250 THE VITAMINS 



the synthesis of vitamin A in the Hvers of rabbits following the admin- 

 istration of carotene. They have attempted to distinguish between 

 carotene, xanthophyll, and vitamin A in food materials by means of 

 (1) spectrographic analysis of color produced by the antimony tri- 

 chloride reaction, (2) the intensity of color of the materials, and (3) 

 separation of carotene from vitamin A and xanthophyll by shaking a 

 petroleum ether solution of the material with 90 per cent alcohol, the 

 carotene remaining in the petroleum ether and the xanthophyll and 

 vitamin A being taken up by the alcohol. Of the materials tested, all 

 of which were of known vitamin A activity by biological tests, egg yolk 

 gave positive results by the methods followed for carotene, xanthophyll, 

 and vitamin A ; butter for carotene and vitamin A ; and extracts of 

 carrots, green cabbage, and spinach for carotene only. 



Meantime continued confirmation of the ability of purified carotene 

 to function as vitamin A was reported from various laboratories. 

 Kawakami and Kimm (1929) reported that carotene purified by re- 

 peated crystallization from carbon disulfide and ethyl alcohol cured 

 xerophthalmia in rats in doses of between 0.03 and 0.05 milligram daily. 

 Javillier and Emerique (1930) found that a sample of spinach carotene 

 which had been preserved for 40 years in an atmosphere of hydrogen 

 in a sealed tube in diffuse light was effective as a source of vitamin A in 

 a dosage of 0.01 milligram per 100 grams weight of rat. 



Green and Mellanby (1930) reported complete protection of rats 

 against infection with purified carotene, m. p. 174° C, in doses of 0.02 

 milligram daily and stated their belief that carotene itself is the specific 

 substance responsible for the vitamin A activity of green vegetables, 

 carrots, and butter, and possibly of tgg yolk. In regard to liver, the 

 possibility was suggested of the presence of a "highly active leuco-form 

 of carotinoid which may be reconverted into carotene and liberated 

 into the circulation as required." 



Formation of Vitamin A in Nature 



Acceptance of the hypothesis that vitamin A occurs in plant material 

 in the form of its precursor carotene necessitates either a restatement 

 of findings as to the origin of this vitamin, or conscious acceptance 

 of the fact that in actual usage the term vitamin A may (and often 

 does) cover both the "primary" or precursor form (carotene) and the 

 "secondary" form immediately active in animal nutrition (vitamin A 

 in the spectrographic sense). The latter alternative is much more ac- 

 cordant with present (1930) usage, and is therefore employed in this 

 monograph. In this sense there is no evidence of the formation of this 



