858 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



increased, the enzyme rate will rise and i^,„ will increase. Since the inhi- 

 bition produced by a competitive inhibitor depends on K^^: 



^^^ (15-124) 



(I) + K^l + {^)lK^-\ 



as iC,„ increases, the inhibition will become greater. This has been very 

 nicely demonstrated for the inhibition of succinic dehydrogenase by mal- 

 onate (Thorn, 1953). The rate constant, k^, can be altered here by changing 

 the concentration of the hydrogen acceptor, e.g. methylene blue. The 

 Ky„ was found to vary over a twenty fold range with changes in the meth- 

 ylene blue concentration and, as a consequence, the inhibition by malonate 

 was found to increase almost linearly with the rate of succinate oxidation. 

 Noncompetitive inhibition, of course, would not be affected by such rate 

 changes. 



It was claimed by Florijn and his colleagues (Florijn et al., 1950) that 

 noncompetitive inhibition in mutual depletion systems is dependent on 

 the rate as influenced by the substrate concentration. However, reference 

 to Eqs. 3-34 to 3-36 shows that the substrate concentration does not enter 

 into the expressions for the inhibition. Actually, a noncompetitive inhi- 

 bitor inactivates a fraction of the enzyme active sites, although it does 

 not affect the binding of the substrate, and it makes no difference what 

 the substrate concentration is or in what zone with respect to substrate 

 the system hajppens to be. 



It would be well to point out the errors in the treatment of Florijn 

 and his co-workers. In the first place, although noncompetitive inhibition 

 is assumed, only the equilibrium between the substrate and the enzyme is 

 considered, whereas a noncompetitive inhibitor doesn't alter this but only 

 decreases F,„ or prevents the substrate from reacting once it has complexed 

 with the enzyme. In the expression for K„ the concentration of free en- 

 zyme is not (E); — (ES) — (EI), but only (E), — (ES), since a noncompeti- 

 tive inhibitor doesn't reduce the sites for the substrate binding. In the 

 second place, they calculate the inhibition as equivalent to {v — v,)/v in 

 an equation involving i and in their table showing the variation of inhibi- 

 tion with the substrate concentration, they initially assume that i = 0.2 

 and then proceed to calculate an inhibition that varies. The experimental 

 results they obtained, which prompted them to develop the treatment, 

 are, however, very interesting, but will be discussed later since they in- 

 volve complex metabolic systems. 



Obvious relationships between the rate and the inhibition occur in some 

 cases. For example, in mutual depletion systems with respect to the inhi- 

 bitor, varying the rate by altering the total enzyme concentration will 

 affect the inhibition, because more or less of the inhibitor will be bound to 

 the enzyme. As the enzyme concentration is increased, the rate will go up 



