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|Section2= |Section6= |Section7= }} Cholecalciferol () (vitamin D3) is one of the five forms of vitamin D.〔("Nomenclature of Vitamin D. Recommendations 1981. IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN) )" reproduced at the Queen Mary, University of London website. Retrieved 21 March 2010.〕 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,〔Norman, Anthony W. (1998) (Sunlight, season, skin pigmentation, vitamin D, and 25-hydroxyvitamin D:integral components of the vitamin D endocrine system. ) Am J Clin Nutr;67:1108–10.〕 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 () 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. The three steps in the synthesis of vitamin D3 are regulated as follows: * Cholecalciferol is synthesized in the skin from 7-dehydrocholesterol under the action of ultraviolet B (UVB) light. It reaches an equilibrium after several minutes depending on the intensity of the UVB in the sunlight - determined by latitude, season, cloud cover, and altitude - and the age and degree of pigmentation of the skin. * Hydroxylation in the endoplasmic reticulum of liver hepatocytes of cholecalciferol to calcifediol (25-hydroxycholecalciferol) by 25-hydroxylase is loosely regulated, if at all, and blood levels of this molecule largely reflect the amount of cholecalciferol produced in the skin combined with any vitamin D2 or D3 ingested. * Hydroxylation in the kidneys of calcifediol to calcitriol by 1-alpha-hydroxylase is tightly regulated: it is stimulated by either ''parathyroid hormone'' or ''hypophosphatemia'' and serves as the major control point in the production of the active circulating hormone calcitriol (1,25-dihydroxyvitamin D3). Click on icon in lower right corner to open. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cholecalciferol」の詳細全文を読む スポンサード リンク
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