730 



14. EFFECTS OF pH ON ENZYME INHIBITION 



The curves will be horizontal over the entire range of pHg as shown in the 

 dashed lines of Fig. 14-22. 



What few experimental data are available do generally conform to the 

 theoretical curves but there are important deviations from the expected 

 behavior. Simon- Bee vers' plots for the depression of yeast respiration by 

 four inhibitors are shown in Fig. 14-23. We may conclude that the general 

 theory is correct and that HI is the dominant permeant form of the inhi- 

 bitor. It may be noted also that the values of log (I;)^ at low pH^'s would 

 indicate at least an appreciable buffering capacity of the yeast cells since 



LOG 

 CONC. 



pHo - 



Fig. 14-22. Simon-Beevers plots for unbuffered 



and completely buffered cellular systems. (I), 



= 1 TuM, pH,o = 6.8, and ^K^ = 5. 



the concentrations of the inhibitors required for 50% inhibition are not 

 much higher than would be expected from work in vitro. Simon and Beevers 

 (1952) produced a generalized curve from the data on all the weak acids 

 studied; from this and from the results with the inhibitors shown in Fig. 

 14-23, we may observe certain deviations from the theoretical curves. The 

 most important are: (a) the mean slope of the log (I^)o-pHo curves at high 

 pHp's is usually less than unity, and in the generalized curve of Simon and 

 Beevers is 0.56, (6) the log (I^)o-pHo curve tends to level off at higher pHo's, 

 usually when the pH^ is several units higher than the pK^, and (c) the 

 log (HI)o-pHo curves almost invariably develop a negative slope in the 

 pHg range above ]^K^ [this is, of course, related to the slope of less than unity 

 in the log (I^)o-pHo curve in this range of pHp]. Attempts must now be 



