Chapter IXD 



RIBOFLAVIN 



In 1933, one of the members of the vitamin B 2 complex was identified 

 as lactochrome, a naturally occurring yellow pigment, 1 first concentrated 

 from milk in 1879, 2 and obtained in crude form from the same source 

 in 1925. 3 This yellow pigment, which has a characteristic green fluores- 

 cence, was obtained in crystalline form from both egg white (ovoflavin) 1 

 and milk (lactoflavin). 1 - 4 Subsequent chemical studies by Karrer and 

 Kuhn and their co-workers limited the structural possibilities for the 

 vitamin to the isomeric 6,7-dimethyl-9-(tetrahydroxyamyl)-isoalloxa- 

 zines. By preparation of the possible isomers, the structure was finally 

 established by Karrer and co-workers 3 > 6 and Kuhn and co-workers, 7 * 8 

 who independently synthesized the vitamin in 1935. The vitamin which 

 has since been termed riboflavin has the following structure: 



CH 2 OH 



I 

 HO— C— H 



HO— C— H 



I 

 HO— C— H 



H-i- 



II 



X 



CH,-^ Y C C=0 



N C 



A 



riboflavin 

 [6, 7-dimethyl-9- (d,1 '-ribityiyisoalloxazine] 



While the purification and proof of structure of riboflavin were being 

 accomplished, simultaneous investigations showed the vitamin to be 

 associated with certain enzymes. In 1932, Warburg and Christian 9 

 isolated a coenzyme essential for the functioning of the yellow enzyme. 

 This coenzyme was found to be related to vitamin B 2 , 10 ' u and in 1936 

 its identity with synthetic riboflavin-5'-phosphoric acid was demon- 

 strated. 12 In vitro, riboflavin can replace the coenzyme, but only at 



