788 PHYSIOLOGY OF DIGESTION AND SECRETION. 



brane of the small intestine (duodenum, jejunum). This something 

 Pawlow supposed is an enzyme, and since its action is on another 

 enzyme, "si ferment of ferments," he designated it as a kinase 

 or enterokinase. The action of the enterokinase is very prompt 

 and decided and was supposed to be specific, but later observers 

 (Delezenne-Zunz) state that an inactive pancreatic secretion 

 may be activated by a number of salts, especially those of calcium 

 and magnesium. The physiological value of this very interesting 

 relation is not clear, but it seems possible that it may serve to 

 protect the living tissues from the powerful digestive action of 

 the trypsin. The other enzymes of the pancreatic juice, the 

 diastase and the lipase, are secreted in part, at least, in active 

 form. 



The Digestive Action of Pancreatic Juice. — The digestive 

 action of the secretion depends upon the three enzymes, trypsin, 

 diastase (amylase), and lipase. The specific effects of each may 

 be considered separately. 



Action oj Trypsin. — The activated trypsinogen causes hydrolytic 

 cleavage of the protein molecule in a manner analogous to that 

 described for pepsin. Its action differs from that of pepsin, however, 

 in several respects. It attacks the protein in aeutral as well as in 

 slightly acid or markedly alkaline solutions. Its effect upon the 

 protein is more rapid and powerful than that of pepsin and the 

 protein molecule is broken up more completely. As was said in 

 describing the action of pepsin, it and the trjrpsin really act in 

 series — the change begun by the pepsin is completed by the tryp- 

 sin. The preUminary action of the pepsin not only hastens that of 

 the trypsin, but to some extent alters it; a protein submitted first 

 to pepsin and then to trypsin is more completely broken up than if 

 the tryjDsin acted alone. The steps in the hydrolysis of the protein 

 molecule by trypsin have been the subject of a very great amount of 

 study, and views as to the details have changed somewhat from 

 time to time. It would seem that the trypsin, like the pepsin, 

 hydrolyzes the simple proteins first to a proteose, and then to a pep- 

 tone stage, but the latter product may be split still further into a 

 variety of simpler bodies, the number and character of which de- 

 pend on the amount of trypsin and the time that it acts. After 

 a prolonged pancreatic digestion no peptone or peptone-like 

 body can be found; in fact, no substance which gives a biuret reac- 

 tion. Under such conditions the protein molecule is broken up very 

 completely into a great number of smaller molecules, many of 

 which have been identified, while some have as yet escaped de- 

 tection so far as their chemical structure is concerned. The actual 

 products formed depend on the length of time the trypsin is allowed 



