626 13. REVERSAL OF INHIBITION 



in the medium) and in turn oxidized tlie enzyme sulfhydryl groups. In ni- 

 trogen, this reaction does not occur. Ghitathione, for example, in air is an 

 inhibitor of succinoxidase by way of its GS-SG form, while simultaneously 

 it will remove the p-CMB from the enzyme. It might be suggested that ideally 

 such reversal experiments with thiols be conducted in nitrogen. Some of the 

 failures noted above could well be due to this phenomenon. 



Protection studies, where the substance binding the inhibitor is added 

 either before or with the inhibitor, are of even less significance in pro- 

 viding useful or reliable information about the inhibition. If a substance 

 reacts with an inhibitor, it will reduce the concentration of free inhibitor, 

 thereby reducing the rate at which inhibition occurs and decreasing the 

 final inhibition achieved. Results of this type showing protection only 

 confirm the obvious and have no relationship to the mechanism of the inhibi- 

 tion. The only interesting thing that might emerge from protection studies 

 is the demonstration that a substance known to bind or react with the inhi- 

 bitor does not influence the inhibition in the expected manner, since in such 

 a case it might indicate that the reactive group or groups on the inhibitor, 

 with respect to the inhibition, were not those involved in the reaction with 

 the reversor. The effects of metal chelators on enzymes or metabolism 

 might profitably be compared with the effects of the metal ion-chelator 

 complexes, in order to determine if the inhibition is indeed through che- 

 lation of an enzymically important metal ion or, on the other hand, is an 

 action dependent on the total structure of the molecule and unrelated to the 

 chelating activity. 



The preceding pessimistic remarks are meant to apply to basic experi- 

 mental studies on enzyme inhibition. Information on protection and re- 

 versal can often be of great pratical importance, particularly as the actions 

 of many poisons and the toxic effects of certain drugs are the result of en- 

 zyme inhibitions. It is also not implied that accurate quantitative work 

 on inhibition reversal is valueless; indeed, some very interesting facts 

 might be derived from such work where reversal rates are measured. Finally, 

 protection or reversal by substances binding the inhibitor has been consid- 

 ered here, and other means of modifying the inhibition may be very infor- 

 mative, as will be seen in the following sections. 



SPONTANEOUS REACTIVATION OF INHIBITED 

 ENZYMES 



Occasionally it has been observed that the inhibition in pure enzyme 

 systems is not maintained but is slowly lost. The inhibition rises to a max- 

 imal value and then declines with time. This usually means that either 

 the free inhibitor is unstable and is inactivated in solution, or the bound 

 inhibitor is reacted by the enzyme. The former nonenzymic inactivation of 



