<unk>  is a secosteroid formed by a photochemical bond breaking of a steroid, specifically, by the action of ultraviolet light (UV-B or UV-C) on ergosterol, a form of provitamin D2. Like cholecalciferol,<unk>  is inactive by itself. It requires two hydroxylations to become active: the first in the liver by CYP2R1 to form 25-hydroxyergocalciferol (ercalcidiol or 25-OH D2), and the second in the kidney by CYP27B1, to form the active 1,25-dihydroxyergocalciferol (ercalcitriol or 1,25-(OH)2D2), which activates the vitamin D receptor. Unlike cholecalciferol, 25-hydroxylation is not performed by CYP27A1 for<unk> .<unk>  and metabolites have lower affinity to the vitamin D-binding protein compared to the D3 counterparts. The binding affinity of ercalcitriol to the vitamin D receptor is similar to that of calcitriol.<unk>  itself and metabolites can be deactivated by 24-hydroxylation.
Ergocalciferol
Cholecalciferol
Calcitriol
Calcium/cholecalciferol
Retinol
Calcipotriol
Β-Carotene
Folate