732 



14. EFFECTS OF pH ON ENZYME INHIBITION 



log (I^)o-pHo curves will be listed and the changes in the curves are indicated 

 in Fig. 14-24. 



{A) Depressant effects due to reduced pH ,. If the acidification of the cell 

 leads to a depression of the metabolism, in addition to the action of the 

 inhibitor itself, a lower concentration of inhibitor than predicted can be 

 used to produce a chosen inhibition. The effects of this acidification must 

 always be borne in mind when the pH, is near the -pK^ or below. The total 

 inhibition under such circumstances can be expressed as: 



''pH 



where i,„h is the fraction due to the inhibitor and i 



pH 



(14-171) 



is the fraction due 



to the fall in pH,. The fraction i^^ is not corrected for by running a con- 



LOGd,) 



pH. 



Fig. 14-24. Illustration of the ways in which 



Simon-Beevers curves can be modified from 



the theoretical. The letters correspond to the 



factors discussed in the text. 



trol at the lowered pH^ because the drop in pH^ is due to the presence and 

 penetration of the inhibitor. The fact that weak acids damage and kill 

 the cells when the pH^ is lowered, independently of whether the acids are 

 specifically toxic or not, indicates the importance of this pH effect. In 

 fact, one possible control to test the occurrence of this pH effect would 

 be to use a noninliibitory acid that is comparable to the inhibitor in other 

 ways, such as piii,,. This factor need not produce a distortion of the shape 

 of the log (I,)o-pHo curve but may only lower the horizontal portion. 



{B) Tlie anion cdso exerts effects. If the anion contributes to the inhibition 

 in any way — it may not penetrate but act on the cell surface — the curve 



