GENERALIZED KINETICS OF REVERSIBLE INHIBITION 55 



tions of the substrate and inhibitor, and the relative affinities of the enzyme 

 for each. Thus the degree of inhibition will depend on (S) and K^. 



Noncompetitive inhibition is assumed to involve reaction of the inhi- 

 bitor with a region other than the active center so that combination of the 

 substrate with the substrate site is unaffected but the breakdown of the 

 ES complex is prevented. In this case the degree of inhibition will not depend 

 on (S) or Kg but only on (I) and K^, the inhibitor dissociation constant. 



In practice these two types of inhibition are distinguished by the char- 

 acteristics of the various kinetic plots of experimental data, as explained 

 later. It should be pointed out that there is often a discrepancy between 

 the kinetic and the spatial concepts of competitive and noncompetitive 

 inhibition. Many inhibitions designated as noncompetitive are actually 

 not according to the definition above. In some cases an irreversible inhibi- 

 tion is claimed to be noncompetitive because it yields the appropriate 

 plots, whereas this procedure is not applicable to irreversible inhibitors. 

 In fact, such an inhibitor can react with the substrate site but competition 

 will not be exhibited because of the failure to reach equilibrium. The sub- 

 strate may, however, delay the development of the inhibition in ^his case 

 because of the protection it affords its site. Occasionally an inhibitor is 

 competitive with coenzyme or activator but if the data are analyzed ki- 

 netically with respect to the substrate only, the inhibition will appear 

 to be noncompetitive. Lastly, in many cases inhibitions have been called 

 noncompetitive because the kinetics do not conform exactly to those of 

 competitive inhibition, although they do not fit noncompetitive criteria 

 either if examined carefully. Some of these inhibitions are partially compet- 

 itive or of a mixed type. It would seem reasonable that pure noncompet- 

 itive inhibition must be rather uncommon, at least with simple enzymes, 

 since it is unlikely that an inhibitor could combine with the apoenzyme at 

 a region close enough to the substrate site to prevent decomposition of the 

 ES complex without hindering to some extent the binding of the substrate, 

 thereby introducing a competitive component to the inhibition. Because of 

 the difficulties, conclusions as to the site of inhibition from kinetic data 

 alone must be made with caution. It cannot be overemphasized that for 

 reliable kinetic interpretations, accurate quantitative data must be obtained 

 under controlled conditions and a sufficient range of substrate and inhibitor 

 concentrations must be utilized to allow accurate plotting of the data. 



GENERALIZED KINETICS OF REVERSIBLE INHIBITION 



No theory or inhibition equation can include all the types of inhibition 

 outlined above but a generalized formulation may be applied to type (I) 

 inhibitions where reaction is with the apoenzyme; the special types of in- 

 hibition, e.g. competitive and noncompetitive, can be derived from this 



