The D Vitamins and Their Precursors

Abstract

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.