The biological action of 1,25(OH) 2D is mediated through the vitamin D receptors (VDRs) and this receptor as well as CYP27B1 are expressed widely in several tissues (fig. The second hydroxylation reaction is stimulated mainly by parathyroid hormone (PTH), and inhibited by calcium, phosphate and fibroblast growth factor-23 (FGF-23). The second step, in the kidneys, requires the P450 enzyme, CYP27B1 (1α,25-hydroxylase) to form the main active metabolite - 1,25(OH)D or calcitriol. The first step, in the liver, requires P450 enzymes such as CYP2R1 and CYP27A1 (25-hydroxylases) to form the major circulating form 25(OH)D. Vitamin D (D 2, D 3) has no biological activity without a two-step hydroxylation process.
Sun-dried mushrooms, UV-B irradiation of the yeast sterol ergosterol, as well as vitamin supplements are considered as main sources of vitamin D 2. Vitamin D 3 can also be obtained exogenously from animal sources such as fish oils or fortified dairy products and vitamin supplements. Vitamin D 3 constitutes around 80-90% of the circulating metabolites and is synthesized mainly in the skin by the action of ultraviolet (UV) light (280-315 nm). Vitamin D refers to two biological precursors - ergocalciferol (D 2) and cholecalciferol (D 3). However, intensive research over the last two decades has indicated that vitamin D may also be a critical modulator of several non-skeletal systems and related diseases. In humans, the active vitamin D metabolite, 1,25-dihydroxyvitamin D (1,25(OH) 2D), has been well recognized for its role in calcium and phosphate homeostasis.
Most plants and animals that are exposed to sunlight have the ability to produce vitamin D.
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