Active form of Vitamin D is: March 2011, March 2013 (a, e, g)
Ans. C: 1, 25 hydroxy Vitamin D (Calcitriol) Calcitriol (1, 25-DHCC) is the biologically active form of vitamin D Calcitriol/ 1, 25-dihydroxycholecalciferol/ 1,25-dihydroxyvitamin D3 It is the hormonally active form of vitamin D with three hydroxyl groups (abbreted 1,25-(OH)2D3 or simply 1, 25 (OH)2D). It increases the level of calcium (Ca2+) in the blood by (1) increasing the uptake of calcium from the gut into the blood, (2) decreasing the transfer of calcium from blood to the urine by the kidney, and (3) increasing the release of calcium into the blood from bone Calcitriol increases blood calcium levels () by promoting absorption of dietary calcium from the gastrointestinal tract and increasing renal tubular reabsorption of calcium thus reducing the loss of calcium in the urine. Calcitriol also stimulates release of calcium from bone by its action on the osteoblasts, causing them to release RANKL, which in turn activates osteoclasts. Calcitriol acts in conce with parathyroid hormone (PTH) in all three of these roles. For instance, PTH also stimulates osteoclasts. However, the main effect of PTH is to increase the rate at which the kidneys excrete inorganic phosphate (Pi), the counterion of Ca2+. The resulting decrease in serum phosphate causes Ca5 (PO4)30H to dissolve out of bone thus increasing serum calcium. PTH also stimulates the production of calcitriol. Many of the effects of calcitriol are mediated by its interaction with the calcitriol receptor, also called the vitamin D receptor or VDR.The maintenance of electroneutrality requires that the transpo of Ca2+ ions catalyzed by the intestinal epithelial cells be accompanied by counterions, primarily inorganic phosphate. Thus calcitriol also stimulates the intestinal absorption of phosphate.Calcitriol also inhibits the release of calcitonin, a hormone which reduces blood calcium primarily by inhibiting calcium release from bone. Calcitriol is produced in the cells of the proximal tubule of the nephron in the kidneys by the action of 25-hydroxyvitamin D3 1-alpha-hydroxylase, a mitochondrial oxygenase and an enzyme which catalyzes the hydroxylation of 25-hydroxycholecalciferol (calcifediol). The activity of the enzyme is stimulated by PTH. The reaction is an impoant control point in Ca2+ homeostasis. The production of calcitriol is also increased by prolactin, a hormone which stimulates lactogenesis, a process which requires large amounts of calcium.
Cholecalciferol
24, 25 hydroxy Vitamin D
1, 25 hydroxy Vitamin D (Calcitriol)
25, hydroxyl Vitamin D