VARIATION OF IXTRACELLULAE INHIBITION WITH pH 721 



which we are concerned is ahnost totally restricted to the mitochondria; 

 the internal mitochondrial pH is 5.8 and the experimental bulk pHj is 

 6.8. When the cells are allowed to come to equilibrium with a weakly aci- 

 dic inhibitor, the concentration of I~, the active form, will vary inversely 

 with the hydrogen ion concentration of the region (Eq. 14-152). In this case, 

 the concentration of I~, in the mitochondria will be one-tenth that calcu- 

 lated on the basis of pHj. Accumulation of an inhibitor in a region within 

 the cell can also occur by this mechanism. What is fundamentally impor- 

 tant is the pH of the region in which the inhibited enzyme is located. When 

 calculations of inhibitor distributions or intracellular inhibitions are made 

 using the bulk cytoplasmic pH,, the results should be considered to be 

 mean values at the best. 



The action of an inhibitor is usually not uniform throughout a cell 

 because of its distribution and the fact that the susceptible enzymes are 

 distributed heterogeneously. The entrance of the inhibitor into the cell 

 and its action there will thus not change the internal pH uniformly. If 

 the inhibitor acted specifically on the mitochondrial enzyme in the above 

 example, the intramitochondrial pH might be changed to a degree depending 

 on the buffering capacity of the mitochondria, without appreciable imme- 

 diate changes in the pH elsewdiere. Local changes in pH could influence 

 markedly the metabolic and functional res])onses of the cells to inhibitors. 



Permeant Forms of Inhibitors 



Weak acids and bases penetrate into cells more rapidly in the uncharged 

 form. This would be expected on the basis of what is known about the per- 

 meability of cells to ions and neutral molecules. There is also adequate evi- 

 dence to support this concept. It has been generally found that the toxicity 

 of weak acids becomes progressively greater as the pH of the medium is 

 lowered and the concentration of the undissociated form rises. Thus Smith 

 (1925) found that salicylate and benzoate anions are without effect on the 

 development of sand dollar {Echinarachnius) eggs in normal sea water, but 

 that cleavage is depressed when the pH is lowered to provide sufficient 

 concentrations of the acid forms. The entrance of sulfide into Valonia cells 

 was shown by Osterhout (1925) to be entirely due to the penetration of 

 the membranes by HgS. Analyses of the total sulfide concentration within 

 the cells at different external pH's are shown in Fig. 14-18 and may be com- 

 pared with the curve which gives the calculated concentrations of H^S in 

 the medium. The actual rates of penetration were measured by Jacques 

 (1936) and these were shown to be proportional to (HaS)^ over the 5-min 

 periods during which the rates were measured. Likewise, the rates of pene- 

 tration of arsenite into the larvae of the flesh fly were found to rise with 

 a decrease in pH (Ricks and Hoskins, 1948) as would be expected if ar- 

 senious acid were the principal permeant form. Many other examples from 



