Site of 25-hydroxylation of cholecalciferol

7-Dehydrocholesterol is the precursor of vitamin D3. Within the epidermal layer of skin, 7-Dehydrocholesterol undergoes an electrocyclic reaction as a result of UVB radiation, resulting in the opening of the vitamin precursor B-ring through a conrotatory pathway. Following this, the previtamin D3 undergoes a antarafacial sigmatropic rearrangement and therein finally isomerizes to form vitamin D3. Cholecalciferol is then hydroxylated in the liver to become calcifediol (25-hydroxyvitamin D3).Ref: Ganong&;s review of medical physiology; 24th edition; page no:-379