Unlike most other vitamins that are dietary essential
nutrients for the human body, vitamin D is synthesized in the human body and
acts like a steroid hormone. Vitamin D metabolism in the human body starts
with the UVB
(ultraviolet light B) photon-stimulated structural change in
7-dehydrocholesterol to yield vitamin D3 in the epidermis of the skin.
Vitamin D3 is then converted to the biologically active
1,25(OH)2D3 through two sequential hydroxylation reactions. Vitamin D3 is first
hydroxylated mainly by the enzyme, CYP2R1, to become 25(OH)D3 in the liver;
then 25(OH)D3 is hydroxylated to become 1,25(OH)2D3 by the enzyme, CYP27B1, in the
epithelial cells of the proximal convoluted tubule in the kidney.
PTH (parathyroid hormone), which is released by the
parathyroid glands in response to decreasing blood calcium level, stimulates
CYP27B1 activity in the epithelial cells of the proximal convoluted tubules in
the kidney, thereby regulating the rate of renal conversion of 25(OH)D3 to
1,25(OH)2D3.
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