fact that it digests proteids in 

 an alkaline medium. Purkinje and Pappenheim as early as 1836, and Cl. 

 Bernard later alluded to the proteolytic action of the pancreatic juice, but 

 Corvisart (1857) must be looked upon as its real discoverer. Later Kiihne, 

 especially, did large service for our knowledge of this enzyme. 



Trypsin as such does not occur in the pancreas, but instead a zymogen, 

 which, like those of the other enzymes, is more resistant toward all kinds 

 of injurious agents than the enzyme itself. But even the secreted juice does 

 not contain any trypsin and is entirely without effect on proteid, if it is not 

 first activated by an enzyme, called enterokinase (Pawlow), found in the 

 intestinal juice. The formation of trypsin from its zymogen presupposes 

 therefore the presence of this special enzyme, and according to Delezenne, 

 Popielski, Bayliss and Starling, there is no other means of bringing about 

 this change. (The unactivated secretion, nevertheless, will digest boiled 

 fibrin and casein, though very slowly.) 



Opposed to these observations however are others according to which a 

 powerfully active extract is obtained, if, for example, the gland be allowed to 

 lie twenty-four hours before extraction. Hekma is of the opinion that this is 

 a case of bacterial action, since with antiseptic fluids no formation of trypsin 

 could be observed. 



According to Schiff and Herzen, the spleen may have much to do with the 

 formation of trypsin, since addition of splenic infusion or of splenic venous 

 blood activates the pancreatic extract. This in Herzen's opinion is due to an 

 internal secretion of the spleen. 



The cleavage of proteids by trypsin goes on in the same way as that pro- 

 duced by pepsin, saving only that the end products are formed in less time 

 in tryptic than in peptic digestion. However, hydrolytic cleavage of proteid 

 may be carried further, if peptic digestion precedes the tryptic digestion 

 (Giirber). Siegfried finds two peptones (C 10 H 17 N 3 5 , CnH^NsO^ molecular 

 weights, 259 and 273 respectively), and Fischer and Abderhalden find a more 

 complex residue containing all the monamino acids, which stubbornly resist 

 further cleavage with trypsin. 



Trypsin also dissolves gelatin, elastic substance and structureless membranes ; 

 likewise the gelatin-forming tissues, if they first be treated with acids or warmed 

 to 90 C. Bokai states that trypsin does not act upon the nucleins; but after 

 autodigestion of the pancreas, Kutscher found xanthin, hypoxanthin, and guanin 

 just the cleavage products of nucleic acids. Blood serum and serum globulin 

 are not attacked by trypsin, although both are digested without difficulty by 

 gastric juice. 



The pancreatic juice of many mammals (human pancreatic juice uncertain) 

 also coagulates milk, and according to Vernon the action is due to a special 

 enzyme. Instead of paracasein, however, the clot contains a substance known 

 as metacasein, which may represent a product of tryptic digestion of casein 

 (Roberts). 





