6 THE BIOCHEMISTRY OF B VITAMINS 



are absent from certain types of cells, then our definition might have to 

 be modified immediately. At present such modification does not appear 

 likely. 



Chemically Recognized B Vitamins 



There are a number of specific chemical substances of known structure 

 which are universally recognized as B vitamins. We may, therefore, gain 

 a more specific idea of what is meant by the term "B vitamin" by dis- 

 cussing briefly these individual compounds. 



Thiamine (aneurin) earlier received the designation "Bi" in keeping 

 with the fact that it was the first B vitamin to be discovered and isolated. 

 There is substantially no confusion resulting from identifying vitamin 



CH 3 

 N=C— NH 2 HC1 C=C— CH 2 — CH 2 OH 



CH 3 — C C— CH 2 N 



II II + \ 



N— CH CI- C— & 



H 



Thiamine chloride hydrochloride 



Bx as thiamine, because thiamine is the only naturally occurring structure 

 which is capable of performing the vitamin functions (p. 684). More 

 complex structures containing the thiamine unit as a part may function 

 nutritionally, as may also products formed by reversible oxidation and 

 reduction, if such exist. The chemistry of thiamine has been reviewed 

 by R. R. Williams. 3 



Riboflavin, earlier called vitamin B 2 , has the structure indicated below 

 and is the only naturally occurring structure possessing the characteristic 

 vitamin activity. Several synthetic flavins have lesser biological activity; 



CH 2 OH 



HO— C— H 



HO— C— H 



HO— C— H 



CH, 



H I 



CNN 



CH 3 — C C C C=0 



I II I I 



CH 3 — C C C NH 



V V V 



Riboflavin 



