106 3. KINETICS OF ENZYME INHIBITION 



the formation of EI would depend on the particular ES involved. The rate 

 equation is the same but {l)KJKj^ is replaced with {\)jK- , since K- = 

 KJK^. However, inasmuch as the slopes of the ljv^ — 1(S) plots would be: 



Classic competition: s = 1 + 



Reaction with ES: 



VJ8) I ' K, 

 K, 



F™(S) 





(3-112) 



(3-113) 



and since K/K^ — K^, it would appear that K^ is determined in either case. 

 Also there is evidence that benzoate does combine with the free enzyme 

 in the absence of substrate. We shall see there are many reasons why the 

 experimentally determined inhibitor constant is not necessarily the true 

 dissociation constant; the reason in the present case is unknown. 



The Inhibition Index 



This term is conducive to confusion since it has been defined according 

 to two unrelated concepts. One definition arose in the field of competitive 

 antagonism of metabolites as applied to growth inhibition in microorgan- 

 isms. Mcllwain (1942) defined the " antibacterial index " as (I)/(M) where 

 (I) is the concentration of inhibitor that is just bacteriostatic in the presence 

 of metabolite at a concentration (M), a metabolite here being either a 

 substrate or cofactor. Shive and Macow (1946) related this more specifically 

 to competitive enzyme inhibition and defined the index as the ratio (I)/(S) 

 when maximal inhibition occurs. Later workers have tended to call this 

 ratio the " index of inhibition " (Albert, 1960, p. 50) or " inhibition index " 

 (Ariens et al. 1955) and it has been generally assumed that this ratio remains 

 constant for any degree of inhibition. Sometimes the inhibition index re- 

 fers to the ratio (I)/(S) for which 50% inhibition is observed. Albert stated: 

 " For each pair of substances there will be a unique index of inhibition, 

 which is defined as the ratio of the number of molecules of analogue per 

 molecule of metabolite required to give 50 per cent inhibition." One of the 

 most frequent errors encountered in work on antagonism and enzyme 

 inhibition is the belief that this ratio or inhibition index is constant. In 

 fact, it is often stated that (I)/(S) = KJKg. It is obvious that this is not 

 generally so and that the true expressions are: 



(3-114) 

 (3-115) 



