Holick M F, Clark M B
Fed Proc. 1978 Oct;37(12):2567-74.
Provitamin D3 (7-dehydrocholesterol) is converted to previtamin D3 by the action of ultraviolet radiation on the skin. Previtamin D3 thermally isomerizes to vitamin D3 in the skin and the vitamin is then transported to the liver on the vitamin D-binding protein. Although there are extrahepatic vitamin D-25-hydroxylases, the liver is the major site for the 25-hydroxylation of vitamin D. In response to hypocalcemia and hypophosphatemia, 25-OH-D is metabolized by a renal-cytochrome. P450-dependent mixed function oxidase system is 1alpha,25(OH)2D. When calcium and phosphate homeostasis prevails the renal 25-OH-D-1alpha-hydroxylase activity is limited and instead a non-cytochrome P450 mixed function oxidase metabolizes 25-OH-D to 24R,25(OH)2D. Parathyroid hormone has clearly been shown to be a trophin for the renal synthesis of 1,25(OH)2D; however, the role and significance of the adrenal steroids, or gonadal and pituitary hormones, on the renal 25-OH-D-1alpha-hydroxylase is not well defined. The regulation of the photometabolism of provitamin D3 to vitamin D3, the role and significance of the side-chain metabolism of 1,25(OH)2D by the small intestine, and the metabolism of 25-OH-D to 24R,25(OH)2D by chondrocytes and its stimulation of protein synthesis in these cells are just a few issues that will require further investigation.
维生素D3原（7-脱氢胆固醇）通过皮肤表面紫外线的作用转化为维生素D3原。维生素D3原在皮肤中热异构化为维生素D3，然后该维生素通过维生素D结合蛋白转运至肝脏。尽管存在肝外维生素D-25-羟化酶，但肝脏是维生素D进行25-羟化的主要部位。为应对低钙血症和低磷血症，25-OH-D由一种肾细胞色素代谢。P450依赖的混合功能氧化酶系统生成1α,25(OH)2D。当钙和磷的稳态占主导时，肾脏25-OH-D-1α-羟化酶的活性受到限制，取而代之的是一种非细胞色素P450混合功能氧化酶将25-OH-D代谢为24R,25(OH)2D。甲状旁腺激素已被明确证明是肾脏合成1,25(OH)2D的促激素；然而，肾上腺类固醇、性腺和垂体激素对肾脏25-OH-D-1α-羟化酶的作用和意义尚未明确界定。维生素D3原向维生素D3的光代谢调节、小肠对1,25(OH)2D侧链代谢的作用和意义，以及软骨细胞将25-OH-D代谢为24R,25(OH)2D及其对这些细胞中蛋白质合成的刺激作用，只是需要进一步研究的几个问题。
