Vitamin D Flashcards
What are the two different structures of Vitamin D? From what are they synthesized?
Ergocalciferol (Vitamin D2; found in plants) and Cholecalciferol (D3; synthesized in skin)
Absorption of Vitamin takes place in the _________ and is transported bound to _____________.
intestine, plasma protein-vitamin D-binding protein
You know that ergocalciferol (D2) and cholecalciferol (D3) are prohormones and so they are not active. In order to make them active, what must happen? Which one is the predominant form of Vitamin D in Plasma?
First hydroxylation: formation of 25-hydroxycholecalciferol
-Takes place in the liver. Creates the predominant form of vitamin D.
Second hydroxylation: formation of 1,25-dihydroxycholecalciferol (a.k.a. calcitriol)
- Takes place in the kidney. Makes Vitamin D physiologically active!
“OH, the liver is where it lies but in the kidneys it is 1,25!”
How is 1-hydroxylase, the enzyme that hydroxlates the prohormones D2 and D3, regulated? What is special about the regulation of hydroxylase by calcium?
1- hydroxylase is STIMULATED by a DECREASE in plasma phosphate, calcium ions and calcitriol.
A DECREASE in Ca2+ leads to an INCREASE in parathyroid hormone, which then INCREASES 1-hydroxylases activity
What is the role of vitamin D? How is it similar to a steroid hormone?
Main function of vitamin D is in the endocrine system: regulation of plasma Ca2+
Works like a steroid hormone because it has a nuclear receptor and regulates gene expression
Where in the body does vitamin D function?
Vitamin D/ Calcitirol acts in the Duodenum, the Distal tubules of the kidney and Destroys bone.
+ stimulates absorption of Ca2+ and phosphate
(increase synthesis of transporters on brush border membrane, calbindin in cytoplasm, calcium ATPase/Na+-Ca2+ exchanger on basolateral membrane)
+ stimulates osteoclastic activity and calcium reabsorption in kidneys
(must have parathyroid hormone for both activities)
In what types of cells does calcitriol induce How may it inhibit cancer?
Hematopoietic cells –> monocytes/macrophages/granulocytes/osteoclasts.
How does calcitriol reduce your risk of cancer?
It inhibits telomerase activity.
What does a deficiency in Vitamin D look like?
Ricket’s in children (incomplete mineralization of collagen matrix of bone)
Osteomalacia (not osteoporosis) in adults (demineralization which INCREASES ratio of unmineralized:mineralized bone)
A patient of yours has a Vitamin D deficiency. What would you expect the patients Ca2+ levels to be? Phosphate? Parathyroid hormone? Calcitriol? Cacidiol? 1-hydroxylase? Alkaline phosphatase?
In a patient with Vitamin D deficiency,
- Ca2+ levels would be normal initially, then LOW
- Phosphate levels would be LOW (demineralization of bone)
- Parathyroid hormone would be HIGH (because not using PTH to stimulate osteoclasts and Ca2+ reabsorption in the kidneys?
- Cacitriol and calcidiol = Vitamin D so LOW
- 1-hydroxylase and alkaline phosphatase would be HIGH (not using 1-hydroxylase to convert prohormones into active Vitamin D)
Where can Vitamin D be found?
UV exposure of skin, fortified dairy products and salt water fish
Symptoms of Vitamin D overdose?
Nausea, thirst, loss of appetite, stupor, hypercalcemia and calcium deposits in arteries and kidneys (calcinosis)
