724 



14. EFFECTS OF pH ON ENZYME INHIBITION 



is without demonstrable action; that is, the action or accumulation is di- 

 rectly proportional to the concentration of the neutral form and in a pH 

 range where this form is negligible, the action is undetectable. In Fig. 

 14-19 it is seen that for most tissues the inhibiting effect of malonate dis- 

 appears as the pH becomes greater than 5.5-7.0. However, the inhibition 

 of the respiration of rhubarb leaves is still appreciable at these higher 



Fig. 14-20. Variations of the inhibitions produced by iodoacetate with the pH. 

 Curve A: yeast fermentation — (lAA) =: 1.1 n\M and time = 20 min (Schroeder 

 et al., 1933 a). Curve B: yeast fermentation — (lAA) = 1.1 m3I and time = 2 h 

 (Schroeder et al, 1933 a). Curve C: yeast fermentation — (lAA) = 1.1 mil/ and 

 time = 6 h (Schroeder et al., 1933a). Curve D: yeast fermentation — (lAA) = 1.1 

 mJf (Briicke, 1933). Curve E: yeast viabihty — (lAA) = 1 mil and time = 30 

 min (Aldous, 1948). Curve F: barley root respiration — (lAA) = 0.04 mM (Laties, 

 1949). Curve G: Avena coleoptile growth — (lAA) = 0.02 m3I (Cooil, 1952). Curve 

 H: yeast fermentation — (lAA) = 1 mil and time = 4.5 min (Lundsgaard, 1932). 



pH's; the inhibition of CO., production actually levels off above pH 5.3. 

 In this case it would seem that an ionic form can penetrate. Some cells are 

 undoubtedly more permeable to ions than others and the permeabilities to 

 cations and anions need not be equivalent or even related. Many types of 

 cells, such as renal tubular cells or erythrocytes, are reasonably permeable 

 to anions. It is not only a matter of membrane permeability. Many cells 



