VARIATION OF INTRACELLULAR INHIBITION WITH pH 



729 



For a constant degree of inhibition. (I), must be constant and the pH^ is 

 constant because the cells are completely buffered. At pH^'s significantly 

 lower or higher than p^, this equation simplifies to: 



Low pHo: 

 High pH,: 



log {It)o = log (I), - pH, + ipKa 

 log (I,), = log (I), - pH, + pH„ 



(14-163) 

 (14-164) 



A plot of log (I^)o against pH^ will be a carve with zero slope at low pH^'s 

 and with a slope of + 1 at high pH^'s (lower solid curve in Fig. 14-22). 

 For unbuffered cells, combining Eq. 14-156 and 14-159, neglecting (H)^^ 

 and writing the result in terms of (I;)o, we obtain: 



at)o = (I); 



1 + 



(H)„ 



Putting this into logarithmic form: 



log (I,), = 2 log (I), + pK„ + log 



1 + 



K. 



(H), 



(14-165) 



(14-166) 



Considering again ranges of pH^ significantly lower or higher than pK,,: 

 Low pH„: log (!,)„ = 2 log (I), + pZ„ (14-167) 



High pH„: 



log (I, 



2 log (I), -FpH„ 



(14-168) 



Thus again a plot of log (I^)^ against pHg, with (I)^ constant, will give a 

 curve with zero slope at low pH^'s and a slope of + 1 at high pH^'s (up- 

 per solid curve in Fig. 14-22). 



It is, therefore, impossible to distinguish between unbuffered or com- 

 pletely buffered cells by such plotting; it follows that intermediate cases 

 of partial buffering will also give curves of this type. The buffer capacity 

 of the cells will shift the curves along the log (I^)o axis, as seen in Fig. 

 14-22 where the values given on the horizontal portions of the curves are 

 derived from a hypothetical example in which (I)^ = 1 raM, pH^o = 6.8, 

 and K^ = 10~^ 31. If the sensitivity of the inhibited system within the cell 

 is known, and the value of (I)j can be estimated for 50% inhibition, some 

 idea of the buffer capacity may be obtained from the height of the horizontal 

 portion of the experimental curve. 



In a similar manner, log (III)^ can be plotted against pH^ for the pro- 

 duction of a chosen degree of inhibition. The equations in this case do not 

 involve pH^: 



Completely buffered cells: log (HI),, = piC^ — pH, + log (I), 



(14-169) 



Unbuffered cells: 



log (HI)„ = 2 log (I), + pZ„ 



(14-170) 



