Vitamin D
Regulation, Metabolism, AndFunction

Interest in vitamin D has dramatically increased over the past several decades. From the beginning, vitamin D was incorrectly named a vitamin when later it was discovered to be a member of the steroid hormone family. Over time, the vitamin D receptor was discovered along with its major circulating form, 25-hydroxyvitamin D, and its the hormonal ligand, 1,25-dihydroxyvitamin D. Classically, vitamin D was known to be important for enhancing intestinal absorption of calcium; however, interest grew in vitamin D when it was determined that vitamin D may be utilized by other tissues of the body. Vitamin D3 is made in the skin from 7-dehydrocholesterol under the influence of UV light. Vitamin D2 (ergocalciferol) is derived from the plant sterol ergosterol. Vitamin D is metabolized first to 25 hydroxyvitamin D (25OHD), then to the hormonal form 1,25- dihydroxyvitamin D (1,25(OH)2D). CYP2R1 is the most important 25-hydroxylase; CYP27B1 is the key 1-hydroxylase. Both 25OHD and 1,25(OH)2D are catabolized by CYP24A1. 1,25(OH)2D is the ligand for the vitamin D receptor (VDR), a transcription factor, binding to sites in the DNA called vitamin D response elements (VDREs). There are thousands of these binding sites regulating hundreds of genes in a cell-specific fashion. VDR-regulated transcription is dependent on modulators, the profile of which is also cell-specific. Analogs of 1,25(OH)2D are being developed to target specific diseases with minimal side effects.(Bikle , 2014)