606 13. REVERSAL OF INHIBITION 



WAYS IN WHICH REVERSAL OF INHIBITION MAY BE 



ACHIEVED 



Reactivation of an inhibited enzyme may occur spontaneously or can 

 be brought about by a variety of techniques. Some of the conditions allow- 

 ing or favoring restoration of enzyme activity are listed here and later will 

 be discussed in greater detail. 



A. Replacement of the medium bathing the enzyme or cells with medium 

 free of the inhibitor, or removal of the inhibitor by dialysis. 



B. Introduction of a substance that binds the inhibitor strongly and 

 removes it from the enzyme. 



C. Displacement of the inhibitor from the enzyme with increased con- 

 centrations of the substrate, in case the inhibition is competitive. 



D. Spontaneous reactivation brought about by the chemical alteration 

 and si^litting off" of the inhibitor through the catalytic action of the enzyme 

 that is inhibited (in such cases the inhibitor is often structurally related 

 to the substrate so that the reversal occurs by a reaction similar to that 

 undergone by the substrate but much slower). 



E. Introduction of a substance that reacts with the EI complex to fa- 

 cilitate the reaction and removal of the inhibitor. 



F. Metabolism of the inhibitor by enzymes in the preparation or spon- 

 taneous inactivation of an unstable inhibitor. 



G. Addition of a cofactor or coenzyme in those cases where the normally 

 present cofactor or coenzyme has been inactivated by the inhibitor. 



H. Redistribution of the inhibitor from the enzyme to other binding sites 

 which do not pick up the inhibitor rapidly but yet possess a high affinity 

 for it. 



/. Synthesis of new enzyme in cells where the inhibition has been truly 

 irreversible and the enzyme permanently inactivated. 



There are, of course, other ways in which inhibition can be reversed but they 

 are usually relevant to special cases only and will be discussed in relation 

 to the inhibitors exhibiting such behavior. For example, the inhibition of 

 cytochrome oxidase and respiration by carbon monoxide can be abolished 

 by light which dissociates the EI complex. Volatile inhibitors, such as hy- 

 drocyanic acid, can be removed from the enzymes by bubbling a gas mix- 

 ture through the medium; indeed, in work with cyanide, care must te tak- 

 en so that the concentration of the inhibitor does not decrease during the 

 experimental period due to the escape of hydrocyanic acid from the liquid 



