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The generation of 1,25(OH)2D3. When UVB-rays from the sun hit the skin, 7-dehydrocholesterol is converted to pre-vitamin D3, which isomerizes quickly to a more stable vitamin D3. Vitamin D3 is rapidly converted to 25-hydroxyvitamin D (25(OH)D) by 25-hydroxylases, which are enzymes from the cytochrome P450 family. 25(OH)D is a circulating metabolite that is converted to the active form, 1,25(OH)2D3, by 1 alpha-hydroxylase enzymes.
