BIOLOGICAL ASPECTS OF ENZYME STUDIES 95 



their effect upon nuclear as upon cytoplasmic systems. 



3. In many cases it is possible to show a decline in the 

 concentration of active enzyme in a particular tissue, 

 subsequent to the administration of a drug. But usu- 

 ally this effect can only be demonstrated after the 

 lapse of an interval from the time of administration of 

 the drug. In such circumstances it is difficult to be 

 sure whether the inactivation is produced by direct 

 action of the drug upon the enzyme, or is an indirect 

 effect ensuing from the action of the drug on some 

 other enzyme system. 



4. When the action of a given substance is studied on the 

 enzyme systems of different organs, it is often found 

 that the main effect appears to be selective for different 

 enzyme systems in different organs. 



5. In many cases some doubt as to an interpretation in 

 terms of enzyme activity appears because a similar 

 physiological effect can be produced by a mechanism 

 which is primarily non-enzymic in its action. Thus 

 vesication, which is produced by substances which 

 are strong enzyme poisons, is also produced by fric- 

 tion, by heat and by cold. 



6. If we attempt a straight-forward application of the 

 results of enzyme studies, we should be tempted to 

 say that drugs acting on the same enzyme system must 

 produce the same effect. But this is seldom true. For 

 example, vesicant substances all appear to poison 

 hexokinase under certain conditions, and we might 



