352 



CH, H 



I 

 G 



\/\ 



/— CH3 



CH, 



CH3H 



CH, 



C 



1 

 H 



C 



X / 



CH, C 



1 

 I 



H 



ANIMAL BIOCHEMISTRY 

 CH3 H CH3 



C 



\, 



G 



% 



\ 



\ 



H H 



trans-vitSLtnin Ai 



GH3 



G H 

 C 



H 



G H 



\ 



G 



G 



\ 



\ 



H 



H 



CH2OH 



C=G 



GH3 



A^-fi's-vitainin Ai 



GH2OH 



has one cis double bond, probably the third. Although unknown at 

 present, others of the possible isomers may exist in nature. 



Although plants do not contain vitamin A, herbivorous animals 

 must either depend on plants for a substitute or synthesize their own 

 vitamin. Actually all plant eaters probably follow a middle course 

 and convert certain plant pigments into vitamin A. The most im- 

 portant pigment in this process, jS-carotene, is already closely related 



-CH3 



CH3 



GH3 



-CH=CH— G=CH— CH=CH— C=GH— CH=CH— CH= 



GHa GH3 



CH3 



CH3 



13 ^'113 GH3 



C— CH=CH— GH=C— GH=GH- 



GH3 CH3 



)3-carotene 



to the vitamin. Since both halves of /3-carotene are alike and match 



