DISSOCIATION or THE ENZYME-INHIBITION COMPLEX 607 



phase. Reversal of the changes brought about by inhibitors in the complex 

 metabolic and functional processes of cells and tissues will be examined aft- 

 er treatment of the simpler reactivations observed with isolated enzymes. 



DISSOCIATION OF THE ENZYME-INHIBITOR COMPLEX 



Reversal of mhibition, either of enzymes in solution or within cellular 

 preparations, can usually be achieved by the removal of the inhibitor from 

 the medium in which the enzymes or cells are studied. If the binding of the 

 inhibitor to the enzyme is reversible, the inhibitor will dissociate from the 

 enzyme and diffuse away along a concentration gradient, the enzyme ac- 

 tivity progressively increasing. The rate of the reversal will depend on how 

 effectively the concentration of free inhibitor can be maintained at a low 

 level and on the rate constant for the dissociation of the EI complex. 



Dissociation of the El Complex When Free Inhibitor Concentration Is Zero 

 The rate of dissociation of the EI complex, according to the reaction: 



EI ^ E -^ 1 (13-1) 



k. 



can be expressed as: 



dm) 

 dt 



A-_i(EI) (13-2) 



if by some means the concentration of free inhibitor is maintained at zero. 

 Since (EI) = i(E,), this may also be written as: 



dt 

 which is directly integratable to: 



di 

 -r = - k_,i (13-3) 



(13-4) 



where i^ is the initial inhibition before reversal begins. Thus if log i is 

 plotted against time, one will obtain a straight line from the slope of which 

 the rate constant may be determined. 



Occasionally it is possible to calculate the rate constant for the com- 

 bination of the inhibitor with the enzyme even though the reaction cannot 

 be followed directly. If K^ is found from the usual graphical procedures, 

 a determination of k_j^ as described above will provide a value for the as- 

 sociation rate constant since ^^ = k_JK^. The reversal of inhibition is of- 

 ten more readily measured than the development of inhibition because 



