VARIATION OF INTRACELLULAR INHIBITION WITH pH 739 



mental conditions, it will be necessary to extend the previous treatment 

 to such situations. The factors that would favor depletion are: a high 

 ratio of the cell or tissue volume to the volume of the medium, a low K^ 

 so that small concentrations of the inhibitor are used, the binding of the 

 inhibitor to nonenzymic components of the cells, a pH^ favoring large pen- 

 etrations of the inhibitor, and a high buffering capacity of the cells, which 

 also favors penetration of larger total amounts of inhibitor. There are var- 

 ious situations, depending on the cause of the depletion, that may be ana- 

 lyzed. 



Case I: depletion is caused by penetration into the cell and not to binding. 

 This is represented by diagram 14-150 and Eqs. 14-151 to 14-155 remain 

 valid; however, (I;)^ does not now remain constant. The cellular fraction 

 of the total reaction volume will be represented by n and thus the fraction 

 that the medium occupies is 1 — n. The convention will be adopted of 

 enclosing over-all concentrations in brackets; thus the concentration of 

 inhibitor in the total volume of medium and cells is [I J. The initial con- 

 centration of the inhibitor in the medium, before penetration into the cells 

 has occurred, is [IJ/(1 — n), so that the over-all concentration and initial 

 concentration can be easily interconverted; it is, however, easier in some 

 respects to use thei over-aU concentrations in expressing the behavior. 



The over-aU concentration in terms of external and internal concentra- 

 tions is given by: 



[/,] = n(hh + (1 - n){h)o (14-176) 



and in terms of (I)(, which is assumed to represent the active form, by: 



[I.] = ah 



(H), (H), 



n H == h (1 — n) 



(14-177) 



Ka ^ (H)o 



Let us examine how this depletion will affect a plot of log [I,] against pHg. 



(H), 



Low pH,,: log [L] = log (I), + log 



n 



(14-178) 



High pH,: log [L] = log (1), + log (1 - n) - pH, + pH, (14-179) 



In the low pH^ range the slope will be zero and in the high pH^ range the 

 slope will be + 1. However, the intermediate region will be affected by the 

 depletion and the inflection between slopes will occur at a pH^ different 

 from pKfj. Such plots for different values of n are shown in Fig. 14-27. 

 As the relative volume of the cells increases, the over-all concentration of 

 the inhibitor must become larger to produce the same degree of inhibition, 

 which one would expect on the basis of depletion from the medium. Also, 



