CHAPTER VIII 



DISTRIBUTION, PROPERTIES, AND 



CHEMISTRY OF THE PROVITAMINS D 



AND VITAMINS D 



A. INTRODUCTION 



There are many similarities between the vitamins D and the vitamins A. 

 The most important one is the fact that both are found in the plant king- 

 dom in the form of inactive precursors, while they are present as such only 

 in animal products. A provitamin D compound, such as 7-dehydrocholes- 

 terol, may occur in the animal body, just as a provitamin A, i.e., /S-caro- 

 tene, may be present in animal tissues. However, in the former case, the 

 provitamin D may be synthesized by the animal, but it is not believed that 

 any provitamin A can be produced de novo; such a provitamin A must have 

 its origin in the diet. 



Moreover, the animal possesses the capacity to transform the pro- 

 vitamins D to their active forms, just as it can change the provitamins A 

 to vitamin A. However, the methods by which these reactions are brought 

 about are quite distinct for the two pro\ntamins. In the case of provita- 

 min D, the activation can be effected by ultraviolet radiation of definite 

 frequency, Avhereas a physical action of this nature causes destruction of 

 the provitamins A, as well as of the active forms of vitamin A. It is not 

 known at present what causes the provitamins A to be converted to vita- 

 min A, although it is presumed that one or more enzyme systems vcmy be 

 responsible for the reaction. 



Another difference between provitamins A and D is in their activation 

 in vitro. While it is possible to convert a provitamin D very effectively to 

 the corresponding vitamin D outside the body by irradiation and by other 

 physical means, no physical or chemical methods are at present available 

 for changing the provitamins A to their active forms. 



^'itamin A and vitamin I) are frequently, although not alwa3's, found 

 associated in animal fats. For this reason, the recognition that vitamin A 

 and vitamin D are two separate and distinct vitamins was long dela3^ed. 

 Cod liver oil, which cures xerophthalmia, is also known to he an especially 

 effective medicinal agent for the treatment of rickets. When these fat- 

 soluble vitamins are concentrated from cod liver oil by saponification, both 



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