Fig. 3.

Cytochrome P450 (CYP) enzymes and the metabolism of vitamin D. Cholecalciferol is formed in the skin from 7-dehydrocolesterol after exposure to sunshine, then hydroxylated in the liver to 25-(OH)D3 (calcidiol) by the P450 enzymes CYP27A1 or CYP2R and subsequently activated in the kidney by CYP27B1 to 1α, 25-(OH)₂ D₃ (calcitriol), the active form of vitamin D. The latter is inactivated by CYP24A1 mainly to 1α,24,25-(OH)₃ D₃ (calcitroic acid). Therefore, active vitamin D levels depend on the relative rates of its synthesis via CYP27B1 and its modifications via CYP24A1