VITAMINS 207 



animal feeding experiments, which proved that carotene is converted into 

 the vitamin in the animal body. Later, pure vitamin Ai was obtained 

 from fish liver oils in the form of lemon-yellow crystals (Fig. 9-2). 

 When the chemical formula of the purified vitamin had been established, 

 its relation to carotene became even more apparent: 



H3C CH3 CH3 CH3 



HiC C— CH:CHC:CHCH:CHC:CHCH20H 

 H2C.^ ^C — CH3 



Vitamin Ai, C19H27CH2OH 



It may be seen that the carbon structure of vitamin Ai is exactly half 

 that of the beta-carotene molecule and that the vitamin is a primary 

 alcohol containing five double bonds. The exact chemical formula of 

 vitamin Ao has not yet been determined. 



Of the many carotenoids of known chemical structure only four (alpha-, 

 beta-, and gamma-carotene, and cryptoxanthin) of common occurrence in 

 foodstuffs are convertible into vitamin A. It was long supposed that 

 this transformation occurred in the liver, but more recent information 

 tends to discredit this view. Carotene injected into the blood stream of 

 rats deprived of vitamin A does not prevent the animals from dying of 

 vitamin A deficiency, even though the livers may contain considerable 

 amounts of carotene at the time of death. Increased vitamin A contents 

 in the intestinal wall and in the lymph, following the injection of carotene, 

 make it appear most probable that the conversion occurs during absorp- 

 tion of the carotene through the intestinal wall. 



The fact that vitamin Ai is almost exactly one-half of a beta-carotene 

 molecule suggests that two molecules of the vitamin might arise from the 

 conversion of one molecule of carotene. This, however, does not seem 

 to be the case, since on a weight basis beta-carotene has only about one- 

 half (or less) of the biological value of vitamin A. Therefore, the con- 

 version is probably not a splitting of the carotene molecule in the middle, 

 but rather a degradation of one end of the molecule until one molecule 

 of vitamin A is produced. 



Other related substances which possess vitamin A activity have been 

 prepared synthetically, as has the vitamin itself. These include the 

 corresponding aldehyde (C19H27CHO, retinene), acid (C19H27COOH), 

 and methyl ether (C19H27CH2OCH3), In a natural state the primary 

 alcohol group of vitamin Ai is frequently involved in ester linkage, and 

 such esters as the acetate 



O 



II 

 C19H27CH2OCCH3 



