734 14. EFFECTS OF pH ON ENZYME INHIBITION 



less permeant, or more or less reactive. The inhibition of certain enzymes by 

 hydroquinone deiDends on the pH because the equilibrium between hydro- 

 quinone and p-benzoquinone is sensitive to the pH. It is the p-benzoqui- 

 none that is active and hydroquinone is oxidized rapidly only in the pH 

 range near neutrality or above. Thus hydroquinone is rather inactive at 

 a pH of 5 (Potter, 1942) but active at pH 7.4 by virtue of its rapid oxidation. 

 Changes in structure, reaction with other components in the medium, 

 dimerization, and alteration in general stability can all be brought about 

 by variations in the pH and such factors can be important in the inhibition. 

 The slope of any part of the log (I,)o-pHo curve may be modified because 

 the changes can occur at any pH^ and can be either to increase or decrease 

 the inhibitory potency. 



The deviation of the high pH portion of the curve from a unit slope is 

 the most difficult to understand. Actually, there is much variation between 

 the different weak acids. For the inhibitors in Fig. 14-23, the slopes of the 

 linear sections of the curves are: fluoride, 1.1; azide, 0.75; dinitro-o-cresol, 

 0.73; and iodoacetate, 0.47. The only reasonable explanation is to attri- 

 bute a slope of less than one to some action of the anion. However, the anion 

 concentration in the medium remains essentially constant as the pHo 

 rises above p£^^ + 2. An increased penetration or effect of the anion as 

 I he ijHq increases would explain the curves but would seem unlikely. The 

 yeast cell might be considered as abnormal in view of its relatively large 

 "extracellular" metabolic space, but slopes of less than unity have been 

 observed in a wide variety of cells (Simon and Beevers, 1952). It may be 

 noted that the same phenomenon is seen in the actions of weak bases, 

 such as phenylhydrazine, except that the slope is now less than one in 

 the low pH portion of the curve. Thus it is not confined to the alkaline 

 range of pH^. Because the slopes are different with the various weak acids 

 tested on the same type of cell, it must be an effect related to the properties 

 of the active substance and not entirely dependent on the cell response. The 

 low slope of the iodoacetate curve could be explained on the basis of a rise 

 in the rate of reaction with SH groups at or near the cell surface as the 

 pHp increases, but this would not hold for the other substances. The an- 

 swer must await further work on a greater variety of substances and cells, 

 and particularly more investigations of the penetration of inhibitors in the 

 higher pH^, range. 



Potent and Weak Inhibitors 



The buffer capacity of a cell has been shown to be an important factor 

 in determining the degree of inhibition produced by an ionizing inhibitor. 

 It was also pointed out that actual cells are partially buffered and would 

 be expected to behave in a fashion intermediate between the unbuffered and 

 the completely buffered ceUs for which equations expressing the distribu- 



