DETERMINATION OF CONSTANTS OTHER THAN iiCj 



181 



DETERMINATION OF CONSTANTS OTHER THAN K, 



Most inhibitions may be adequately characterized by two types of con- 

 stants: dissociation constants indicating the binding energy and interaction 

 constants describing the effects of the bound inhibitor on either the binding 

 of the components of the reaction or the rate at which the active complex 

 breaks down to products. It is obvious that the inhibition is not quanti- 

 tatively expressed by K^ alone but that the effect of the inhibitor on the 

 reaction must also be evaluated. These effects and the values of the inter- 

 action constants are implied when the inhibition is stated to be completely 

 competitive or noncompetitive but insufficient attention has been paid to 

 situations where the inhibition is partial, mixed, or more complex. 



Determination of Interaction Constants 



The general reaction scheme 3-2 for an enzyme not involving activators 

 or coenzymes presents an inhibition characterized by three constants 

 — a, /3 and K^ — and is dependent also on the uninhibited enzyme con- 

 stants Vy„ and Kg. If the inhibition is not of the simple types where a = 1 

 or 00 or where /? = 1 or 0, it is possible to determine these constants by 

 the usual graphical procedures. In the general case, the slopes and inter- 

 cepts of the type A double-reciprocal plot are given by: 



Slope 



aK. 



IjVi Intercept 



1/(S) Intercept 



(5-25) 



(5-26) 



(5-27) 



(5-28) 



From these equations, or comparable ones from other types of plotting, or 

 from combinations of different plots, it is possible to determine a, /?, and 

 K^. For example, determination of the slopes and 1/Vj intercepts for two 

 different concentrations of inhibitor will lead to a value for a: 



1 



(5-29) 



from which ^ and K^ may also be calculated. If the inhibition is obviously 

 of a type in which either or or /5 assume extreme values, the calculations 



