628 X. VITAMINS D 



cally devoid of activity as a vitamin D after irradiation.^" 7-Dehydrosito- 

 sterol, which also carries an ethyl group on C24, was originally found to be 

 capable of activation.^^-^^ However, Ruigh^^ later presented evidence that 

 the positive effects were to be ascribed to the presence of a small amomit of 

 7-dehydrocampesterol, or other provitamins D which were present as im- 

 purities in the 7-dehydrositosterol. Although there are no data as to 

 whether or not 7-dehydrocUonasterol, which is a C24-epimer of 7-dehydro- 

 sitosterol, is a potential provitamin D, Bills^ is of the opinion that it is not 

 active, since the ethyl group on the C24 carbon would preclude this possi- 

 biUty. 



Table 1 



The Distribution of Provitamins D in 



Phanerogams and Cryptogams" 



" Adapted from W. H. Sebrell, Jr., and R. S. Harris, 2Vie Vitamins, Academic Press 

 Inc., New York, 1954, Volume II, p. 149. 



b. Occurrence of Provitamins D in General. Although the provitamins 

 D are widely distributed in nature, they are seldom found in sufficient 

 amounts and suflBciently free from other sterols to make it possible to 



9» O. Linsert, Z. physiol. Chem., Ul, 125-128 (1936). 

 9' W. Grab, Z. physiol. Chem., US, 63-89 (1936). 



