BIOCHEMICAL REACTIONS AND THEIR CATALYSTS 111 



lished. Biotin activity is associated with several molecules more complex 

 than the simple vitamin itself and it would indeed be unusual if one or 

 more of these complexes were not a coenzyme for some type of reaction. 

 Although a great many facts have been learned about the metabolic 

 processes in which p-aminobenzoic acid, folic acid, and vitamin Bi 2 

 participate, no work has been reported which justifies drawing any con- 

 clusions concerning the chemical relationship of these vitamins to their 

 respective coenzymes. Inositol has recently been shown to be an active 

 constituent of a-amylase. It has not been definitely demonstrated that 

 this simple molecule is identical with the coenzyme, but it would appear 

 that this could be so (p. 125) . 



Coenzymatic Activity of the Simple Vitamins. Can the B vitamins, 

 in their simplest chemical forms, ever serve as coenzymes? With the 

 exception of inositol, just mentioned, there is no evidence that any B 

 vitamin participates in vivo in a catalytic function until it has been 

 transformed into its corresponding coenzyme (s). 



It is possible in some cases to demonstrate, in vitro, chemical trans- 

 formations in which a particular vitamin behaves in a manner analogous 

 to the behavior of its coenzyme, in vivo. An example is the conversion 

 (transamination) of pyridoxal to pyridoxamine by heating the former 

 with amino acids. The conditions required for a reasonable yield make 

 it appear most unlikely that within the cell such nonenzymatic reactions 

 of the free vitamin could be important. It is also possible to demonstrate 

 that free riboflavin can take part directly in oxidation and reduction 

 processes. These reactions are so sluggish in an environment comparable 

 to that found intracellularly that they would undoubtedly be useless. 



Some of the free vitamins produce very distinctive pharmacological 

 responses (Chapter VC) ; however, these are entirely independent of the 

 vitamin function of the compound and have no necessary connection 

 with processes in cellular metabolism for which the vitamin is required. 

 There are two possible ways in which a free vitamin can conceivably 

 influence the metabolism of a cell. It is possible for a vitamin to act as 

 an inhibitor of its own coenzyme (Section D) . Because of the structural 

 similarity between a vitamin and its coenzyme, it is possible to produce 

 effects which can best be explained by assuming that the vitamin com- 

 petes with its coenzyme for the apoenzyme. In in vitro studies with 

 bacteria and yeast, however, the concentrations of a vitamin required 

 to produce inhibition are entirely outside the limit which would be found 

 in natural circumstances. The ability of an organism to detoxify the 

 inhibitor (the vitamin) by converting it to the coenzyme would also 

 reduce the probability of achieving concentrations producing inhibition 

 within the cell. 



