632 13. REVERSAL OF INHIBITION 



EI complex so that less than one-thousandth the concentration of the 

 unsubstituted analog is required for reactivation. This is believed to be 

 due to the availability of the anionic site in TEPP-inhibited cholinesterase, 

 the electrostatic attraction between this group and the N"'"-group facilitat- 

 ing the binding of the reactivator. In cholinesterase inhibited by diisopro- 

 pylfluorophosphate (DFP), the anionic site is not available and the pres- 

 ence of a positively charged atom on the reactivator is not so important. 

 In the second place, the nucleophilic group should be within a certain dis- 

 tance of the positive charge in the reactivator, and perhaps also in a fa- 

 vorable direction relative to the positive charge (if the rest of the pyridine 

 ring is constrained to one orientation on the enzyme surface). The reactivat- 

 ing potency of 2-PAM has been attributed to its ability to react with the 

 enzyme in such a way that the nucleophilic oxygen atom falls about one 

 bond length from and is directed toward the phosphorus atom of the inac- 

 tivated enzyme (Wilson and Ginsburg, 1958) and the differences between 

 the 2-, 3- and 4-aldoximes of iV-methylpyridine have been ascribed to re- 

 lated steric factors (Wilson et at., 1958). 



y\ 0- 



H 



CH, 



The group attached to the pyridine-N+ is also of great importance, as 

 may be seen in the relative potencies given in Table 13-1. This group may 

 either increase binding through its van der Waals interaction with the en- 

 zyme surface or interfere with the binding sterically. The reason for the 

 very high activity of the bis(pyridinium) aldoximes is not clear as yet but 

 it could be the result of the tight binding of these compounds to the enzyme 

 surface in a position that is near optimal for the nucleophilic attack on 

 the phosphorus atom. Comparison of large numbers of reactivators has led 

 to the conclusion that the potency is not correlated witli the ability to 

 inhibit or bind to the normal enzyme, nor with the reactivity toward the 

 organophosphorus inhibitors (Hobbiger et al., 1960). 



The reactivation reactions and kinetics may be expressed in different 

 ways. At least three forms of the enzyme must be assumed in inhibitions 

 by the organophosi^horus compounds: the initial reversible EI complex, 

 the phosphorylated enzyme EX, and the final inactivated enzyme after 

 the phosphate group has been shifted to another amino acid. Reactivation 

 seems to be exerted only on the EX form (Sanderson and Edson, 1959). 

 The rate at which EX passes into the irreversibly inactivated form varies 

 with the inhibitor and the enzyme; in those cases where this reaction is 



