716 14. EFFECTS OF pH ON ENZYME INHIBITION 



pH in a manner opposite to the external concentration. The pH dependence 

 of cellular enzyme inhibition may thus be quite different from that of the 

 same enzyme when isolated and in solution. The second point is that the 

 intracellular concentration of the inhibitor may be much higher than in 

 the medium; in the example chosen in Fig. 14-16, it is 63.5 times as great. 

 It is evident that (I,)o = (I^)^ when pH^ = pH,; below pH, the internal 

 concentration will be greater and above pH, it will be smaller than outside. 

 The inhibition of an enzyme within the cell may thus be much greater or 

 much less than would be expected from the external inhibitor concentra- 

 tion and studies of the enzyme in vitro. 



Case II: cells unbuffered. In addition to Eq. 14-151 and 14-152, there 

 is now a further relationship: 



(H), = (H)» + (I), (14-156) 



where (H)*^. is the initial hydrogen ion concentration in the cell before the 

 inhibitor is added. This equation results from the fact that for every I 

 released in the cell, a corresponding proton arises. Substituting for {\){. 



(H), = (H)« + ^;^^^° (14-157) 



(H), 



and, since (HI)^ = {IXlofh/- 



(H)^ - (H)° (H), - ^°f;^° = (14-158) 



oJhi 



When (H)'' is negligible — and we shall see that it usually is — this may 

 be simplified to: 



(H). = ]/^f^ (14-159) 



f oJhi 



The value of (I)^ can be obtained from Eq. 14-156 using (II)j; from either 

 Eqs. 14-158 or 14-159 and (III)j is still given by Eq. 14-154. The concentra- 

 tions of the forms of the inhibitor and the intracellular pH are plotted in 

 Fig. 14-17 for the same system as in the previous figure. 



The internal concentration of inhibitor does not rise above that in the 

 medium and over most of the ])H range is less. It is, furthermore, interesting 

 to note that the internal pH is reduced appreciably over a wide range of 

 external pH. That is, even at a pH far above pH^, sufficient HI will enter 

 the cell to lower pH,; of course, very little HI is required when the initial 

 hydrogen ion concentration is as low as it is in most cells. In the example 

 chosen, (H)" may be ignored up to pH^ = 9 with less than a 5% error in 

 using Eq. 14-159. 



