METABOLITE ANTAGONISTS 241 



overcome by increasing the concentration of the normal metabohte. 

 Antagonists are known which inhibit the functioning of vitamins, amino 

 acids, and other metabohtes. 



It is postulated that a metabolite and its antagonists compete for the 

 active surface of specific enzymes. The ratio of inhibitor to metabolite 

 required to reduce growth to one-half its normal value is called the inhibi- 

 tion index. Effective inhibitors have small inhibition indexes. The 

 same compound may act as an antagonist for some fungi and as a metabo- 

 lite for others; e.g., desthiobiotin. The medium used for investigating 

 inhibition is important, for the presence of a normal metabolite, or a 

 secondary metabolite derived from it, may prevent inhibition, A given 

 compound may be considered as an antagonist, but it is only an antagonist 

 for certain species, and then only under certain conditions. Organisms 

 may acquire a tolerance or resistance to inhibitory agents and become 

 fast. In extreme instances they become dependent upon the inhibitor, 

 which then acts as a kind of growth factor. 



The competitive nature of many inhibitions is firmly established. In 

 most instances there is a close structural relation between a metabolite 

 and its antagonists. The theories which have been advanced to explain 

 these phenomena have been useful in correlating the results of research 

 and for increasing our insight into metabolic processes, 



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