158 VITAMIN D GROUP 



dihydrositosterol and ergosterol to the commonest vegetable sterol, but, 

 unfortunately for this hypothesis, sitosterol and ergosterol were later found 

 to contain 29 and 28 carbon atoms, respectively.^^* If ergosterol is the 7-de- 

 hydro derivative of anything, it would be of brassicasterol, but this sterol 

 is much less widely distributed than sitosterol. Also, against the hypothesis 

 of origin by oxidation-reduction is the fact that no provitamin except er- 

 gosterol (and 22-dihydroergosterol) has been found in the vegetable king- 

 dom. Until a better explanation is offered, it will have to suffice to regard 

 ergosterol as a primary metabolic product of the higher plants, as well as 

 of the lower. 



In animals the provitamins D are partly absorbed from food and partly 

 formed within the body. The ergosterol in hens' eggs is a clear example of 

 an exogenous provitamin. The ergosterol of worms, snails, and bivalves is 

 probably also derived from the food, but the 7-dehydrocholesterol of these 

 species may be endogenous. In carnivorous and omnivorous animals it is 

 probable that some of the 7-dehydrocholesterol is absorbed along with the 

 cholesterol of their prey. In a word, it appears that all ergosterol originates 

 in plants, and all 7-dehydrocholesterol in animals; almost nothing is known 

 about the origin of the still unidentified provitamins of the invertebrates. 



Herbivorous mammals, which normally never ingest cholesterol after 

 weaning, and also the herbivorous lower animals, apparently must syn- 

 thesize their 7-dehydrocholesterol as well as their cholesterol, and it is 

 probable that all animals do this to some extent. It is not known whether 

 total synthesis from simple compounds of carbon or partial synthesis from 

 cholesterol takes place. There is no evidence of total synthesis other than 



the analogy of the well-established fact that animals synthesize cholesterol 

 itself."- "4a 



There is considerable evidence that 7-dehydrocholesterol can be formed 

 in vivo from cholesterol. Schoenheimer"* in 1931 applied Wieland's dehy- 

 drogenation theory to the explanation of the ubiquitous occurrence of de- 

 hydrocholesterol and dihydrocholesterol in cholesterol. According to this 

 view, dehydrogenation and hydrogenation of cholesterol occur simultane- 

 ously as a general biological process in which cholesterol functions as hydro- 

 gen acceptor. (It is immaterial that Schoenheimer spoke of ergosterol as 

 the provitamin in this system, for at that time ergosterol was thought to 

 have the same number of carbon atoms as cholesterol; his argument ap- 

 plies equally well to 7-dehydrocholesterol). In keeping with this theory 

 are the observations that dehydrogenation of cholesterol can be brought 

 about in vitro by an enzyme^^ and by bacterial action.^^ The constant forma- 



■34 A. Windaus, F. von Werder, and B. Gschaider, Ber. 65, 1006 (1932). 

 i3toK. Bloch and D. Rittenberg, J. Biol. Chem. 145, 625 (1942). 

 136 R. Schoenheimer, Science 74, 579 (1931). 



