PENETRATION INTO CELLS AND THE EFFECTS OF PH 169 



Ehrenfest, 1932), yeast respiration (Beevers and Simon, 1949), yeast growth 

 (Schroeder et al., 1933 a; Kapkine, 1937; Aldous, 1948), Tonda growth 

 (Hansen, 1956 a), algal respiration and transport (Bergqiiist, 1958), CJdo- 

 rella photosynthesis (Eraser, 1954), Avena coleoptile growth (Cooil, 1952), 

 and barley root respiration (Laties, 1949 a). A typical inhibition-pH curve 

 for yeast fermentation is given in Fig. 1-14-21. Since the pH affects the rate 

 of penetration of the inhibitor and the intracellular concentration, inhibi- 

 tion will occur at the higher pH's if sufficient time is allowed, as shown in 

 Fig. 1-14-20. In the examples above, the inhibition is roughly proportional 

 to the concentration of undissociated iodoacetic acid and the same relation- 

 ship can be obtained by varying the total concentration at a constant pH 

 (Aldous, 1948). However, Simon-Beevers curves for the effect on yeast res- 

 piration (Fig. 1-14-23) show that the situation is not quite this simple, the 

 curve for the undissociated acid not being horizontal, indicating either a 

 penetration or membrane effect by the iodoacetate anion, or a reduction in 

 the inhibition resulting from a fall in the intracellular pH at low external 

 pH's. Although little work has been done with iodoacetamide with respect 

 to pH, it is worth noting that Kohn (1935) found a decrease in the pH from 

 9.2 to 5.3 to increase the inhibition of photosynthesis by iodoacetamide, an 

 effect which is difficult to explain unless one assumes some change in the 

 permeability properties of Chlorella. Failure of iodoacetate to inhibit can 

 often be attributed to lack of penetration at an unfavorable pH. For exam- 

 ple, Lewin and Mintz (1955) reported that respiration and photosynthesis 

 are scarcely affected by 50m M iodoacetate in CJdamydomonas, but the pH 

 was 8.5 and the inhibitor was added from the side-arm of the vessels, so 

 that probably iodoacetate simply did not get into the cells. 



The marked pH effects discussed above were all observed in yeast and 

 plant cells. Quite different results have been obtained in the few studies 

 on mammalian tissues. The hemolysis of erythrocytes by iodoacetate was 

 related to an inhibition of glycolysis by Wilbrandt (1937), but the effect is 

 weakened as the pH is decreased from 7.4 and is abolished at pH 5.9. This 

 indicates an action on the membrane rather than one requiring penetration 

 into the cells. The respiration of rat brain suspensions is inhibited by iodo- 

 acetate and this is independent of pH from 6.7 to 7.8 (Bernheim and Bern- 

 heim, 1941), but there is a question whether this was a cellular preparation. 

 Certainly many mammalian tissues are metabolically and functionally de- 

 pressed by iodoacetate at physiological pH, but the concentrations necessary 

 for inhibition are often rather high (1-10 mM) and this may indicate that 

 penetration is poor. If one assumes that iodoacetate at 0.01-0.1 mM inhib- 

 its the EM pathway, it is reasonable to estimate that around 100 times 

 these concentrations must be present in the extracellular medium. With 

 respect to the concentration of undissociated iodoacetic acid, increasing the 

 total concentration 100 times is essentially the same as keeping the total 



