92 THE PHILOSOPHY OF BIOLOGY 



element, or compound which cannot act as a 

 catalyser." * 



Enzymes, then, are agents which are produced by 

 the organism, and which act by influencing (accelerat- 

 ing or retarding) chemical reactions. An enzyme, as 

 such, need not exist in a tissue; it is there as a zymogen, 

 a substance which may become an enzyme when 

 required. An enzyme need not be active : it may be 

 necessary that it should be " activated " by a kinase, 

 another substance produced at the same time. Asso- 

 ciated with many enzymes are anti-enzymes, substances 

 which undo what their corresponding enzymes have 

 done. Finally some, perhaps most, enzymes are re- 

 versible, that is, if they produce a change in a certain 

 substance they can also produce the opposite kind of 

 change : the meaning of this will become clearer a 

 little later on. We have spoken of enzymes as 

 ' agents " or " substances," but it is not at all certain 

 that they are definite chemical compounds. In the 

 preparation of an enzyme what the bio-chemist obtains 

 is a liquid, a glycerine or other extract which possesses 

 catalytic properties. An actual catalytic substance, 

 like platinum black, cannot be obtained from this 

 liquid. A white powder may be obtained, but this 

 usually proves to be proteid in composition ; it is not 

 the actual enzyme itself but is the impurity associated 

 with the latter. Now the very great number of 

 enzymes " isolated " by the physiologists has rather 

 destroyed the original simplicity of the idea of enzyme 

 activity and suggests a parallel statement to that made 

 by Ostwald about catalysts : any tissue substance 

 may influence the reactions that may possibly occur in 



1 A statement of interest in view of the enormous number of " ferments " 

 or enzymes discovered by physiologists. It would appear that any tissue in 

 any organism is capable of yielding an enzyme to modern investigation. 



