634 13. EEVERSAL OF INHIBITION 



and both spontaneous and induced reactivation: 



i = i^e^-k-k-<n)}t (13-53) 



If log i is plotted against t, the slope will provide a means for calculating 

 the rate constants. 



Plots of this type were made for the reactivation of acetylcholinesterase 

 carbamylated by dimethylcarbamylcholine and dimethylcarbamylfluoride. 

 These inhibitors react with the enzyme active center in a manner similar 

 to the organophosphorus compounds (Wilson et at., 1960) and the EX 

 form is the same for both the inhibitors. Hydroxylamine reactivates but 

 2-PAM and other agents effective against the enzyme inhibited by the 

 organophosphorus inhibitors do not. The plots of log i against t are shown 

 in Fig. 13-11 for both the spontaneous reactivation and that induced by 

 hydroxylamine. The monomolecular spontaneous reactivation rate constant, 

 k, was found to be 0.032 min~^. 



If the reactivators form a complex with the inactivated enzyme: 



EX + R ^ EXR ^ E -^ XR (13-54) 



the kinetics of reactivation would be expected to conform to ordinary en- 

 zyme kinetics (Green and Smith, 1958 a, b). The rate at which the reacti- 

 vation proceeds will be given by: 



^•2(EX,)(R) 



Vt = --^^ ^ — (13-55) 



(R) + K,r 



where (EX,) = (EX) + (EXR) and K^^ is the Michaelis constant, which 

 in the general case is equal to {k_-y + ko)lki. The relative values of the con- 

 stants are not known and hence it is not certain if K^.^ is ever a true disso- 

 ciation constant. The reactivation has been shown to follow first-order 

 kinetics when the reactivator is in excess, and the release of phosphorus 

 from the enzyme has been shown to accompany the reduction in the inhi- 

 bition (Jandorf et at., 1955). 



It is of interest now to express the steady-state inhibition in a system 

 in which a reactivator is present. The following reactions may be written: 



E + I ^ EI 4 EX (13-56) 



EX + R ^ E + XR (13-57) 



