108 PHYSIOLOGICAL ACTIVITIES 



3. They produce very great chemical changes without 

 themselves being appreciably affected. 



Enzymes will not continue to act indefinitely but are 

 used up in the process (combination with products). The 

 amount of change is so great in proportion to the amount of 

 enzyme that the above statement is justified in the rela- 

 tive sense. Thus a milk-curdling enzyme has been prepared 

 that would precipitate 100,000,000 times its own weight of 

 caseinogen. 



4. Their action is specific in that each enzyme acts on 

 one kind of chemical substance only, and the products are 

 always the same. 



The substance may be combined with a variety of other 

 chemical substances so that the action appears to be on 

 several, but in reality it is on a definite group of molecules 

 in each instance. For example, emulsin attacks several 

 different glucosides but always sets free dextrose from them. 



5. The action is inhibited and eventually stopped, and in 

 some cases the enzj^me is destroyed by an accumulation of 

 the products of the action. If the products are removed the 

 action will continue if the enzyme is not destroyed. This 

 effect is explained partly because the enzyme probably 

 combines with some of the products, since it does not act 

 indefinitely, and partly because of the reversibility of the 

 reaction. 



6. Like many chemical reactions those of enzymes are 

 reversible, that is, the substance broken up may be reformed 

 by it from the products produced in many instances. Thus: 



maltose + maltase ^ glucose + glucose + maltase. 

 fat + lipase ^ glycerin + fatty acid + lipase. 



7. The presence of certain mineral salts seems to be 

 essential for their action. These and other substances which 

 are necessary are sometimes called co-eiizymes. A salt of 

 calcium is most favorable for a great many. 



8. They may be adsorbed like other colloids by " shaking 

 out" with finely divided suspensions Hke charcoal or kaolin, 

 or by other colloids like aluminum hydroxide or proteins. 



