Cholecalciferol

Cholecalciferol is a fat-soluble vitamin precursor whose main function is to maintain calcium and phosphate balance in the body and promote skeletal health. It can be synthesized in the skin itself. Under ultraviolet B irradiation (UV-B, 290-315 nm), 7-dehydrocholesterol in the skin is converted into cholecalciferol through photochemical reactions and thermal isomerization. In the liver, cholecalciferol is converted by cytochrome P450 2R1 (CYP2R1, vitamin D 25-hydroxylase) into 25-hydroxycholecalciferol (calcidiol), which is the major circulating form of vitamin D in the blood. Then, in the kidney, 25-hydroxycholecalciferol is converted by cytochrome P450 27B1 (CYP27B1, 25-Hydroxyvitamin D3 1-alpha-hydroxylase) into 1,25-dihydroxycholecalciferol (calcitriol). This active form binds to the vitamin D receptor in many organs, regulates the transcription of numerous genes, increases intestinal absorption of Ca²⁺ and PO₄³⁻, helps the kidney maintain blood calcium levels, and indirectly promotes osteoclast differentiation and activation to mobilize calcium ions by stimulating osteoblast expression of RANKL and related factors, thereby releasing Ca²⁺ when needed.