696 VII. VITAMINS A 



The provitamin A activity of vitamin A2 is further substantiated by the 

 work of Lederer and Rathmann^*'' who found that the livers of rats and frogs 

 had vitamin A activity after vitamin A2 had been fed. 



Although a number of other homologues of vitamin A are known — the 

 apocarotenols (see chapter VI, pages 531 and 532) — no other types of vita- 

 min A are found as natural products. However, stereoisomeric forms of 

 vitamin A have been discovered in fish liver oils, differing from vitamin A 

 only in the tr-ans or cis configuration of the double bonds. 



(3) Stereoisomeric Forms of Vitamin A 1 



The possibility that stereoisomeric forms of vitamin A may exist has been 

 recognized only recently. Zechmeister^^ has pointed out that, in addition 

 to all-^rans-vitamin A, which is the usual form, three other isomers are pos- 

 sible: 3-cis-, 5-cis-, and 3,5-di-cis-vitamin A. The structure of these pos- 

 sible isomers is illustrated here. 



/ \ 



H,C CCCHj), 3 5 



I I H 



H,C C C 



H H i H 



/ u V i C C I G 



\ ^ \ //\ //\ //\ y/\ 



C C C C C CHjOH 



I H I H I 



CHj CH3 CHj 



All-irans-vitamin A 



HjC C(CH5)2 



I I H 



C C A A S--. .CHzOH 



iH3 Hh^C^VVV 

 H H H 



3-cis- Vitamin A 



A 



HjC C(CH3)2 



I I H H H 



"2^^ ,C^ ^C^ .C^ ^C. 



(y C^ C^ C^ C, CHzOH 

 I H I H„ / -^_/ 



CH3 CHj HjC C 



H 



5-c2's- Vitamin A 



/ \ 



HaC CtCHj), 



I I H H 



"^\ /\ /n H^'\ /\ , 



? s / w ^"^^'^ 



CH3 H,C C C 



3,5-Di-cis-vitamin A 

 i« E. Lederer and F. H, Rathmann, Biochem. J., 32, 1252-1261 (1938). 



