EXPRESSION AND INTERPRETATION OF INHIBITION 



105 



K,JKi axis was interpreted to indicate the value of KJK^ since at this 

 point V and hence ko would be zero. A similar method was suggested by 

 Slater (1955) and the ensuing discussions of this procedure and its limita- 

 tions should be consulted (Slater, 1955, pp. 301-308). The important point 

 is that KyJK^ is frequently meaningless and varies widely with the con- 

 ditions of the experiment. The affinity ratio KJE^ is however interpre- 

 table thermodynamically and from it can be calculated the difference in 

 free energy of binding of substrate and inhibitor. 



Reaction of the inhibitor with the substrate or with the enzyme-coenzyme 

 complex leads to types of inhibition superficially competitive. If such me- 

 chanisms are not recognized, the determined K/s will bear no physical 

 relationship to the KJs and the ratio KJK^ is meaningless. In partially 

 competitive and mixed inhibitions, K^ is altered by the binding of substrate 

 to the same degree that Kg is altered by inhibitor; the determined K^ may 

 thus not represent the true dissociation constant of the EI complex if 

 certain methods of evaluating the constants are used (see Chapter 5). 



Variation of K, with the Nature of the Substrate in Competitive Inhibition 



According to the classic formulation of competitive inhibition, the value 

 of Ki, as determined by the various graphical procedures, should be inde- 

 pendent of the substrate used for a particular enzyme. However, Klein 

 (1960) found that in the inhibition of pig kidney D-amino acid oxidase by 

 benzoate, determinations of K^ from \li\ — 1(S) plots led to different 

 values for each substrate (see tabulation). Furthermore, there appears to 



be no correlation between K^ and either F„; or K^. One possible explana- 

 tion for this behavior was suggested. If the inhibitor reacts only with the 

 ES complex to release S: 



ES + 1 — EI +S 



K,- 



(ES)(I) 

 (EI)(S) 



(3-111) 



