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Unlike the other twelve vitamins, vitamin D is only conditionally essential - in a preindustrial society people had adequate exposure to sunlight and the vitamin was a hormone, as the primary natural source of vitamin D was the synthesis of cholecalciferol in the lower layers of the skin’s epidermis, triggered by a photochemical reaction with ...
This is the next-to-last step of cholesterol biosynthesis. [4] Defective synthesis results in the human inherited disorder lathosterolosis resembling Smith–Lemli–Opitz syndrome. [4] Mice where this gene has been deleted lose the ability to increase vitamin D 3 in the blood following UV exposure of the skin. [5]
Calcitriol is a hormone and the active form of vitamin D, normally made in the kidney. [8] [9] [10] It is also known as 1,25-dihydroxycholecalciferol.It binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes. [11]
The three steps in the synthesis and activation of vitamin D 3 are regulated as follows: Cholecalciferol is synthesized in the skin from 7-dehydrocholesterol under the action of ultraviolet B (UVB) light.
CYP2R1 is cytochrome P450 2R1, an enzyme which is the principal vitamin D 25-hydroxylase. [5] [6] In humans it is encoded by the CYP2R1 gene located on chromosome 11p15.2. [7]It is expressed in the endoplasmic reticulum in liver, where it performs the first step in the activation of vitamin D by catalyzing the formation of 25-hydroxyvitamin D. [8]
The first step involves a photochemically induced conrotatory ring opening of 7-dehydrocholesterol to form pre vitamin D3. A [1,7]-hydride shift then forms vitamin D 3 . Another example is in the proposed biosynthesis of aranotin, a naturally occurring oxepine, and its related compounds.
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Ergosterol is a biological precursor of vitamin D 2, the chemical name of which is ergocalciferol. Exposure of white button mushrooms to UV-C irradiation produces time-dependent increases in vitamin D2 concentrations in the mushrooms.