Biology and Biochemistry of Vitamin

The D-vitamins are a group of fat soluble materials with high antirachitic activity. Rickets is a disease of infancy caused by faulty calcium hydroxy apatite deposition in the growing bone. It was Palm 140) in 1890 who found that the disease responded favorably to irradiation of patient s food or skin by sunlight or ultraviolet light. Mellanby 113) in 1919 first reported the presence of an antirachitic factor in cod-liver oil. This discovery encouraged research on the isolation of the active principle in fish-liver oil, but the low natural concentration of the vitamin made isolation very difficult. 

Early investigators showed that superficially purified cholesterol and activatable foods contained a provitamin, or precursor, which was converted into the vitamin on irradiation. Ultimately, two vitamin D precursors were identified. Provitamin D2 was found to be ergosterol (la). WiNDHAUS 194) and Askew (6) independently isolated pure crystalline vitamin D2 (2 a) via its 3,5-dinitrobenzoate from the complex and unstable irradiation products of ergosterol. Provitamin D3 was more difficult to identify, but it was found by Windhaus 195) to be 7-dehydro-cholesterol (lb). On irradiation crystalline vitamin D3 (2b) was obtained, which was identical with that isolated from tuna-liver oil by Brock-MANN 29). Vitamins D4 and D5 have been found in nature, each one differing from the other and the ones mentioned above in the constitution of the side chain. 

The provitamins are more abundant in nature than the pure vitamins. Ergosterol is found in plants and yeast. 7-Dehydrocholesterol is the provitamin of man it is also found in animal skins. The most concentrated source, however, is in the snail Baccinum undatum, which contains about 27 percent 7-dehydrocholesterol in its dry weight. The D-vitamins themselves are not very widely distributed in nature. The richest sources are fish-liver oils, milk, and bird eggs. In consequence, the main source of the vitamins D2 and D3 is by irradiation of the corresponding pro- 

The last decade has seen a startling advance in our understanding of the biochemical mechanism of action of vitamin D. Ten years ago little was known concerning the absorption or tissue localization of this classical nutritional substance, nor was there available in the literature any very clear postulation of its mode of action. Some workers preferred to think of vitamin D as a cofactor of some unidentified enzyme (57). It is now clear, however, that vitamin D acts in a manner analogous to 

The early investigators in the 1930 s and 1940 s attempted to follow the metabolic fate of vitamin D3 in animals. Unfortunately, their efforts were hampered by the smaU natural concentration of vitamin D and its metabolites. Kodicek (93) in 1955 synthesized radiolabeled vitamin D2. However, the material was of such low specific activity that it was of little value in characterizing the metabolism of vitamin D itself. 

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