Final answer:
Cholecalciferol, or vitamin D3, becomes 25-hydroxycholecalciferol in the liver with the help of the enzyme D3-25-hydroxylase. It is then further processed in the kidneys to form calcitriol, the active form of vitamin D.
Step-by-step explanation:
Cholecalciferol, also known as vitamin D3, has a hydroxyl group added in the liver to become 25-hydroxycholecalciferol, or calcidiol. This metabolite is the primary form of vitamin D circulating in the blood, and its production is crucial for maintaining adequate levels of vitamin D within the body.
Liver enzymes use a process involving the enzyme D3-25-hydroxylase to add the hydroxyl group to cholecalciferol, which is synthesized in the skin from 7-dehydrocholesterol under the influence of UVB sunlight. Subsequently, another hydroxylation takes place in the kidneys, where the enzyme 25(OH)D3-1-hydroxylase converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, or calcitriol. Calcitriol is the biologically active form of vitamin D that plays a key role in calcium and phosphate homeostasis.