VARIATION OF INTRACELLULAR INHIBITION WITH pH 741 



The log [I J-pHo curves will be similar to those in Fig. 14-27 but the effect 

 of the binding will be to raise the necessary inhibitor concentration in the 

 low pHg range and to shift the inflections further to the right. The slopes 

 in the high pH^ range will remain + 1. 



Case III: depletion is caused by penetration into the cell and to combi^iation 

 with the enzyme. We can simply replace P by E and K^ by K^ in the equa- 

 tion above. However, now we wish to find the over-all concentrations of 

 the inhibitor that will maintain the inhibition constant at different values 

 of pHg. Equation 14-181 may be rewritten as: 



[I,] = m(E,), + (I), \n + -^-1^ + (1 - n) ^^^' 



Ka ' ' ' (H), 



(14-182) 



where the niCEi)^ term represents the effect of the binding to the enzyme 

 and the other term, as in Eq. 14-177, represents the penetration into the 

 ceUs. Of course, (I)^ may be substituted by iKJ{l — i) for noncompetitive 

 inhibition. The log [I J — pH^ curves will be similar to those for the other 

 cases. 



It is clear that the depletion of inhibitor from the medium, from what- 

 ever cause, does not change the slope in the high pH^ range from + 1. 

 The reason for this is that at a high pH^ very little HI is present and the 

 amount of inhibitor entering the cell is so small that depletion is minimized 

 or abolished (see Fig. 14-16). These conclusions are thus in disagreement 

 with those of Cowles and Klotz (1948), who stated that such depletion 

 would decrease the slope in the high pH^ range. However, their equations 

 would actually lead to a slope of + 1 at high pH^, and agree with those giv- 

 en above. Their expression for the slope: 



^'""^ = K. + (H)S + iPm '"■^''' 



where P and k are parameters related to the binding, would provide a slope 

 of + 1 when (H),, became sufficiently low. The 1 + {Pjk) factor only shifts 

 the inflection to higher pH^'s, just as described for the three cases treated 

 in this section. Therefore, one cannot explain the deviation of the slope by 

 the phenomenon of inhibitor depletion. Nevertheless, the effects of deple- 

 tion are interesting and important in other respects. 



Inhibitors That Are Dibasic Acids 



Singly ionizing inhibitors have been treated for convenience and the 

 extension to doubly ionizing inhibitors is straightforward. If only the H2I 

 form can penetrate into the cells: 



(HJ)„ = (HJ), (14-184) 



