748 VIII. PROVITAMINS D AND VITAMINS D 



Ergosterol and 7-dehydrocholesterol are the only provitamins D which 

 occur as such in nature. Considerable amounts of the provitamins have 

 been demonstrated in the sterols of various vertebrates, invertebrates, and 

 plants, but the nature of the compound is uncertain. These sterols are 

 listed in Table 1, which has been assembled by Rosenberg." 



2. Properties of the Provitamins D 



All of the provitamins are characterized by solubility in fat solvents and 

 insolubility in water. They can be readily crystallized from the lower 



TABLE 2 

 Absorption Characteristics of Provitamins D2 and D3 in Ethanol"-'' 



" T. II. Hogness, A. E. Sidwcll, Jr., and F. P. Zscheile, Jr., J. Biol. Chem., 120, 239- 

 256(1937), p. 242. 



*" J. R. Loofbourow, Vitamins and Hormones, 1, 109-155 (1943), p. 118. 



alcohols, from which they separate on the addition of a molecule of water 

 or solvent of crystallization. 



The provitamins ergosterol, 7-dehydrocholesterol, 7-dehydrositosterol, 

 and 7-dehydrostigmasterol produce almost identical absorption spectra'"* 

 with 4 well-defined maxima. These occur at 2625, 2710, 2815, and 2930 



1°* H. Brockniann, Ergeh. Vitamin-Hormonjorsch., 2, 55-103 (1939). 



