726 



14. EFFECTS OF pH ON ENZYME INHIBITION 



form that reacts with the enzyme and the un-ionized form that pene- 

 trates. The difference is clearly illustrated by the effects of iodoacetate on 

 yeast (Schroeder et al., 1933 b). The rate of reaction of iodoacetate with 

 the SH groups of glutathione increases with pH (Fig. 14-21, curve A) 

 while the inhibition of intracellular fermentation decreases with pH (curve 

 B). Although iodoacetate does not depress fermentation by reacting with 



100 



pH ► 



Fig. 14-21. Effect of the pH on the reaction of iodoacetate with glutathione (curve 



A), on the inhibition of yeast fermentation by iodoacetate (curve B), and on the 



reaction between iodoacetate and glutathione within yeast cells (curve C). (From 



Schroeder et al, 1933 6.) 



glutathione, the pH dependence of its inhibition on phosphoglyceraldehyde 

 dehydrogenase is probably similar. The permeant form here is iodoacetic 

 acid and the active form is the iodoacetate anion. 



Buffer Capacity of Cells 



The degree of enzyme inhibition that will be produced by an inhibitor 

 that is a weak acid or base will depend on the buffer capacity of the cell 

 interior in many cases. This was seen in the two extreme cases discussed 

 previously. In the completely Iniffered cells, the accumulation of I~ was 

 much greater than in the unbuffered cells. The greater the buffer capacity 



