Vitamine D - Metabolism
The active molecules of the vitamin D “group” are calcifediol and especially calcitriol. Their precursors are: cholecalciferol also called vitamin D3 and calciferol also called ergocalciferol and vitamin D2 .
Cholecalciferol, also written colecalciferol, has an endogenous and exogenous origin. Cholecalciferol is synthesized in the skin by conversion of 7-dehydrocholesterol, under the influence of ultraviolet radiation, into cholecalciferol which is also supplied by the diet. Ergosterol, provitamin D2, supplied in the diet or in the form of drug, is converted into ergocalciferol and then into cholecaciferol.
In the liver, cholecalciferol is transformed into calcifediol or 25- (OH) D3 which passes in blood circulation. This hydroxylation requires oxygen and NADPH (nicotinamide adenine dinucleotide phosphate).
In the kidney, at the level of tubular cells, calcifediol is hydroxylated into calcitriol also called 1, 25- (OH) 2 D3. This hydroxylation is achieved by an enzymatic complex, cytochrome P-450-flavoprotein-ferrodoxine, in the presence of oxygen and NADPH. The activity of this complex is controlled: it is increased when the concentration of the vitamin D, calcium or phosphate decreases. It is influenced by PTH, prolactin and estrogens. It is inhibited by the intake of high dose of vitamin D.
The plasma half-life of calcitriol is approximately 5 hours.
Calcitriol is inactivated by conversion into calcitroic acid and by conjugations.
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