Figure 1.

Synthesis and activation of vitamin D. Vitamin D₃ is synthesized in the skin upon UVB exposure. The UVB exposure of provitamin D₃ (7-dehydrocholesterol) in the skin breaks the B-ring to form previtamin D₃, which undergoes thermally induced rearrangement to vitamin D₃. Vitamin D₃ is transported to the liver where it is hydroxylated at C-25 by the enzyme 25-hydroxylase producing 25OHD₃, which is the major circulating form in vertebrates. The 25OHD₃ is hydroxylated a second time at C-1 in the kidneys to the active metabolite 1,25(OH)₂D₃. Figure adapted from Jäpelt et al. (2012).