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Calcifediol is strongly bound in blood by the vitamin D-binding protein. [5] Measurement of serum calcifediol is the usual test performed to determine a person's vitamin D status, to show vitamin D deficiency or sufficiency. [4] [5] Calcifediol is available as an oral medication in some countries to supplement vitamin D status. [4] [6] [7]
Vitamin D deficiency is typically diagnosed by measuring the concentration of the 25-hydroxyvitamin D in the blood, which is the most accurate measure of stores of vitamin D in the body. [ 1 ] [ 7 ] [ 2 ] One nanogram per millilitre ( 1 ng/mL ) is equivalent to 2.5 nanomoles per litre ( 2.5 nmol/L ).
The resulting metabolites, such as 20-hydroxyvitamin D3 and 20,23-dihydroxyvitamin D3, act as inverse agonists for RORα and RORγ2. This interaction leads to effects such as the downregulation of IL-17 signaling, which influences the immune system. [ 210 ]
24,25-dihydroxyvitamin D 3 is formed from 25-hydroxyvitamin D 3 by the action of CYP24A1 (25-hydroxyvitamin D3-24-hydroxylase). CYP24A1 appears to be "a multicatalytic enzyme catalyzing most, if not all, of the reactions in the C-24/C-23 pathway of 25-OH-D 3 metabolism."
CYP24A1 was identified in the early 1970s and was first thought to be involved in vitamin D metabolism as the renal 25-hydroxyvitamin D3-24-hydroxylase, modifying calcifediol (25-hydroxyvitamin D) to produce 24,25-dihydroxycholecalciferol (24,25-dihydroxyvitamin D).
Calcitriol is produced in the cells of the proximal tubule of the nephron in the kidneys by the action of 25-hydroxyvitamin D 3 1-alpha-hydroxylase, a mitochondrial oxygenase and an enzyme which catalyzes the hydroxylation of 25-hydroxycholecalciferol (calcifediol) in the 1-alpha position. The activity of this enzyme is stimulated by PTH.
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