lecture 18: vitamin D

Question 1

vitamin D

Answer 1

D3 can be synthesized by humans through sunlight on exposed skin
fat-soluble

Question 2

7-DHC

Answer 2

immediate precursor of cholesterol synthesis, synthesized by skin cells

Question 3

vitamin D binding protein (VDBP)

Answer 3

major plasma carrier of all vitamin D metabolites

Question 3

synthesis of vitamin D process

Answer 3

7-DHC in skin absorbs UVB photons
transformation of 7-DHC to previtamin D3
thermal isomerization to vitamin D3
translocation of D3 from skin cells to bloodstream

Question 4

metabolism of vitamin D in the liver

Answer 4

circulating vitamin D in chylomicron remnants and VDBP-bound are taken up by liver
hydroxylated in liver by 25-OH-ase > 25OHD > circulates in plasma with VDBP, no feedback regulation, best measure of vitamin D status

Question 5

metabolism of vitamin D in the kidney

Answer 5

25OHD from liver converted to 1,25(OH)2D in the kidneys

Question 6

megalin

Answer 6

membrane protein that mediates endocytic uptake of 25OHD-VDBP from kidney

Question 7

1a-hydroxylase

Answer 7

25OHD to 1,25(OH)2D

1,25(OH)2D inhibits 1a-hydroxylase, which inhibits PTH. Therefore, PTH is not inhibited and plasma calcium increases

Question 8

catabolism of vitamin D

Answer 8

catabolism to more polar and readily excretable inactive metabolites
24-hydroxylation of 25OHD or 1,25(OH)2D via 24-hydroxylase

Question 9

1,25(OH)2D

Answer 9

biologically active hormonal form of vitamin D
increases efficiency of intestinal phosphorous absorption, maintains calcium levels by activating osteoclasts

acts on intestines, bones, and kidneys to elevate plasma calcium

Question 10

vitamin D in gene expression

Answer 10

vitamin D binds to VDR> recruited by RXR > heterodimer
heterodimer binds to promoter region of specific genes at VDREs (binding sites)

1,25(OH)2D promotes genes for osteoblasts in bone and CaT1 in small intestine