The first reaction
is driven by sunlight acting on the skin [4] to photolyze 7-
dehydrocholesterol to vitamin D3 (cholecalciferol), which is transferred to the liver where a hydroxyl group is added to the C-25
position by a vitamin D-25 hydroxylase (encoded by the CYP27A1
gene) to form 25-hydroxyvitamin D3 [25(OH)D3] that is the immediate precursor for active Vitamin D