VARIATION OF INTRACELLULAR INHIBITION WITH pH 743 



where the apparent substrate and inhibitor constants are related to the 

 true constants as follows: 



Ki' = Ki 



KJ = K, 



(H), 

 (H)„ 



(H), 

 (H)„ 



1 + 



(H), 



(H)„ 



Ka. 



(14-192) 



(14-193) 



When the internal pH^ does not change, an experiment performed at a 

 certain external pH^, using different concentrations of substrate and inhi- 

 bitor, will give results comparable to competitive inhibition on isolated 

 enzymes. However, the constants determined by graphical methods will 

 be pH-dependent in the manner given above. When piiC^^ and ])K^. differ 

 by a significant amount, the behavior is somewhat abnormal. Let us as- 

 sume that pK^ < pK^ . In the pH^ range below i:>K^., both the substrate 

 and the inhibitor will penetrate readily, but if the pH^ is raised above ipK^ . 

 but is still lower than p^^ , the inhibitor will penetrate less readily while 

 the substrate will enter readily, so that the inhibition will fall in the expected 

 way; however, when the pH^ is raised to 1>K^^, the substrate will cease to 

 penetrate readily and the inhibition may not be decreased as much as ex- 

 pected. If pK^ < piCg ., as the pH^ is raised above pK^ , the substrate will 

 not enter readily and the inhibition may actually increase until pH^ ap- 

 proaches P-fiC^.. The behavior of an hypothetical system where p^^ = 4 

 and pK^. = 6 is shown in Fig. 14-28 and the effect of substrate exclusion 

 between pH^'s 4 and 6 is clearly evident in the rise in the inhibition. 



Complex Effects of pH on Cellular Inhibition 



Effects of changes in pH^ not described or predicted by the simple treat- 

 ment given in this chapter should be expected occasionally. For one thing, 

 most inhibitions are not specific. The changes of over-all metabolism or 

 function that are measured may depend on actions of the inhibitor on sev- 

 eral different systems, each with its particular j^H dependence. Even 

 though the inhibition of each intracellular enzyme follows the predicted 

 behavior, the process measured may respond in quite a different manner. 

 The presence of multienzyme systems in most of the important areas of 

 metabolism also must make cellular inhibition pH^ dependence more com- 

 plex in many instances. Furthermore, we have not considered quantita- 

 tively in this chapter the possible effects of the inhibition on the perme- 

 ability properties of the cell membrane. It is quite possible that metabolic 

 depression would lead to an increased permeability to the ionic forms of 

 the inhibitor and alter the pH relations. One may imagine many factors 

 that would lead to deviation of the behavior from that described by the 

 basic equations. 



