Figure 2: Vitamin D metabolism pathway. Ingested vitamin D, be it either vitamin D2 or D3, is incorporated into lipoprotein particles known as chylomicrons. These are transported by the lymphatic system into venous circulation, where the vitamin binds to the vitamin D binding protein (DBP). Skin derived vitamin D is also carried in the circulation by DBP. Vitamin D is transported to the liver where it is converted by vitamin D-25-hydroxylase to 25-hydroxyvitamin D (25(OH)D). In this form, the vitamin is biologically inactive and is therefore converted in the kidneys by 25-hydroxyvitamin D-1α- hydroxylase to 1,25-dihydroxyvitamin D (1,25(OH)2D). The chemical structures of 25(OH)D and 1,25(OH)2D are shown in the figure. In the diagram, ‘vitamin D’ represents either vitamin D2 or D3.
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