Vitamin D Flashcards
Form in skin
7 dehydrocholesterol. precursor of cholecalciferol in skin
previtamin D3–> isomerization –> vitamin D3
D2
ergocalciferol. in plants, fungi, yeast, snails, worms
formed by irradiation of ergosterol
lumisterol/ tachysterol
biologically inactive. prevents D3 overproduction + provides a means for adequate supply when exposure is short
Sun Toxicity?
sustained heat on skin will photodegrade vit D and PD3 as its formed. thermal activation of PD3 –> inc of non VD3 forms –> limits formation of D3
1,25 (OH)2 D mechanism
1,25 (OH)2 D - VDR (nuclear receptor) –> heterodimer - RXR –> (+) VDRE (on DNA) –> (+) calbindin –> inc Ca absorption in Small intestine
vitamin D activation
Active 1,25 (OH)2 D
circulating 25 (OH) D
rate limiting 1a hydroxylase (+) by PTH
liver: 25 hydroxylase –> 25 (OH) D
kidney: 1a hydroxylase –> 1,25 (OH)2 D
(-) by 24 hydroxylase (prevent over production)
Ca absorption in SI
Ca + VD RXR complex –> (+) Ca channel proteins, (+) CaBP –> inc Ca blood
Ca resorption in bone
RANKL- RANK –> mature OC –> Ca, HPO4 released –> inc Ca blood levels
Plasma Ca
low Ca –> (+) PTH –> (+) active VD –> inc Ca reabsorption –> inc Ca resorption
Regulation
Calcemic, central regulator of Ca and Pi homeostasis. maintains normal blood levels and (+) bone mineralization
Rickets
big epiphyseal growth plates, stunted growth, frontal and occipital bossing
Nutritional Rickets
hypocalcemia, hypophosphatemia, inc PTH, inc serum alkaline phosphatase activity
Osteomalacia
weak bones, bone pain, muscle weakness
Minimal erythemal dose of UVB
5-30 min from 10-3PM, 2x/week without sunscreen
depends on time of day, length of day, season, cloud, smog, skin melanin content, sunscreen
Current issues
preventing osteoporosis. low Vit D –> low Ca absorption. RDA is 15-20 mcg
cancer. possible role in prevention of colon, prostate, breast cancer
