METABOLISM 85 



work within the cell and others without. The idea 

 that the action of enzymes is physical and mechani- 

 cal is strengthened by the fact that finely divided 

 platinum (platinum black) will produce catalytic 

 effects similar to those produced by oxidizing en- 

 zymes. 



The most wonderful feature of all, perhaps, is 

 the fact that the protoplasm makes its own enzymes, 

 since they are, of course, the secreted products of 

 the activity of the living substance, developed in 

 just the places and apparently at just the time when 

 needed. 



Since the action of enzymes is mechanical, the 

 question has often arisen, How is it controlled ? 

 Why, for example, does not the stomach digest 

 itself? We hardly know enough about enzymes to 

 answer such a question in detail, but we have learned 

 that many enzymes when produced are incapable of 

 performing their offices until supplemented by 

 another element, usually produced in a different 

 region. The pancreatic secretion has no power to 

 digest proteins until it has been " activated " by 

 the secretion of the lining wall of the intestine, a 

 secretion induced by the flow of acid liquid from 

 the stomach. The action is due to the presence of 

 a complementary body, enterokinase, which ap- 

 parently combines with the zymogen (or enzyme- 

 former), called trypsinogen, to form trypsin, the 

 proteid-cleaving enzyme of the pancreatic secretion. 

 Similarly it has been discovered that the muscles 

 cannot reduce the glycogen which is necessary as 



