REVERSAL BY A SUBSTANCE BENDING THE INHIBITOR 



619 



variations appear when (R^) approaches (I^). Curve D gives the limiting case 

 where the affinity between inhibitor and reversor is very high, so that 

 (I) = (I^) — (R,). Where the affinity is high: 



at) 



1 



io(l - *"/) 



(R,)mM- 



(13-27) 



iOQ 



Fig. 13-6. Reversal of the inhibition by a substance binding the inhibitor for the 

 general case in which the concentration of the free reversor is reduced by this 

 binding, i.e., (R<) = (R) + (RI). The inhibition was calculated from the inhibitor 

 concentrations derived from Eq. 13-26. K^ = 1 mM, (I) = 5 mJil, and (E^) = 1 

 mM. Curve A: X, = 10 mM; curve B: Z^ = 1 mM; curve C: K^ = O.l mM; 



curve D: K- = 0. 



SO that when the inhibition is reduced to 1 1 nth. of the initial level, i.e. 



if= 0.1 V 



I rt.i n — I 



(13-28) 



(R*) 



(I,) 



and thus in this case also the ratio depends on the initial inhibition. 



It is possible to visualize the relationships between inhibitors and re- 

 versors on a pi scale. The representation is the same as that used for 

 showing the relationships between acid-base and oxidation-reduction coup- 

 les (Fig. 13-7). This is possible because one can write the following equa- 

 tions for the reactions of the inhibitor with enzyme and with reversor: 



pi = p^, - log 



pi = pKr - log 



(EI) 

 (E) 



(RI) 

 (R) 



(13-29) 



(13-30) 



