Metabolism
As Vitamin D passes through the liver, it is hydroxylated to 25-hydroxyvitamin D (25-hydroxycholecalciferol), the inactive form of vitamin D.
25-Hydroxyvitamin D travels by the circulation to the kidney, where proximal tubule cells contain the enzyme 1 α-hydroxylase, which converts the molecule to 1,25-dihydroxyvitamin D, the most active form of vitamin D.
The activity of the 1 α-hydroxylase is tightly controlled by PTH and plasma phosphate levels.
PTH and hypophosphatemia stimulate 1 α-hydroxylase activity, resulting in elevated vitamin D levels and the maintenance of calcium (and phosphate) balance.
In contrast, low PTH levels and hyperphosphatemia inhibit the enzyme, reducing the production of vitamin D.
[[25-Hydroxyvitamin D3 1-alpha-hydroxylase (VD3 1A hydroxylase) also known as cytochrome p450 27B1 (CYP27B1) or simply 1-alpha-hydroxylase is a cytochrome P450 enzyme that in humans is encoded by the CYP27B1 gene.[5][
VD3 1A hydroxylase is located in the proximal tubule of the kidney and a variety of other tissues, including skin (keratinocytes), immune cells,[8] and bone (osteoblasts).[9] ]
The enzyme catalyzes the hydroxylation of calcifediol to calcitriol (the bioactive form of Vitamin D):[10]]]
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