ENZYMES AND THEIR ACTION 199 



lase " has been reported as occurring in artichoke tubers; " pro- 

 lipase," in castor beans; " proinvertase," in several species of 

 fungi; and, probably, " prooxidase," in tobacco leaves. In the 

 case of the last-named zymogen, it has been observed that after 

 the zymogen has been once activated, as in response to the need 

 for increased activity due to the entrance of the germs of certain 

 leaf-diseases, it can once again produce a second supply of the 

 enzyme, but the process cannot again be repeated. 



Calcium salts, or very dilute acids, are usually energetic acti- 

 vators of proenzymes. 



PHYSIOLOGICAL USES OF ENZYMES 



There can be no doubt that enzymes exert a tremendously 

 important influence in vital phenomena, by determining the rate 

 at which the chemical changes which are involved in these phe- 

 nomena shall proceed. S nee they do not initiate reactions, and 

 since they may catalyze reversible reactions in either direction, it 

 cannot be said that they determine the type of reactions which 

 will take place in any given mass of protoplasm; but, undoubtedly, 

 they do exert a determining influence upon the rate at which the 

 reaction will proceed, after the protoplasmic activity has deter- 

 mined the direction in which it shall go. 



Without the intervention of these catalyzing agents, it would be 

 impossible for reactions between these non-ionized organic com- 

 ponents of the cell contents to come to completion with anything 

 like the marvelous rapidity with which these changes must take 

 place in order to permit the organism to grew, to perform its neces- 

 sary vital functions, or to adjust itself to the changes in its environ- 

 mental conditions. 



Since the number of different reactions which take place 

 within a living cell is very great, and since these chemical changes 

 are extremely variable in type, it follows that the number of dif- 

 ferent enzymes which must exist in either a plant or an animal 

 organism is likewise very large. For example, fourteen different 

 enzymes have been isolated from the digestive system, and at 

 least sixteen from the liver, of animals. They are ,universally 

 present in living protoplasm of every kind, from the most minute 

 bacterium to the largest forest trees, in the plant kingdom; and 

 from the amoeba to the whale, in animals. 



