Chapter II B 



COENZYMES DERIVED FROM B VITAMINS 



The problem of determining the specific chemical reactions catalyzed 

 by a coenzyme derived from a B vitamin is not one which can be attacked 

 in a straightforward manner. There is no standardized procedure the 

 utilization of which would assure an investigator of success in determin- 

 ing the function of a newly discovered vitamin. The relationships between 

 B vitamins and their enzyme systems have been established through the 

 correlation of information obtained from many types of investigation of 

 metabolic processes. 



In a critical examination of biochemical processes, there are logical 

 reasons for studying the enzymatic reactions in their natural environment, 

 i.e., within intact cells and as an integral part of a series of reactions. 

 On the other hand, it is impossible to characterize a reaction completely 

 — to determine the specific compounds reacting and the component parts 

 of the catalyst — unless an isolated system free from all interfering 

 phenomena is studied. Obviously, it is not possible to achieve both of 

 these goals with the same techniques. This difference in purpose has led 

 to some argument concerning the relative merits of establishing the 

 existence of biological processes in cells by the use of isotopes, inhibitors, 

 mutants, etc., and of studying specific isolated enzymatic reactions in 

 detail. An adequate explanation of metabolism can ultimately be achieved 

 only by approaching the individual problems from both directions. 



The coenzymatic functions of some of the B vitamins were discovered 

 by investigators whose primary interests were in the enzyme systems they 

 were isolating; in such cases, specific vitamins have fortuitously been 

 found to be component parts of particular enzymes — that is, the investi- 

 gators up to time of finding the presence of a given vitamin, may have 

 had no particular interest in that or any other vitamin. 



In other cases the known functions of B vitamins have been ascertained 

 as a result of observing the effect of vitamins upon metabolism. In such 

 direct attempts to study the chemical changes catalyzed by the vitamins, 

 normal biological systems are compared with those in which the vitamin 

 has been prevented from functioning. The most direct method of limiting 

 the reactions catalyzed by a vitamin is to produce a deficiency of that 



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