EXPRESSION AND INTEEPRETATION OF INHIBITION 107 



This ratio will thus depend on (S) and is not a constant, as pointed out by 

 Quesnel (1956). but will approach KJK, at high substrate concentrations 

 (generally greater than 5Kg) when the enzyme would be mainly saturated, 

 a fact pointed out by Shive and Macow for the antibacterial index. Figure 

 3-21 illustrates the marked changes in (I)/(S) with substrate concentration. 

 In fact, it is possible by plotting (I)/(S) against 1/(S) for any chosen degree 

 of inhibition to obtain values for K^andK^, as can be seen fromEq. 3-114. 

 It may also be noticed that even at higher substrate concentrations, (I)/(S) 

 at 50% inhibition will equal KjIK,,, in the general case and may not relate 

 to the relative affinities of substrate and inhibitor for the enzyme. 



1.0 — 



0.8 



0.6 



0.4 



0.2 



0.5 





0.01 0.1 I '""iM 



(S ) ^ 



Fig. 3-21. Variation of [(l)/(S)]o.5 '^^'ith the substrate concentration for competitive 

 inhibition. K,= I mM and X, = 0.1 milf . 



An entirely different definition of the inhibition index was made by 

 Kistiakowsky and Shaw (1953 a) to facilitate expression of complex inhi- 

 bition behavior. 



V — Vi i i 



Vi 1 — i a 



(3-116) 



where v is uninhibited rate, v^ the inhibited rate, i the fractional inhibition, 

 and a the fractional activity. The inhibition index here is not a constant 

 but another way of expressing the degree of inhibition. 



Noncompetitive inhibition: rp = {1)1 Ki = (!') (3-117) 



Competitive inhibition: <p = ^^^—j = ~k) ^^'^^^^ 



