CHAPTER X 



AVIAN CAROTENOIDS 



OCCURRENCE 



Birds resemble mammals in having the property of converting 

 carotene into vitamin A which is stored in the liver and the eggs. 

 They differ from mammals in preferentially storing xanthophylls in 

 the liver, eggs, body fat, skin, feathers, shanks, etc. In this they resem- 

 ble marine animals with which they also share the common ability of 

 altering absorbed carotenoids and storing these altered products in 

 some special structure such as feathers ; especially interesting in this 

 connection is the appearance in this phylum of astaxanthin. 



Eggs 



The first avian pigment to be investigated was that of the yolk of 

 the domestic hen's egg. Willstatter and Escher^ in 1912, considered 

 that it was a homogeneous pigment which was an isomer of " leaf 

 xanthophyll ; " they named it lutein. This statement was left un- 

 disputed for eighteen years until Karrer and Helfenstein ^ showed that 

 the yolk pigment, although having the same m.p. as leaf xanthophyll 

 (lutein), differed from it in optical rotation. Kuhn, Winterstein, and 

 Lederer^ then showed that the yolk pigment of hens on common 

 rations ^^'as a mixture consisting of 70 per cent, of lutein (xanthophyll) 

 the remainder being principally zeaxanthin. The xanthophylls are 

 stored in the egg mostly in the free form, only about 8 per cent, being 

 esterified. * Carotene is stored only to a small extent, constituting on 

 the average about 2-10 per cent, of the total carotenoids present. ^- ^ 

 It is not known whether the observations that hens' eggs contain less 

 carotenoids than ducks' eggs, which in their turn contain less than 

 gulls' eggs, ^" ^ ° are of physiological importance. In general, it can be 

 said that hens carry to their yolks at least part of any carotenoid fed 

 to them. Apart from lutein (xanthophyll), zeaxanthin, and carotene, 

 this has been proved in the cases of cryptoxanthin, 1 1- 1 ^ capsan- 

 thin, 1 3, 1 4 lycopene, i s, i e neoxanthin, flavoxanthin, and isolutein ; ^ ® 

 violaxanthin is apparently an exception. ^ ^ It should be noted here that 

 a discrepancy exists between the claims of Karrer and Krause-Voith^' 



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