COMPETITIVE ANALOGUE-METABOLITE INHIBITION 463 



[S] are varied, the inhibition index varies approximately directly with 

 the total effective enzyme concentration. 



The substances which can produce such an increase in the effective 

 enzyme concentration include (a) a limiting coenzyme or a precursor of 

 a limiting coenzyme when excess apoenzyme is synthesized by the biologi- 

 cal system; and (b) a limiting second substrate which is involved in the 

 utilization of the inhibited substrate in the enzymatic reaction. Since the 

 biological system normally synthesizes these substances in near optimal 

 amounts, these effects usually are relatively moderate in comparison with 

 the other types. 



(5) Destruction of the Inhibitor. Substances which allow rapid destruc- 

 tion of an inhibitory analogue by the biological system will render the 

 analogue ineffective as an inhibitor; however, if destruction occurs more 

 slowly, other effects, e.g., one similar to a precursor effect, may be ob- 

 served. 



Other Theoretical Considerations. Reversing agents of type 1 are 

 easily distinguished from those of types 2, 3, and 4 by determination of 

 the inhibition index over a wide range of concentrations. Since substances 

 of the latter three types exert their action by increasing the inhibition 

 index, methods whereby these groups can be further differentiated are 

 desirable. A number of substances exerting such an effect on competitive 

 inhibition can usually be classified into the various groups. Since reversing 

 agents of type 2 but not of types 3 and 4 involve more than one enzyme, 

 data which involve the dissociation and other constants will permit classi- 

 fication of substances of type 2. Hence, the ratio of the inhibition index 

 for maximum to that for half-maximum inhibition would be expected to 

 change with the addition of an agent of type 2, but not of types 3 and 4. 

 Reversing agents of type 3 and type 4 exert their effects independently 

 of each other and are synergistic; but neither exerts an effect on the in- 

 hibition in the presence of the product of type 2. 



When two analogues prevent enzymatic reactions at two successive 

 stages in a biosynthetic sequence, a synergistic action of the inhibitions 

 is usually obtained. When the first biosynthetic transformation is inhib- 

 ited to such an extent that the biosynthesis of the immediate product is 

 reduced to half the normal rate, the amount of an analogue of the product 

 necessary to inhibit completely the response of the biological system is 

 reduced to half. However, the amount of the first analogue necessary to 

 reduce the rate of the reaction to half the normal rate is often only a small 

 fraction of that necessary to inhibit completely the response of the bio- 

 logical system. Consequently, the effects of the inhibitors are not additive 

 but synergistic. Demonstration of such synergistic effects can often be 

 useful in indicating such a biosynthetic sequence. 



