294 IV. DIGESTION OF LIPIDS OTHER THAN FATS 



However, the demonstration of a provitamin A activity on the part of 

 the carotenes, as demonstrated by growth-promoting action or by the 

 deposition of vitamin A in the liver, cannot be considered as absolute 

 proof of the absorption of the carotenes as such. One can argue that 

 the provitamins A are converted into vitamin A in the lumen of the gut; 

 under such conditions, vitamin A and not the carotenes would be the 

 component absorbed. This argument carries no weight in the case of 

 man or of other animals in which carotene is a normal constituent of the 

 blood and tissues. 



c. The Absorption of Lycopene. Lycopene, C 4 oH 5 6, is a hydrocarbon in 

 which both /3-ionone rings have been ruptured; consequently it is not con- 

 vertible to vitamin A. The absorption of this compound is predicated by 

 the fact that it has been found in human fat, 295 - 296 human liver, 297 - 298 and 

 in the blood serum of man. 299 -** 00 Gillam and Heilbron 301 also reported the 

 isolation of lycopene from butter. The occurrence of lycopene in tissues 

 is believed to be adventitious and to reflect the absorption of this caro- 

 tenoid from that consumed in the food. 



d. The Absorption of Oxycarotenoids. Oxycarotenoids having one or 

 more alcohol groups might be expected to be more readily absorbable, 

 since alcohols in general are more soluble in aqueous media than are the 

 corresponding hydrocarbons. In the case of the entire group of oxycaro- 

 tenoids, only indirect methods have been employed to prove absorbability; 

 for this reason no comparative rates of absorption can be indicated. 



(a) Cryptoxanthin. Cryptoxanthin, C40H55OH, is the only common 

 member of the oxycarotenoids which can be listed as a provitamin A. 

 It has been reported to be a component of butter, 301 of eggs, 302 and of the 

 blood serum of cattle. 303 In all these cases, it is believed the origin of the 

 carotenol can be ascribed to its absorption from food. 



Another proof of the absorption of cryptoxanthin is its ability to serve 

 as a provitamin A. It is converted to vitamin A in an amount somewhat 

 in excess of 50% of that of jS-carotene. 304,308 This indicates that it is ab- 

 sorbed as well as /3-carotene when administered in the small dosages re- 

 quired for bioassay studies. Since cryptoxanthin has only one intact 

 (8-ionone ring, and since it has a molecular weight slightly in excess of that 



296 L. Zechmeister and P. Tuzson, Z. physiol. Chem., 231, 259-264 (1935). 



296 L. Zechmeister and P. Tuzson, Bull. soc. chim. biol, 17, 1110-1118 (1935). 



297 L. Zechmeister and P. Tuzson, Z. physiol. Chem., 234, 241-244 (1935). 



298 H. Willstaedt and T. Lindqvist, Z. physiol. Chem., 240, 10-18 (1936). 



299 E. V. Daniel and G. J. Scheff, Proc. Soc. Exptl. Biol. Med., 33, 26-30 (1935). 



300 E. V. Daniel and T. Beres, Z. physiol. Chem., 238, 160-162 (1936). 



301 A. E. Gillam and I. M. Heilbron, Biochem, J., 29. 834-836 (1935). 



