PathwayStep1657 PathwayStep1655 PathwayStep1656 PathwayStep1653 PathwayStep1654 PathwayStep1651 PathwayStep1652 PathwayStep1646 PathwayStep1647 PathwayStep1648 PathwayStep1649 PathwayStep1650 1,25-dihydroxyvitamin D3 is deactivated Further hydroxylation of calcidiol in kidney to form calcitriol Vitamin D3 translocates to the cytosol and binds to IDBP Legumain degrades DBP DBP:Calcidiol translocates into lysosomes Calcidiol binds to DBP in the bloodstream and is delivered to the kidney 25-Hydroxylation of vitamin D3 in liver Vitamin D3 translocates into the cytosol Vitamin D3 translocates to the blood serum and is transported to the liver Photolytic cleavage and thermal isomerization of 7-dehydrocholesterol DBP:Calcidiol is sequestered by cubilin on the cell surface Megalin internalizes the cubilin-DBP:Calcidiol complex Vitamin D (calciferol) metabolism Authored: Jassal, B, 2008-10-01 13:18:42 Edited: Jassal, B, 2008-06-02 10:50:11 GENE ONTOLOGYGO:0042359 Pubmed15951480 Reactome Database ID Release 43196791 Reactome, http://www.reactome.org ReactomeREACT_13523 Reviewed: D'Eustachio, P, 2008-05-28 08:30:54 Vitamin D3 (cholecalciferol) is a steroid hormone that plays a role in regulating calcium and bone metabolism. It is obtained from the diet and produced in the skin by photolysis of 7-dehydrocholesterol and released into the bloodstream. Only a few food sources have significant amounts of vitamins D2 and D3 but many foodstuffs nowadays are fortified with vitamin D. The metabolites of vitamin D3 are carried in the circulation bound to a plasma protein called vitamin D binding protein (VDBP). Vitamin D3 undergoes two subsequent hydroxylations to form the active form of the vitamin, 1,25(OH)2 vitamin D3 (calcitriol). The first hydroxylation takes place in the liver and subsequent transport to the kidney allows the second hydroxylation. Calciferol acts by binding to nuclear vitamin D receptors.