510 VI. CAROTENOIDS AND EELATED COMrOTINDS 



and Heilbron^^ presented the modern viewpoint on the chemistry and 

 physiology of vitamin A. An over-all picture of the chemical and physio- 

 logical relationships of the provitamins A and vitamins A is included in 

 Rosenberg's treatise. '- 



B. THE CAROTENOIDS 



1. Introduction 



The carotenoids represent a group of pigments which have an extremely 

 wide distribution in nature. Not only do they occur in a wide variety of 

 plants, ranging from the bacteria — which may be classed as the lowest 

 forms of cryptogams — to the dicotyledons — which may be considered as 

 the highest level of phanerogams — but they are found to be equally widely 

 distributed in the animal kingdom. Thus, they are present in both inverte- 

 brates and vertebrates and, in fact, in most species from the protozoa to 

 man. 



Much confusion has arisen in naming the carotenoids. In many cases 

 identical pigments have not been recognized when prepared from two 

 sources, with the result that two names have been applied to the same sub- 

 stance. This is in part due to the failure to prepare pure pigments which 

 were not admixed with other carotenoids. In some cases contamination 

 with fat changed the properties. In other cases, the examination of the 

 pigment in different solvents showed variations in properties, a fact which 

 has not always been recognized. Finally, a considerable number of stereo- 

 isomers with markedly different properties can be prepared from the same 

 parent carotenoid when it is subjected to relatively mild treatment. It 

 has been possible to obtain a clean-cut separation of these latter isomers 

 from each other only by the use of chromatography. 



The first classification of carotenoids was proposed by Thudichum,'' 

 who suggested the name, luteine or luteins, for the yellow pigments found in 

 animal and plant tissues. He included in this group not only the yellow 

 pigment from the corpus luteum of mammals, from which the name was 

 derived, but also that in the blood serum, adipose tissue, and butter, as 

 \\^\\ as the chromogenic material of egg-yolk. According to Thudichum, 

 such pigments as that of yellow maize {Zea mays), of anatto seeds {Bixa 

 oreUana), of the carrot (Daucus carota), of such yellow leaves as the Coleus, 

 and of the stamens and petals of many flowers, comprise the plant luteins. 



31 I. M. Heilbron, J. Chem. Soc, 1948, 386-393. 



'^ H. R. Rosenberg, Chemistry and Phi/siology of the Vitamins, Interscienoe, New York, 

 1945. 



" J. W. L. Thudichum, Proc. Roij. Soc. London, 17, 253-256 (1869). Cited by L. S. 

 Palmer, Carotinoids and Related Pigments, Ohemical Catalog Co., New York, 1922, pp. 

 14. 87. 



