736 VII. VITAMINS A 



5500,^^^ corresponding to a value of 4800^^ shown by vitamin Ai and 

 antimony trichloride. Crystalline anhydrovitamin A melts at 76-77 °C. 



Anhydrovitamin Ai is quite stable in oil solutions, but purified concen- 

 trates and crystalline preparations deteriorate quite rapidly, even when 

 kept at — 35°C. When anhydrovitamin A is allowed to stand in solvents 

 for any length of time, a pale yellow, rubbery precipitate is deposited which 

 is probably a polymerization product. A yield of 50% of the theoretical 

 amount of this material results, in 48 hours, from a petroleum ether solu- 

 tion of anhj^drovitamin A which has been standing at room temperature. 

 Anhydrovitamin A may be distilled in the molecular still, where it exhibits 

 an elimination maximum 19° lower than that characteristic of vitamin A 

 alcohol. 



Anhydrovitamin A2 shows the same absorption peaks as does anhydro- 

 vitamin Ai.^^'"" However, these anhydrovitamins are probably not 

 identical, since they are adsorbed with different affinities on a chromato- 

 graphic column. 



(c) Distribution of Anhydrovitamin A. Anhydrovitamin A is believed to 

 occur naturally in some fish oils. Castle et alr^^ obtained a yellow sub- 

 stance from halibut oil with an ultraviolet absorption spectrum similar to 

 that given by cyclized vitamin A, while Heilbron et al}'' found that the 

 spectra of most volatile fractions of a prolonged vacuum distillation had a 

 fine structure which followed a similar spectrometric pattern. Pritchard 

 et al}^^ demonstrated that, on fractionation of a mammalian liver concen- 

 trate, the portion insoluble in 83% alcohol had slight inflections which sug- 

 gested the presence of the anhydrovitamin A. Embree^^^ prepared a 

 typical sample of this product from tuna liver oils in the low-boiling frac- 

 tions distilled in the molecular still. IVIoreover, on molecular distillation of 

 samples of halibut, pollack, and cod liver oils, anhydrovitamin A was 

 found in the lowest boiling fractions to the extent of about 2%. 



{d) Vitamin A Activity of Anhydrovitamin A. The early results of 

 Embree,^^^ obtained on crude concentrates, indicated that anhydrovitamin 

 A possesses no detectable vitamin A activity. However, the crystalline 

 product investigated by Shantz et alr^° was shown to have a biological 

 potency equivalent to 17,500 U.S. P. units per gram, which is 0.4% that of 

 vitamin A. Samples of anhydrovitamin A, prepared in such a manner as 

 to preclude the presence of vitamin A, assa.yed at 15,200 and 10,900 U.S. P. 

 units per gram.-^^ Anhydrovitamin A is apparently not converted to vita- 

 min A under such conditions l)ut to a vitamin A isomer, rehydrovitamin A, 

 which is stored in the liver as an ester. -^"-^^^ 



b. Rehydrovitamin A. According to the recent work of Shantz, -^^ 



2*° E. M. Shantz, Report of the 114th Meeting of the American Chemical Society, 

 Washington, D. C., 1948, Abstracts, pp. 16c-17c. 



251 E. M. Shantz, J. Biol. Chem., 182, 515-524 (1950). 



