FIGURE 13.1 Pathway of native hormone or 1α,25-dihydroxyvitamin D 3 production. The major source of vitamin D 3 is through ultraviolet irradiation of 7-dehydrocholesterol in the skin. The liver 25-hydroxylase enzyme (encoded by CYP2R1) then converts vitamin D 3 to 25-hydroxyvitamin D 3 (25-OH-D 3 ), the major circulating form of the vitamin. Generation of 1α,25-dihydroxyvitamin D3 occurs primarily in the kidney by the 25-OH-D-1α- hydroxylase enzyme (encoded by CYP27B1). See text for more details. Source: Plum and DeLuca (2010). Nat Rev Drug Discov 9(12):941–55. 2
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