B VITAMINS: WHAT THEY ARE 7 



esters and other simple derivatives of riboflavin may possess full activity, 

 and conjugated forms such as the nucleotides or flavoproteins may be 

 nutritionally effective in proportion to their content of combined ribo- 

 flavin. The (reversibly) reduced form of riboflavin and its conjugates 

 are also physiologically active. Since the establishment of its constitution 

 and its synthesis in 1935 by Kuhn and Karrer and their co-workers, there 

 have been few advances in the organic chemistry of riboflavin. A com- 

 plete review of this topic may be found in the literature. 4 



Nicotinic Acid, Nicotinamide. These two compounds interchangeably 

 possess vitamin activity and the only other naturally occurring com- 

 pounds which can function nutritionally in the same manner are more 

 complicated derivatives which may act because they contain the essen- 



H H 



C C 



HC C— COOH HC C— CONH 2 



II 1 II i 



HC CH HC CH 



\ X \ • 



N N 



Nicotinic acid Nicotinamide 



tial structure in combined form. Combined forms, even naturally occurring 

 ones, are not necessarily wholly effective. The chemistry of nicotinic 

 acid is too old a topic in the field of organic chemistry to require com- 

 ment here. 



Pantothenic Acid. Probably this compound was mainly responsible for 

 what was first designated "vitamin B 3 "; it is the only naturally occurring 

 one capable of performing the nutritional function. Conjugated forms 



CH 3 OH O 



HO— CH 2 — C^— CH— C— NH— CH 2 — CH 2 — COOH 



CH 3 



Pantothenic acid 



may possess vitamin activity because they contain the fundamental 

 structure in combination. The chemistry of pantothenic acid has been 

 reviewed 5 and detailed material on the subject will be found in the 

 literature cited in the review. Other material dealing with the chemistry 

 of pantothenic acid will be found elsewhere in this volume (p. 464). 

 Pyridoxal, Pyridoxamine, Pyridoxine. For animals these three forms 

 are nutritionally interchangeable and are often thought to be in equilib- 

 rium in vivo. Lactic acid bacteria often show incomparably greater 

 response to pyridoxal or pyridoxamine or their phosphates 6 than to 

 pyridoxine, so these forms of the vitamin which were discovered later 



