Cholecalciferol

Cholecalciferol (/ˌkoʊləkælˈsɪfərɒl/) (vitamin D₃) is one of the five forms of vitamin D. It is a secosteroid, that is, a steroid molecule with one ring open. This and all forms of vitamin D are misnamed: vitamins by definition are essential organic compounds which cannot be synthesized by the body and must be ingested; cholecalciferol is synthesized by the body, and functions as a prehormone. Cholecalciferol is inactive: it is converted to its active form by two hydroxylations: the first in the liver, the second in the kidney, to form calcitriol, whose action is mediated by the vitamin D receptor, a nuclear receptor which regulates the synthesis of hundreds of enzymes and is present in virtually every cell in the body.

Synthesis

7-Dehydrocholesterol is the precursor of cholecalciferol. Within the epidermal layer of skin, 7-Dehydrocholesterol undergoes an electrocyclic reaction as a result of UVB radiation, resulting in the opening of the vitamin precursor B-ring through a conrotatory pathway. Following this, the pre-cholecalciferol undergoes a [1,7] antarafacial sigmatropic rearrangement and therein finally isomerizes to form vitamin D₃. Cholecalciferol is then hydroxylated in the liver to become calcifediol (25-hydroxyvitamin D₃). Calcifediol is then hydroxylated in the kidney, and becomes calcitriol (1,25-dihydroxyvitamin D₃) or active vitamin D₃.