630 13. REVERSAL OF INHIBITION 



The steady-state concentration of the complex EI may also be similarly 

 determined: 



(i)(E,) r k, 



(I) + K, [ k, + kS + K.liD] 



(EI). = -j^^^ [l - ,. , , ,r, .. .... I (13-46) 



The total inhibition, given by i= [(EX) + (EI)]/(E,), is thus: 

 • - (I) Ft ^2 ] 



'" ~ (I) + K, [ + h[a)iK,] + k,[i + (i)/^,]J ^^^"^^^ 



This is the total inhibition in the steady state. A slightly different treatment 

 of this system has been given by Wilson et al. (1960) for the carbamylation 

 of cholinesterase and the rate constants for the inhibitors were calculated. 



INDUCED REACTIVATION OF INHIBITED ENZYMES 



Inhibitions resulting from a substratelike chemical reaction of a sub- 

 stance with the enzyme active site, in which the breakdown of the EI 

 complex is slow as described in the previous section, may be reversed by 

 compounds reacting with the EI complex and splitting the inhibiting group 

 from the enzyme.* The normal substrates for cholinesterase form acyl- 

 enzyme intermediates which react with water to complete the hydrolysis; 

 the inhibitors form instead alkylphosphoryl-enzyme complexes which react 

 very slowly or not at all with water. The introduction of a substance that 

 reacts with the EI complex as does water, but much more readily, may split 

 the phosphoryl group from the enzyme and restore the activity. Since 

 the phosphorus atom is electrophilic, it was postulated by Wilson (1951) 

 that strongly nucleophilic gi'oups might attack the phosphorus and disso- 

 ciate it from the enzyme, and it was indeed found that hydroxylamine would 

 reactivate the acetylcholinesterase of electric tissue after inhibition by tet- 

 raethylpyrophosphate (TEPP). Since then many more effective reacti- 

 vators have been found and most contain the nucleophilic hydroxamate 

 (R-CO-NHO-) or aldoxime (R-CH=NOH) groups. The reaction with 

 an hydroxamate may be written as: 



O E O OR 



Hi I! I 



R-C-NHO- + P-=0 + H+ ^ R-C-XH-0-P=0 + EH (13-48) 



1 1 



(OR)^ OR 



* The inhibited enzyme in such cases will be designated by EI although strictly 

 speaking this is inaccurate, since the inhibitor is chemically modified upon reaction 

 with the enzyme. The inhibiting group on the enzyme is only part of the original 

 inhibitor molecule; in the case of the organophosphorus compounds, the X-group 

 has been lost. However, this simplification should not introduce any confusion into 

 the treatment. 



