Module 7: Figure vitamin D metabolism

Metabolism of vitamin D to form the Ca2+ homoeostatic hormone 1,25-dihydroxyvitamin D3 [1,25-dihydroxycholecalciferol or 1,25(OH)2D3].

Synthesis of 1,25-dihydroxyvitamin D3 [1,25-dihydroxycholecalciferol or 1,25(OH)2D3] occurs through three steps carried out in separate tissues. The sequence begins in the skin with the UV-dependent photolysis of 7-dehydrocholesterol to Vitamin D3 (chlolecalciferol). The next step occurs in the liver, where a vitamin D-25 hydroxylase adds a hydroxy group to chlolecalciferol to form 25(OH)D3. The final reaction occurs in the kidney where a 1α-hydroxylase adds another hydroxy group to form the active hormone 1,25(OH)2D3.