ENZYME INHIBITION AND CHANGES IN CELL FUNCTION 471 



Quantitative Relations between Functional Change and Inhibition 



Very little is known of the mechanisms by which energy is transferred 

 to the functional systems or utilized in the functional acts; indeed, one is 

 generally ignorant of the basic relationship between the level of functional 

 activity and the energy supply. However, we can be reasonably certain that 

 in most cases there are several processes between the generation of the 

 energy and the final cell activity. This may be crudely expressed by an ar- 

 bitrary series of equations, such as: 



Function = A (ATP) 

 (ATP) = f,v, ~ Uu 



Vj, = fx Vo 



V, = A(ATPase)(X) 

 Vo = /6(S) - /,(!) 



where the /'s indicate the functional dependences, v^ is the phosphorylative 

 rate, v^ is the rate at which ATP is hydrolyzed by ATPase, v^ is the oxida- 

 tive rate, (X) represents the concentrations of various ions influencing 

 ATPase activity, (S) the concentration of substrate for oxidation, and (I) 

 the concentration of an inhibitor that interferes with oxidation. The de- 

 pendency of function on the inhibitor concentration would then be given 

 by: 



Function = ^{^^[/elS) + /,(!)] + /3A(ATPase)(X)} (9-20) 



so that it would be extremely fortuitous for any simple relationship to e- 

 merge. Thus if one is measuring the functional response to an inhibitor, it is 

 unduly optimistic to expect to be able to elucidate enzymic mechanisms of 

 inhibition from the changes observed; i.e., an inhibition cannot be labeled 

 as competitive on the enzyme level because an increase of substrate con- 

 centration tends to overcome the effect of the inhibitor on cell function. 

 The concept that is being emphasized is simply that the relationship be- 

 tween cell function and enzyme inhibition is, in essentially all cases, very 

 complex and this complexity should not be obscured by apparently sim- 

 ple experimental results. 



Establishing Correlations between Functional Changes and Enzyme Inhibition 



One of the primary reasons for using inhibitors in living tissue is to de- 

 termine the specific metabolic basis for functional activity. Thus uncoup- 

 lers of oxidative phosphorylation are useful in showing a dependence of 

 some cell activity on high energy phosphate compounds, while inhibitors 

 of the tricarboxylic acid cycle (such as fluoroacetate) or glycolysis (such 

 as iodoacetate) are commonly used to demonstrate the presence or absence 



