METABOLISM 83 



the starch in any strong acid and produce the same 

 change. In this case, however, a strong reagent 

 and a high degree of heat are necessary. The mildly 

 alkaline saliva at ordinary temperature thus seems 

 to be able to accomplish the same result with a 

 minute fraction of the expenditure necessary in 

 the second experiment. 



The prompt and effective action of the saliva is 

 due to the presence therein of a minute quantity of 

 a substance called ptyalin, one of a large group of 

 somewhat similar substances which we call enzymes. 

 A similar enzyme, called pepsin, splits up proteins 

 in the stomach ; another, lipase, splits fats, in various 

 parts of the body; another clots blood, and so on. 

 None of these enzymes has been isolated in a pure 

 state, and we know little of their composition, al- 

 though it has been surmised that they are allied to 

 the proteins. They are certainly not alive in the 

 usual sense of the word, for they may be precipitated 

 without injury with absolute alcohol and other 

 reagents. They are, however, destroyed by boiling 

 and do not " work " well below a certain minimum 

 temperature. Most of them need a special environ- 

 ment (e.g. hydrochloric acid for pepsin) to work 

 best, and each of them is specific in its action; 

 that is, affects only one kind of substance. The 

 most remarkable thing about them, however, is 

 that although a minute quantity will produce a 

 relatively enormous effect, the enzyme does not ap- 

 pear to be used up at all, and the process may go on 

 indefinitely, provided the products of its action are 



