Final answer:
The enzyme responsible for converting vitamin D3 to calcidiol in the liver is D3-25-hydroxylase. Calcidiol is then further metabolized in the kidneys to the active form of vitamin D, calcitriol, under the influence of other metabolic factors.
Step-by-step explanation:
The enzyme that converts vitamin D3 to calcidiol in the liver is known as D3-25-hydroxylase. This enzyme plays a critical role in the metabolism of vitamin D. Cholecalciferol, or vitamin D3, is synthesized in the skin from a cholesterol derivative using UV radiation from the sun. Once in the liver, D3-25-hydroxylase metabolizes cholecalciferol to 25-hydroxycholecalciferol, also known as 25(OH)D3 or calcidiol, which is the main form of vitamin D circulating in the blood.
Calcidiol is then transported to the kidneys, where another hydroxylation reaction occurs, this time by the enzyme 25(OH)D3-1-hydroxylase, converting it to calcitriol, the active form of vitamin D. This conversion is influenced by several factors including parathyroid hormone (PTH), calcium and phosphate serum concentrations, and the vitamin D status of the individual.