478 9. INHIBITION IN CELLS AND TISSUES 



be seen by evaluation of (IJ(1 — i)li (see Fig. 5-3). Thus in these methods, 

 and in the others as well, the determination of K^ may be seriously in er- 

 ror if the internal and external inhibitor concentrations differ appreciably. 



The uptake of 2,4-dinitrophenol by yeast cells follows this simple re- 

 lationship over the range of concentrations generally used, since an increase 

 in the external concentration from 0.0495 mM to 0.198 niM (fourfold 

 increase) led to a rise in the internal concentration from 5.2 to 20.1 (3.9- 

 fold increase) (Kiesow, 1959). These concentrations produced a strong in- 

 hibition of the respiration and an inhibition of the Pasteur effect. However, 

 the simple situation in which (Ij)/(I^) remains constant for any external 

 concentration of inhibitor is probably rare. This may well hold true over 

 a limited range of concentration but in most cases the internal concen- 

 tration reaches a maximal level as the external inhibitor concentration is 

 increased. When active transport or exclusion of the inhibitor occurs, the 

 mathematical expression of (Ij) in terms of (I^) will be impossible, unless 

 the actual concentrations can be determined. For inhibitors that are weak 

 acids or bases, the relationship between inner and outer concentrations 

 may be complex and will, of course, depend on the external and internal 

 pH (see Chapter 14). 



Cornpetitive inhibition will involve in addition the intracellular con- 

 centration of the substrate, (S,), which may be related to the external 

 concentration, (S^), in a manner similar to or different from the inhibitor. 

 The simplest expression would again be (S^) = rg{Sg). The rate and inhi- 

 bition equations would then be given by: 



F,„(Se 



(Se) + 



r. 



1 + 



K. J 



(le 



(IJ 





1 + 



r.(S, 



(9-25) 



(9-26) 



to correspond to Eqs. 3-12 and 3-13. The analytical plots may differ even 

 more from the isolated enzyme situation than those discussed above for 

 noncompetitive inhibition. If one could be certain of these simple rela- 

 tionships and the similarity of the extracellular and intracellular enzyme, 

 one might use such methods to obtain an idea of the values of r, or r^, but 

 the assumptions are generally too untenable. At least the possibility of 

 distinguishing between competitive and noncompetitive inhibitions within 

 cells is a reality since the usual analytical plots will remain linear and have 

 the characteristics shown in Fig. 5-1 and 5-3 when the relation between 

 internal and external concentrations is a simple function. 



