440 IX. CAROTENOIDS AND VITAMINS A 



the xanthophylls are oxidized completely or to colorless products which are 

 not as yet known, and this oxidation presumably occurs in the intestinal 

 wall. Group B would thus have a less efficient carotenase system than 

 exists in Group C, but the ''xanthophyll oxidase" continues to be as 

 effective as in Group C. Finally, in Group A, both the carotenase and the 

 xanthophyll oxidase systems are relatively inefficient. In the case of the 

 fowl (referred to as Group D) , the carotenase enzyme system works rather 

 well, but no "xanthophyll oxidase" occurs. It is possible that another 

 transformation may occur in the gut wall of Group D, namely the trans- 

 formation of carotene to the carotenols (oxycarotenoids) . For a further 

 discussion of the distribution of the carotenoids, the reader is referred to 

 another section of this chapter (see pages 476 to 520) . 



d. Sites Other Than the Intestinal Wall for the Conversion of Carote- 

 noids into Vitamin A. Considerable variations in the results of experi- 

 ments designed to demonstrate the conversion of carotene to vitamin A in 

 the liver have been noted. Some of these discrepancies may well be 

 attributed to the species variation in carotenoid metabolism to which refer- 

 ence has already been made. Zechmeister^'*^ recognized these variations in 

 the metabolism of the carotenoids. On the basis of an extensive investiga- 

 tion of the carotene content of the livers of thirty-three mammals compris- 

 ing twenty-one species, forty-one birds comprising thirty-six species, four 

 reptiles belonging to two species, and of eight different human samples, 

 Jensen and With^*^ called attention to the fact that carotenoids were com- 

 pletely absent from many of the livers examined, or present only in small 

 amounts. The variation in the amount of vitamin A and of carotenoids in 

 the livers of the various species was considerable. No simple proportion- 

 ality could be noted between the amount of vitamin A and carotene in the 

 food ingested and the concentration in the liver. Considerable amounts 

 of carotenoids were found in the livers of beasts of prey, birds, and her- 

 bivora. They also occurred in the case of chickens, horses, and humans.'''^ 

 On the other hand, the carotenoids have not been detected in appreciable 

 amounts in the internal organs or blood of mice, rats, guinea pigs, pigs, 

 sheep, or goats, although the wild roe deer (Capreolus capraea) stores a 

 considerable amount. Of the wild birds examined, the shaggy-footed 

 buzzard {Archibuteo lapogus) and the garganey {Anas querquedula or Quer- 

 quedula circia) presented the highest vitamin A content, the Amazon parrot 

 (Amazona (Chrysotis) aestiva) the lowest. Among the reptiles, no appreci- 

 able decrease in vitamin A. stores was noted during hibernation or inani- 



i« L. Zechmeister, Ergeb. Physiol., 39, 117-191 (1937). 



'« H. B. Jensen and T. K. With, Biochem. J., S3, 1771-1786 (1939). 



