The D Vitamins and Their Precursors
Part of the Monographs on Endocrinology book series (ENDOCRINOLOGY, volume 13)
It must be recognized that there is not a single vitamin D compound but that there is a family of compounds that exhibit vitamin D activity. The most important of the vitamin D compounds are vitamins D2 and D3 (Fig. 1), and the more important of these is vitamin D3 or cholecalciferol. The basis for this statement is that vitamin D3 is the vitamin produced in the skin by ultraviolet irradiation. Vitamin D2, however, is produced by ultraviolet irradiation of the plant sterol ergosterol (Fig. 2). The photolysis reaction that converts the 5,7-diene sterols to the corresponding D vitamins, as shown in Fig. 3 has been examined intensively. It is known that 250–310 nm ultraviolet light brings about the photolytic conversion of vitamin D to the previt-amin D (Havinga, 1973, Velluz and Amiard, 1949a). Further activation by light photons brings about the formation of tachysterol, a relatively unstable irradiation product, and lumisterol. The previtamin D is in thermal equilibrium with vitamin D3 and upon heating will rapidly convert to the vitamin D structure. The equilibrium is temperature dependent, and the relative concentrations of the forms of vitamin D at equilibrium are a function of temperature (Velluz and Amiard, 1949b; Velluz et al., 1949). At room temperature the conversion of the previtamin to the vitamin is slow, and even at skin temperatures it is a very slow process.
KeywordsHunt Diene Photolysis Ster Ergosterol
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© Springer-Verlag Berlin, Heidelberg 1979