4 THE BIOCHEMISTRY OF B VITAMINS 



but rather a matter of degree, we have not included the loose term "water 

 solubility" as an essential characteristic of a B vitamin. Indeed, if the 

 idea of the existence of a distinctive group of vitamins which function 

 catalytically in all living cells is a valid one, it is conceivable that we 

 may eventually come to include some of the "fat-soluble" compounds 

 among the B vitamins. Of such compounds already recognized, possibly 

 the most likely candidates for inclusion are the vitamins E, which appear 

 to be widely distributed in diverse organisms and tissues. 2 If these 

 vitamins should be found to be part of the catalytic machinery of all 

 cells, there would seem to be no very valid reason for excluding them 

 from the B vitamins. The fact that a separate letter designation has been 

 used for them would not preclude this possibility. Biotin, which was early 

 called "vitamin H," is now recognized as a member of the B family. The 

 question of the universal occurrence of B vitamins in relation to other 

 vitamins is discussed further in Chapter IIA. 



One of the interesting observations which in a measure appears to 

 differentiate the B vitamins from the members of the so-called fat-soluble 

 group is the fact that in the case of the fat-soluble vitamins there are 

 in every instance several naturally occurring and distinct chemical struc- 

 tures which possess the specific vitamin activity, whereas among the 

 B vitamins the physiological activity is more specifically associated with 

 a single chemical structure. It is true that in several instances among 

 the B vitamins, modified structures possess specific vitamin activity, but 

 in general each B vitamin is represented by a single substance or at 

 least by a few very closely related structures. As an instance of the 

 latter, in case a specific vitamin {e.g., nicotinamide, riboflavin) is in- 

 volved as a catalytic hydrogen carrier, it is reasonable that both the 

 oxidized and reduced forms should have physiological activity; likewise, 

 if a vitamin is a catalyst for ammonia transfer (e.g., pyridoxal), it is 

 not surprising that an aminated form (pyridoxamine) should possess 

 physiological activity. 



We may now profitably consider in a critical manner the meaning and 

 significance of the word "vitamin" — a term the meaning of which we 

 have so far in this discussion taken for granted. It involves historically 

 a nutritional concept and has come to be applied almost exclusively to 

 certain organic substances which function in the nutrition of higher 

 animals. While a considerable number of the B vitamins were discovered 

 and isolated using yeasts and bacteria as test organisms, they have not 

 been admitted to the family of vitamins unless they have been found to 

 be nutritionally effective for higher animals. 



A substance which counteracts a vitamin deficiency is not, however, 

 necessarily designated a vitamin. If this were so, thiamine pyrophosphate, 



