Chapter II D 



UTILIZATION OF COMPETITIVE ANALOGUE-METABOLITE 



INHIBITION IN THE ELUCIDATION OF BIOCHEMICAL 



PROCESSES INVOLVING VITAMINS 



Rapid methods of developing relatively specific assays for metabolites 

 of biochemical systems and of elucidating the biochemical processes in- 

 volving these substances are desirable in the study of biochemistry. One 

 of the recently developed methods whereby this can be accomplished 

 involves a study of the effects of known metabolites and of biological 

 extracts on a specific competitive analogue-metabolite inhibition of a 

 biological system. These effects can be studied by a variety of testing 

 techniques to determine the interrelationship of the metabolite to the 

 substances affecting the inhibition. The ability of unknown substances in 

 natural extracts to exert an influence on the system can be used as a basis 

 for assay of unknown naturally occurring substances directly related to 

 the metabolite. Consequently, this method, which has been termed inhibi- 

 tion analysis, offers a direct approach to specific problems of biochemistry 

 which may otherwise be difficult to solve. 



Theoretical Considerations 



From a theoretical standpoint, 1-5 exogenous substances other than the 

 metabolite which are capable of preventing the inhibitory effect of the 

 analogue in competitive analogue-metabolite inhibitions of biological sys- 

 tems include (1) substances which increase the effective concentration 

 of the metabolite, e.g., precursors of the metabolite; (2) the product, or 

 its equivalent, of the inhibited enzymatic reaction; (3) substances which 

 decrease the quantity of the product necessary for normal functioning of 

 the biological system, i.e., substances exerting a "sparing action" on the 

 product of the inhibited enzyme system; (4) agents increasing the effec- 

 tive concentration of the inhibited enzyme; and (5) substances which 

 assist in the destruction of the inhibitory analogue. 



(1) Precursor Effect. The effective concentration of the metabolite 

 may be increased in a biological system by supplementation of the system 

 with several types of substances other than the metabolite. The most 

 common is a limiting precursor, an additional amount of which allows 

 the organism to synthesize an increased concentration of the metabolite. 



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