Figure 2.

Chemical structure and activation of vitamin D. The vitamin D obtained by the exposure of skin to sunlight or consumed in food or supplements is transported to the liver and converted to 25(OH)D (25-hydroxyvitamin D) by the enzyme 25-hydroxylase (CYP2R1). Thereafter, a second hydroxylation occurs in the kidneys by the enzyme 1-α-hydroxylase (CYP27B1) generating the active vitamin D (1,25-(OH)₂D or 1,25-dihydroxyvitamin D), which biological functions are mediated by the VDR (vitamin D receptor). The VDR bounded to 1,25-(OH)₂D forms a heterodimer with the retinoic acid receptor (RXR), which in turn attaches to the vitamin D-response element (VDRE) acting as a nuclear transcription regulator.