SPECIFICITY OF INHIBITION 869 



bitor concentration in Fig. 16-3. As pointed out above, it is not always 

 advisable to use the concentration of the inhibitor giving maximal speci- 

 ficity. In the situation chosen for illustration, a specificity of 0.7 or greater 

 may be obtained over a range of inhibitor concentration from 2.7 raM 

 to 37 mM (see Fig. 16-3). At the former concentration t^ = 0.73 and i^ = 

 0.03 while at the latter concentration i^ = 0.97 and ^2 = 0.27. Either con- 

 centration or the concentration giving maximal specificity may be expe- 

 rimentally optimal, depending on the systems being tested and the purpose 

 of the investigation. As a general rule, it is perhaps best to work at the 

 lower end of the inhibitor concentration range in order that the inhibition 



Fig. 16-3. Variation of the specificity with the inhibitor concentration for the 

 case shown in Fig. 16-2. 



on the second enzyme be minimal. It is not always necessary for the enzyme 

 that is being studied to be inhibited strongly and correlations between the 

 enzyme and some cell metabolism or function may often be made with 

 inhibitions less than 50%. 



Inhibitions Involving Three or More Enzymes 



Many systems commonly studied may contain several enzymes whose 

 susceptibilities to the inhibitor must be considered. The use of inhibitors on 

 the tricarboxylic acid cycle in mitochondrial suspensions, for example, im- 

 pels one to take into account at least a dozen possible inhibition sites, 

 unless the conditions are such that only a fraction of the cycle is being 

 studied. In structurally organized multienzyme systems, there are usually 

 more possibilities for inhibition than the number of enzymes involved, be- 

 cause the inhibitor may alter spatial relationships between enzymes by a 



