228 



PHYSIOLOGY OF THE FUNGI 



of phenylalanine (a normal metabolite) by Neurospora crassa (Mitchell 

 and Neimann, 1947). 



The effect of an antagonist will depend upon the concentration of the 

 normal metabolite present in the medium and cells and upon the organ- 

 ism. In general, enzymes have a greater affinity for metabolites than 

 for antimetabolites. Since both metabolite and antagonist compete 

 for the same enzyme, the amount of inhibition will depend upon the 

 relative concentrations rather than upon the absolute amounts of these 

 compounds present. The amount of an inhibitor required to reduce the 



nAP 



Substrate 



+ 



Products 

 free enzyme 



Enzyme -substrate 

 complex 



Fig. 47. A diagrammatic illustration of Fischer's simile that an enzyme and its 

 substrate are related as are a lock and its key. (Courtesy of McElroy, Quart. Rev. 

 Biol. 22 : 26, 1947. Published by permission of The Williams & Wilkins Company.) 



amount of growth to one-half will depend upon the ratio of inhibitor and 

 metabolite present. In simple instances, at least, this ratio is equal to a 

 constant and is called the inhibition constant, or index. The amounts 

 of sulfadiazine and p-aminobenzoic acid required to reduce the amount 

 of growth of Streptococcus faecalis R to one-half the normal value gave an 

 inhibition index of 333 (Lampen and Jones, 1946). 



The inhibition index is valid only for the particular conditions used 

 in an experiment and for the particular strains of the organism used. In 

 the case of self-sufficient organisms the use of an amount of inhibitor less 

 than that required for total inhibition will only decrease the rate of 

 growth, and thus the inhibition index will change with the time of incuba- 



