538 IX. CAROTENOIDS AND VITAMINS A 



rat and in the chick, and that this potency is of a lower order than that of 

 (S-carotene. Pro-7-carotene, which is a naturally occurring polycis com- 

 pound, is considered in the following main section (see page 542). 



Other naturally-occurring provitamins A about which less is known in- 

 clude citroxanthin (or mutatochrome),^* myxoxanthin,-^ aphanin, isolated 

 by Tischer^^^ from the freshwater blue-green alga, or "water-bloom" 

 (Aphanizomenon flosaquae) (potency about 50% of that of jS-carotene),^^^ 

 aphanacene (potency 25% that of /3-carotene) ,''^- torularhoden (C,-i7H48- 

 Oo),^'* and a-carotene epoxide.'^^^ Mention should also be made of luteo- 

 chrome, which Karrer and co-workers^*^ and Karrer and Jucker'^^'* isolated 

 and which, according to Karrer et al.,''^^ possesses full vitamin A activity 

 when fed in daily doses of 18 ^g- 



Synthetic 16,16'-homo-|8-carotene, C42H58, which has two additional car- 

 bons centrally located on the aliphatic chain, has been reported to have 

 20% of the biologic activity of jS-carotene.'^^^ Two additional members of 

 this series, decapreno-/3-carotene, CaoHeg,^^^'^^^ and dodecapreno-/3-carotene, 

 CeoHso/" have been synthesized, but their biologic activity has not been 

 reported. One would expect them to be active provitamins at a level 

 somewhat below that of 16,16'-homo-/3-carotene. The synthesis and prop- 

 erties of the homologues of /3-carotene have been reviewed by Inhoffen 

 and Siemer.''"^ 



(a) The Extent of Transformation of ^-Carotene to Vitamin A. (3- 

 Carotene contains two intact (S-ionone rings, while other carotenoids possess- 

 ing provitamin A activity (a-carotene, 7-carotene, cryptoxanthin, echine- 

 none) have only a single intact /3-ionone structure. It is known that the 

 iS-ionone structure must be unaltered if the substance is to be active as a 

 source of vitamin A. Since ;S-carotene is at least tAvice as potent as a source 

 of vitamin A as are the other carotenoids which possess provitamin A ac- 

 tivity, it is only natural to ascribe this to the fact that it contains two P- 

 ionone rings while the other active provitamins A contain only one such 

 structure. This correlation is obviously a perfectly sound one. 



The mechanism by Avhich carotene is transformed into vitamin A is not 

 yet completely elucidated. One theory which has been proposed, but 



T62 A. Scheunert and K. H. Wagner, Z. phtjisol. Chetti., 260, 272-275 (1939). 



7" p. Karrer, J. Rutschmann, and K. Steinlin, Helv. Chim. Acta, 28, 1146-1156 

 (1945). 



-"•* P. Karrer and E. Jucker, Helv. Chim. Acta, 28, 427-436 (1945). 



'55 H. J. Deuel, Jr., H. H. Inhoffen, J. Ganguly, L. Wallcave, and L. Zechmeister, 

 Arch. Biochem. Biophys., 40, 352-357 (1952). 



"6 P. Karrer and C. H. Eugster, Helv. Chim. Acta, 34, 28-33 (1951). 



'" P. Karrer and C. H. Eugster, Helv. Chim. Acta, 34, 1805-1814 (1951). 



"8 H. H. Inhoffen and H. Siemer, Fortschr. Chem. org. Naturstoffe, 9, 1-40 (1952). 



