29G IV. DIGESTION OF LIPIDS OTHER THAN FATS 



canary (Serinus canarius) and of the yellow-hammer (Emberizia citrinella). 

 Lutein occurs in the green water-frog (Rana esculenta) , nz as well as in a 

 number of lower forms. The possible effect of xanthophylls (lutein) on 

 the absorption of /3-carotene and vitamin A has been discussed earlier. 



(c) Zeaxanthin. Zeaxanthin and lutein have been isolated from similar 

 sources. Zeaxanthin occurs in human fat, 308 - 314 in human liver, 298 in the 

 yolk of hen's egg, 314-316 and in the egg of the Atlantic dogfish (Squahis 

 acanthias) . 317 This carotenol likewise occurs along with lutein in the 

 feathers of the wild canary (Serinus canarius) , zu and in the green water- 

 frog. 313 



(d) Other Oxycarotenoids. Zechmeister and Tuzson 296 demonstrated 

 the interesting fact that capsanthin was found in the depot fat of man. 

 These authors 308 consider the presence of the pigment to be adventitious, 

 since it is present chiefly in the fatty tissues of Hungarian peasants who 

 consume large quantities of paprika; the latter vegetable is the principal 

 natural source of the capsanthin pigments. 



e. Stereoisomeric Forms of Carotenoids. No direct absorption studies 

 of the stereoisomeric m-forms of the naturally occurring all-trans caroten- 

 oids are available. A number of the isomers have been proved to be sources 

 of vitamin A; it is not certain, however, whether the vitamin A arises di- 

 rectly from the cis form of the carotenoid or from a portion of the natural 

 all-trans compound which has been produced by isomerization in the intes- 

 tine or after absorption in the tissues. 



In general, the provitamin A potency of the stereoisomers varies from 

 15 to 50% of that of the corresponding all-trans forms, although pro-7- 

 carotene has a bioactivity equal to that of its all-trans isomer. 



The following provitamin A activities have been recorded for the stereo- 

 isomers tested on rats, all-trans-(3-earotene being assigned a potency of 100: 

 neo-0-carotene B, 53% 318 ; neo-0-carotene U, 38%, 319 and 24% 320 ; a- 

 carotene set (all-trans, 53%) 290 ; neo-a-carotene B, 16% 318 ; neo-a-raro- 



313 L. Zechmeister and P. Tuzson, Z. physiol. Chem., 238, 107-203 (19.36). 



314 P. Karrer and E. Jucker, Carotinoide, Birkhauser, Basle, 1948, p. 183. Although 

 Karrer and Jucker 314 list zeaxanthin as a component of human fat, authorities whom they 

 cite in support of their statement 295 ' 308 do not mention this compound. 



316 R. Kuhn, A. Winterstein, and E. Lederer, Z. physiol. Chem., 197, 141-160 (1931). 



316 H. H. Strain, Chromatographic Adsorption Analysis, Interscience, New York- 

 London, 1945. 



317 H. v. Euler and U. Gard, Arkir Kemi Mineral. GeoL, B 10, No. 19, 1-6 (1931). 



318 H. J. Deuel, Jr., C. Johnston, E. R. Meserve, A. Polga>, and L. Zechmeister, Arch. 

 Biochem., 7, 247-255 (1945). 



319 H. J. Deuel, Jr., C. Johnston, E. Sumner, A. Polgdr, and L. Zechmeister, Arch. 

 Biochem., 5, 107-114 (1944). 



330 A. R. Kemmerer and G. S. Fraps, J. Biol. Chem., 161, 305-309 (1945). 



