448 THE BIOCHEMISTRY OF B VITAMINS 



decrease in concentration of enzyme-substrate complex induced by the 

 inhibitor may possibly affect the biosynthesis of the enzyme by a system 

 of multiplying cells. Any such change in concentration of enzyme under 

 these conditions would be expected to be a function of [ES] and of time. 

 It has been shown that even if such changes in the intracellular concen- 

 tration of the enzyme do occur during the course of an experiment, there 

 still is a specific unique value of [/]/[S] which must be initially estab- 

 lished in order to obtain the specified response of the system at the end 

 of a constant experimental period. 16 



It can likewise be demonstrated that even when a growing system 

 develops a resistance to an inhibitor during the course of an experiment, 

 there is still a definite initial ratio of inhibitor to substrate concentration 

 for the particular system which will produce the specific response for the 

 constant experimental period. 



For experimental conditions in which the concentrations of inhibitor 

 and substrate must be regulated in a medium outside the biological sys- 

 tem wherein the reaction takes place, i.e., bacterial growth experiments, 

 etc., the ratio of the concentrations of the inhibitor to the substrate 

 within the cell is a function of this ratio of the concentrations existing 

 outside the cell. Consequently, this latter ratio is also constant for a 

 defined inhibition after a specific period of time and is termed the inhibi- 

 tion index for such systems, even though it does not represent the actual 

 intracellular ratio. 



For biological systems in which high concentrations of inhibitor and 

 substrate cannot be employed, e.g., embryonated eggs, the amount of 

 the substrate synthesized by or present in the system must be considered. 

 The inhibition index would be [/]/([Si] + [S2]), where [Si] represents 

 the contribution of the biological system and [S2] represents the con- 

 tribution of the exogenous supply to the total concentration of substrate. 



If an inhibitor reacts with the enzyme-substrate complex, a constant 

 inhibitor-substrate ratio for a defined inhibition is not obtained, and the 

 inhibition is of the noncompetitive type. 



Even though a vitamin is initially utilized as substrate in the forma- 

 tion of a coenzyme, an analogue may prevent the combination of this 

 coenzyme with an apoenzyme. This can be illustrated by the following: 



P+E a Co 



where E a , Co and / represent the apoenzyme, coenzyme and inhibitor, 

 respectively. E a Co, and E a I represent the complete enzyme and the 

 apoenzyme inhibitor complex, respectively. S, E a CoS and P represent the 



