REVERSAL BY A SUBSTANCE BINDING THE INHIBITOR 615 



the acetylcholine were taken in 10-15 min whereas equilibration of the sub- 

 strate with the free enzyme occurred in a matter of seconds probably. 



REVERSAL OF INHIBITION BY A SUBSTANCE BINDING 



THE INHIBITOR 



The presence of a substance having an affinity for the inhibitor and 

 which by reacting with the inhibitor prevents its binding to the enzyme 

 will protect the enzyme, both slowing the development of the inhibition and 

 usually reducing the final equilibrium inhibition. Such a substance will 

 also reverse an established inhibition if the EI complex is dissociable. The 

 outstanding example of this type of protection and reversal is the antag- 

 onism exerted by sulfhydryl compounds (such as cysteine, glutathione, and 

 especially dimercaprol or BAL) on inhibitions produced by arsenicals, mer- 

 curials, and certain other heavy metal ions. The inhibitor}^ effects of some 

 metal ions can also be reduced by appropriate chelating agents, such as 

 ethylenediaminetetraacetate. Advantage has has been taken of this prin- 

 ciple in the clinical treatment of poisonings by heavy metals. Actually, 

 for many years the treatment of cyanide poisoning has often involved the 

 administration of a substance (such as a nitrite) that would cause the for- 

 mation of methemoglobin, which has a high affinity for cyanide and can 

 partially remove it from the tissues and its combination with cytochrome 

 oxidase. These examples cited illustrate such protection and reversal phe- 

 nomena in a clear-cut form and the agents mentioned are frequently used 

 experimentally in the study of enzyme inhibition, but it must also be 

 remembered that less striking instances of inhibitor binding occur frequently 

 and may go unnoticed. Many inhibitors — fluoride, malonate, parapy- 

 rvivate, pyrophosphate, and arsenate are a few — form complexes with 

 ions, such as calcium or magnesium, commonly present in the media, and 

 most inhibitors react to varying extents with any nonenzymic proteins in 

 the enzyme preparation. In studies with cellular preparations the possibili- 

 ties for nonenzymic binding of the inhil^itor become so great that care 

 must be exercised in the interpretation of kinetic data. It is important to 

 realize that the substance binding the inhibitor need not have a greater 

 affinity than the enzyme for the inhibitor to produce a significant effect. 



Equilibrium Inhibitions in the Presence of a Substance Binding the Inhibitor 



It will be necessary to consider final equilibrium states before rates of 

 inhibition and reversal are treated. The reactions involved are: 



E + S ^ ES ^ E + P A', = (E)(S)/(ES) 



E + I — EI K,= (E)(I)/(EI) 



R + I ^ RI K, = (R)(I)/(RI) 



