METABOLISM OF CAROTENOIDS AND VITAMINS A 541 



many years as a property of organic compounds which possess a double 

 bond, the reahzation that this behavior also applies to carotenoids dates 

 back only to 1935, when Gillam and El Ridi'^^^ reported geometric isomer- 

 ism in the C40 carotenoids. These authors were able to separate the 

 several geometric isomers on an alumina column. "Pseudo-a-carotene," 

 one of the isomers of a-carotene, was shown to have distinct properties 

 differing from those of the all-^ran.s isomer. Gillam and co-workers^" ■''^*' 

 later demonstrated the reversibility of the cis-trans conversion. 



Although the British workers attributed this isomeric change to the ac- 

 tion of the adsorbent in the chromatographic column on the carotenoid, 

 the subsequent investigations of Zechmeister and his colleagues proved that 

 the reactions were spontaneous, and independent of the process of adsorp- 

 tion. It was proved that capsanthin solutions undergo these spontaneous 

 and reversible changes.^^^ Zechmeister and Tuzson"""^ later described 

 similar stereoisomeric changes in the case of /3-carotene, lycopene, and cryp- 

 toxanthin which were not related to the adsorption process. 



With few exceptions, the natural forms of the carotenoids are the ail- 

 erons compounds in which every double bond capable of cis -^ trans rear- 

 rangement is in the trans position. Pro-7-carotene and prolycopene repre- 

 sent two naturally occurring carotenoids which are polycis compounds. 

 Not all double bonds are capable of undergoing geometric isomerism, but 

 only those in which no methyl group is attached to either of the carbon 

 atoms adjacent to the atoms composing the double bond system. The 

 nomenclature employed by Zechmeister"^ for the several isomers is based 

 upon relative ease of adsorbability on the chromatographic column, as 

 compared with the sdl-trans form. Isomers with a low degree of adsorb- 

 ability, and which consequently appear below the all-trans form on the 

 chromatographic column, are called neo-A, neo-B, neo-C, etc., while those 

 with a higher adsorbability than the all-trans isomer, and which hence 

 appear above the all-trans isomer, are called neo-T, U, V, etc. For a fur- 

 ther discussion of cis-trans isomerism and stereochemistiy of the carotenoids 

 the reader is referred to The Lipids, Vol. I, pages 621 to 666, to the review 

 of Zechmeister,"^ and to that of Zechmeister and Polgar."* 



766 A. E. Gillam and M. S. El Ridi, Nature, 136, 914-915 (1935). 



767 A. E. Gillam and M. S. El Ridi, Biochem. J., 30, 1735-1742 (1936). 



'68 A. E. Gillam, M. S. El Ridi, and S. K. Kon, Biochem. J., 31, 1605-1610 (1937). 



'69 L. Zechmeister and L. v. Cholnoky, Ann., 530, 291-300 (1937). 



"0 L. Zechmeister and P. Tuzson, Nature, I4I, 249-250 (1938). 



"1 L. Zechmeister and P. Tuzson, Biochem. J., 32, 1305-1311 (1938). 



"2 L. Zechmeister, Chem. Revs., 34, 267-344 (1944). 



"3 L. Zechmeister and A. Polgdr, /. Am. Chem. Soc, 66, 137-144 (1944). 



