458 IX. CAROTENOIDS AND VITAMINS A 



form.^^'^'^^' The provitamin A, cryptoxanthin, has also been noted under 

 some conditions.^^ The level of plasma carotenoid is considerably higher 

 in cows than in bulls. *^'-^^ Carotene is the predominating carotenoid in 

 the blood plasma of cows and horses. ^^^'-®* even after the feeding of large 

 amounts of xanthophylls. Although Palmer and Eckles ascribed the com- 

 parative absence of the xanthophyllic fraction in cow plasma to its destruc- 

 tion in the gut wall,^*®--^^ Zechmeister and Tuzson^^ are of the opinion that 

 its non-appearance is due to a selectivity in absorption whereby the caro- 

 tenes but not the xanthophylls gain entrance to the blood stream. This 

 hypothesis is based upon the fact that a higher proportion of xanthophylls 

 occurs in horse dung than was present in the f odder.^^ 



Peirce^^^ recorded levels of 2 /xg./lOO ml. for carotene in the blood of 

 sheep in Australia during the dry season, and values as high as 16 /xg- % 

 after green fodder, while Paulsen et al}^^ found values in the range of to 

 17 /ig./lOO ml. for sheep in South Dakota. However, in striking contrast 

 to these results, a number of workers have been unable to demonstrate 

 any appreciable amount of carotene in the plasma of sheep.^"-^'^'^"-^^^ 

 Carotenemia did not occur in the case of rabbits and guinea pigs given in- 

 travenous injections of this substance, ^^^ and only traces of carotene are 

 normally present in the plasma of dogs.-^^ In the case of hedgehogs 

 {Erinaceus europaeus), only 5% of the plasma carotenoids are xantho- 

 phylls. ^^o 



In the case of animals which do not accumulate carotenoids, the blood 

 contains neither carotenes nor xanthophylls. Thus, carotene has been 

 found to be absent from the portal and systemic blood of goats. ^^^ Even 

 after high doses of carotene, this pigment did not appear in the blood of 

 rats,"^'"2 swine,"^'"^ or rabbits. ^^^ The fate of ingested xanthophylls in 



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