1,25(OH) 2D is the principal hormonal form of vitamin D, responsible for most of its biologic actions. 25OHD is then further metabolized to 1,25(OH) 2D principally in the kidney, by the enzyme CYP27B1, although other tissues including various epithelial cells, cells of the immune system, and the parathyroid gland contain this enzymatic activity. Several enzymes have 25-hydroxylase activity, but CYP2R1 is the most important. The liver and other tissues metabolize vitamin D, whether from the skin or oral ingestion, to 25OHD, the principal circulating form of vitamin D. D 3 is preferentially removed from the skin, bound to DBP. ![]() ![]() Pre-D 3 isomerizes to D 3 or with continued UV irradiation to tachysterol and lumisterol. ![]() Vitamin D 3 is produced in the skin from 7-dehydrocholesterol by UV irradiation, which breaks the B ring to form pre-D 3.
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