OrrURRENCE OF PROVITAMINS D 745 



into the corresponding form of vitamin D. All of the provitamins which 

 are known at present are cyclopentanophenanthrene derivatives containing 

 the steroid nucleus. All have an hydroxy group in position 3, which is 

 usually esterified in the natural product; they have 2 double bonds in 

 conjugate positions in ring B at C's-Ce and Ct-Cs. The hydrocarbon chain 

 on Ci7 is of great importance in establishing the nature of the vitamin D 

 activity. Without such a side chain, the molecule cannot be changed to an 

 active vitamin D. Moreover, slight changes in the nature of the side chain 

 will greatly alter the physiological response of the resultant vitamin D 

 formed from the provitamin D. Vitamin Do made from ergosterol, which 

 has the side chain -C'H(CH3)CH:CHCH(CH3)CH(CH.o)2, on Cn, has 

 only a low percentage of the vitamin D activity in chickens that is possessed 

 by vitamin D3, in which the side chain is -CH(CH3)CH2CH2CH2CH- 

 (CH3)2. The saturation of the double bond on carbons 22 and 23 in ergo- 

 sterol results in a reduction of the potency of the resultant vitamin to 10% 

 of its original level. 



When the provitamins D are activated by any one of the various proce- 

 dures into the active forms of vitamin D, the invariable change involves 

 the rupture of the B ring of the steroid nucleus between carbons 9 and 10. 

 The methyl group attached to Cio is changed to a methylene group, while 

 the extra hydrogen thus rendered available is used to saturate Cg. 



1. Occurrence of the Provitamins D 



(1) Ergosterol 



Ergosterol, or provitamin D2, was discovered by Tanret**^ in the sclero- 

 tium of the parasitic fungus ergot {Claviceps purpurea), in 1879. This 

 investigator named the compound ergotinine, and later ergosterine.^^ He 

 reported that it differs in composition from cholesterol and the phyto- 

 sterols.*" Gerard,^ ^~^^ after examination of the sterols from a number of 

 plants, was able to demonstrate that a taxonomic relationship exists in the 

 distribution of ergosterol in nature. Ergosterol was shown to be charac- 

 teristic of the cryptogams, just as the phytosterols are the prevailing sterols 

 in the phanerogams (see Chapter IV). 



Ergosterol is present in lower plants such as brown algae, slime fungi, 

 bacteria (Staphylococcus spp.), Mucor spp., yeast, PeniciUium spp., and 



«8 C. Tanret, Ann. chim. phys. [5], 17, 493-512 (1879). 



89 C. Tanret, Cotnpt. rend., 108, 98-100 (1889). 



^ C. Tanret, Ann. chim. phys. [6], 20, 289-297 (1890). 



91 E. Gerard, Compt. rend., II4, 1544-1546 (1892). 



92 E. Gerard, Compt. rend., 121, 723-726 (1895). 



93 E. Gerard, /. pharm. chim. [6], 1, 601-608 (1895). 

 9^ E. Gerard, Compt, rend., 126, 909-911 (1898). 



