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The 1-hydroxy group is in the alpha position, and this may be specified in the name, for instance in the abbreviation 1α,25-(OH) 2 D 3. [6] Calcitriol is, strictly, the 1-hydroxylation product of calcifediol (25-OH vitamin D 3), derived from cholecalciferol (vitamin D 3), rather than the product of hydroxylations of ergocalciferol (vitamin D 2).
A third important effect of PTH on the kidneys is stimulation of the conversion of 25-hydroxy vitamin D into 1,25-dihydroxy vitamin D (calcitriol). [216] This form of vitamin D is the active hormone which promotes calcium uptake from the intestine via the action of calbindin. [218] Calcitriol also reduces calcium loss to urine. [215]
The name 25(OH)D refers to any combination of calcifediol (25-hydroxy-cholecalciferol), derived from vitamin D 3, and ercalcidiol (25-hydroxy-ergocalciferol), [1] derived from vitamin D 2. The first of these (also known as 25-hydroxy vitamin D3) is made by the body, or is sourced from certain animal foods or cholecalciferol supplements.
The level of serum 1,25(OH)D (calcitriol) is not usually used to determine vitamin D status because it often is regulated by other hormones in the body such as parathyroid hormone. [53] The levels of 1,25(OH)D can remain normal even when a person may be vitamin D deficient. [53] Serum level of 25(OH)D is the laboratory test ordered to indicate ...
It is converted to its active form by two hydroxylations: the first in the liver, by CYP2R1 or CYP27A1, to form 25-hydroxycholecalciferol (calcifediol, 25-OH vitamin D 3). The second hydroxylation occurs mainly in the kidney through the action of CYP27B1 to convert 25-OH vitamin D 3 into 1,25-dihydroxycholecalciferol (calcitriol, 1,25-(OH) 2 ...
25-Hydroxyvitamin D 1-alpha-hydroxylase (VD 1A hydroxylase) also known as calcidiol 1-monooxygenase [5] or cytochrome p450 27B1 (CYP27B1) or simply 1-alpha-hydroxylase is a cytochrome P450 enzyme that in humans is encoded by the CYP27B1 gene.
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The vitamin D receptor (VDR also known as the calcitriol receptor) is a member of the nuclear receptor family of transcription factors. [5] Calcitriol (the active form of vitamin D , 1,25-(OH) 2 vitamin D 3 ) binds to VDR, which then forms a heterodimer with the retinoid-X receptor .