326 



I 



RIBOFLAVIN 



of serving as the sole source of flavin in the growth of Lactobacillus caseif^ 

 however, this flavin has an antiriboflavin activity in the growing rat.^^^ 

 7-Ethyl-9-(D-l'-ribityl)isoanoxazine is effective only in the presence of 

 suboptimal amounts of riboflavin. ^^ 



H3C 



\^ 



^ 



H3C 



H3C 



Riboflavin 



H3C 



6-Methyl 

 flavin 



H3C 



HsCs 



H5C5 



HsCs 



Riboflavin 

 antagonist : 



H3C 



CHa 



7-Methyl- 

 flavin 



6-Ethyl-7- 

 methylflavin 



Biologically Active Riboflavin Homologs 



6,7-Diethyl- 

 flavin 



Isoribo- 

 flavin 



I 



Absence of substituents in both the 6 and 7 positions of riboflavin is con- 

 nected with high toxicity.^ ^ For biological activity, the imino group in the 

 3 position has to be unsubstituted. 



Substituents in any other than the 6 and 7 positions on the benzene 

 ring destroy and in certain cases reverse the vitamin activity. For instance, 

 isoriboflavin, an isomer of riboflavin, which has its two methyl groups in 

 the 5 and 6 positions, is an antagonist of vitamin B2 . 



From the examples given, it is obvious that only slight alterations of 

 the ring substituent in the riboflavin structure can be made without the 

 loss of vitamin activity. 



OHH H 



I I I 

 The following L-araboflavins, R — -0112 — ^C — -C — -C — ■CH2OH, possess 



I I I 

 H OH OH 



some little stimulating activity for rats and lactic acid bacteria in the 

 presence of suboptimal amounts of riboflavin: 6,7-dimethyl-9-(L-l'- 

 arabityl)isoalloxazine (L-araboflavin), 6 ,7-trimethylene-9-(L-l'-arabityl)- 

 isoalloxazine, 6 , 7-tetramethylene-9-(L-l'-arabityl)isoalloxazine,^^ 6-meth- 

 yl-7-amino-9-(L-l'-arabityl)isoalloxazine; the D-arabityl derivative corre- 

 sponding to the latter substance is said to be about half as effective as 

 riboflavin in promoting the growth of B2-avitaminotic rats.^^ 



92 J. p. Lambooy, J. Biol. Chem. 188, 459 (1951). 



92" H. V. Aposhian and J. P. Lambooy, Proc. Soc. Expil. Biol. Med. 78, 197 (1951) ; J. 

 Nutrition 47, 539 (1952). 



93 P. Karrer and T. H. Quibell, Helv. Chim. Acta 19, 1034 (1936). 

 9^ R. Kuhn, H. Vetter, and W. Rzeppa, Bcr. 70, 1307 (1937). 



96 Sumi Nishida, C.A., 45, 7127 (1951); 46, 6715 (1952). 



