170 1. lODOACETATE AND lODOACETAMIDE 



concentration constant and reducing the pH by 2 units. Although plant 

 cells may generally be less anion-permeable than mammalian cells, there is 

 no good evidence to indicate a marked difference in the effects of pH on 

 iodoacetate inhibition. 



lodoacetamide was introduced to facilitate penetration, and little effect 

 of pH on the entrance of this substance into cells would be anticipated. 

 This was confirmed by Stannard (1938 a) for frog muscle, iodoacetamide 

 inhibiting respiration more rapidly than iodoacetate at physiological pH 

 whereas in acid media the two inhibitors are equally effective. However, 

 there are very good reasons for some effect of pH on the actions of iodo- 

 acetamide. First, the permeabilities of cells are modified by pH and, sec- 

 ond, the intracellular pH and hence the rate of iodoacetamide reaction 

 with SH groups may be changed by the change in external pH. 



A nonspecific intracellular acidification due to the penetration of undis- 

 sociated iodoacetic acid at low pH's has seldom been considered as playing 

 a role in the effects observed. The concentrations of iodoacetic acid at dif- 

 ferent pH's and at a total concentration of 1 mM are shown in the accom- 

 panying tabulation. Iodoacetic acid will enter the cells and dissociate into 



pH Iodoacetic acid (M) 



iodoacetate and H+. As the iodoacetate reacts with cellular components, 

 more iodoacetic acid will enter until at equilibrium the concentrations in- 

 side and outside are the same. In addition, 11+ may be released during the 

 carboxymethylation of SH groups. The effect on intracellular pH will de- 

 pend on the buffering capacity of the cell, and at external pH above 6 there 

 is likely to be little change in intracellular pH. However, at lower pH's 

 substantial decreases in intracellular pH are very likely to occur, when one 

 considers the amounts of iodoacetic acid entering the cells and the final 

 equilibrium concentrations. Such intracellular pH alterations may have two 

 important effects: (1) a direct effect on metabolic or functional processes, 

 and (2) a reduction in the rate of inhibition by iodoacetate, since acidifi- 

 cation slows the reaction of iodoacetate with SH groups. The cell pH may 

 also be modified by the metabolic disturbance produced by iodoacetate 

 (see page 82). The marked inhibitions of several intracellular yeast en- 



