INTRODUCTION AND THEORETICAL CONSIDERATIONS 445 



This report prompted a widespread search for chemotherapeutic agents 

 among compounds which were structurally related to catalytic metab- 

 olites, but which inhibited the biological functioning of the metabolite. 

 Many investigators directed their efforts toward the discovery of new 

 and effective chemotherapeutic agents. Others prepared and utilized 

 analogues of metabolites in the study of biochemical transformations. 

 Out of these efforts a new field and new tools for the study of biochemistry 

 have been developed. 



Inhibition of Enzymatic Action 



Inhibition of the functioning of an enzyme by an inhibitor results in a 

 decrease in the rate of the enzymatic reaction and of the biological process 

 in which the enzyme is involved. The mode of action of substances in- 

 hibiting biological processes has in general been determined either by a 

 study of the relative concentrations of the inhibitor and substrate (metab- 

 olite) which are necessary to obtain a defined degree of inhibition after 

 the lapse of a specified time, or by a study of the effect of the substrate 

 on the rate of the process at a specified concentration of inhibitor. For a 

 competitive inhibition, the ratio of the concentration of inhibitor to the 

 concentration of substrate necessary for a defined degree of inhibition of 

 a biological process in a specified time is constant over a range of con- 

 centrations and is termed the inhibition index. The effect of the substrate 

 on the rate of the biological process inhibited by a competitive analogue 

 is such that at high concentrations of substrate the rate approaches that 

 of the normal process in the absence of the inhibitor. Such data are usually 

 presented graphically, as subsequently indicated. 



The Inhibition Index. Competition of an inhibitor I (analogue) with 

 a substrate S (metabolite) for an enzyme is frequently represented by 

 the following equations analogous to those first developed by Michaelis 

 and Menten, 15 where P represents the product and ES and EI represent 

 the enzyme-substrate complex and enzyme-inhibitor complex, respec- 

 tively : 



E+S =^= ES — > E+P 

 E+I =f=*= EI 

 By the Law of Mass Action: 



[E][S] K 



lESj = Ks (1) 



where K 8 is the dissociation constant of the enzyme-substrate complex, 

 and 



[E] [I] 

 [EI] ' 



K I (2) 



