The active hormonal form of vitamin D requires several enzymatic steps. The final step in the synthesis of 1,25-dihydroxycholecalciferol occurs in which of the following tissues?

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Quizplus · Accepted Answer

The final step in the synthesis of the active form of vitamin D, 1,25-dihydroxycholecalciferol (calcitriol), occurs in the kidney, where 25-hydroxyvitamin D is hydroxylated by the enzyme 1-alpha-hydroxylase.  Active calcitriol (1,25-dihydroxycholecalciferol) is derived from ergosterol (produced in plants) and from 7-dehydrocholesterol. Upon exposure to ultraviolet (UV) light from the sun and following thermal isomerization, 7-dehydrocholesterol is nonenzymatically converted to pre-vitamin D3 (cholecalciferol) which then enters the bloodstream and is taken up by the liver. In the liver cholecalciferol is hydroxylated at the 25 position by a specific D3-25-hydroxylase generating 25-hydroxy-D3 [25-(OH)D3]. Conversion of 25-(OH)D3 to its biologically active form, calcitriol, occurs through the activity of a specific D3-1-hydroxylase present in the proximal convoluted tubules of the kidneys

The Active Hormonal Form of Vitamin D Requires Several Enzymatic