CONVERSION OF CAROTENOIDS INTO VITAMIN A 



synthetic and do not occur naturally. Goodwin ^^ has recently re- 

 viewed this work. 



CH,OH 



Anhydrovitamin A 



Rehydrovitamin A 



Another factor which controls the vitamin A activity of carotenoids 

 is spatial isomerism. It will be recalled {see p. 9) that carotenoids 

 generally occur in nature in their all-trans forms. Deuel and Zech- 

 meister and their collaborators 39-50 j^^ve assayed many m-isomers of 

 a- and (3-carotenes for vitamin A activity (see Zechmeister ^ ^ for a 

 review) ; all these isomers except one (see below) are less active than 

 are the corresponding parent all- trans compounds ; confirmatory 

 results have recently been obtained in India. ^ ^ To account for this 

 loss of activity these investigators suggest that a trans — > cis rotation 

 will result in a carotenoid losing its straight shape ; this will then have 

 difficulty in fitting on to the enzyme system(s) necessary to convert it 

 into vitamin A. The importance of this straight chain is illustrated 

 in the case of neo-[3-carotene U ; in this compound only one peripheral 

 double bond has undergone trans — > cis rotation but its activity is 

 somewhat less even than that of a-carotene, which can be considered 

 to be derived from ^-carotene by migration of one terminal double 

 bond out of conjugation. 



An apparent deviation from this rule is pro-y-carotene. This cis 

 isomer which occurs naturally {see p. 30) is now considered as active a 

 vitamin A precursor as all-^raw^-y-carotene, both being more active 

 than neo-y-carotene P.** Originally it was thought that pro-y- 

 carotene was the most active of the series. ^ ° In this case Deuel, 

 Zechmeister et al. consider that by undergoing the maximum number 

 of trans — ^ cis rotations a carotenoid molecule can recover a straight 

 chain shape. Pro-y-carotene is a poly cw-carotenoid and in all 

 probability is 3:5:7:9 : ii-penta-c/s-y-carotene (the numbers refer 

 to the double bonds) thus : 



Me /=^ 



Or 



3:5:7:9:1 /-penta-cis-ycarotene 

 273 



