Vitamin D, Calcium Homeostasis and Biomineralisation Flashcards
how is vitamin D made
in the skin
what supplement amount shall we take of vitamin D
10 micrograms of vitamin D
what can high doses of vitamin D cause
v high doses
Nausea and muscle weakness
very high doses cause calcium absorption and bone resorption
what is the issue with calcium absorption
can lead to calcification of the arteries organs and kidney failure
what are some sources of vitamin d
EGG YOLK FISH OIL
WHAT IS THE plant vitamin D PRECURsor CALLED
ergosterol (provitamin D2)
what is the inactive precursor for vitamin d from our liver
7 dehydrocholesterol (vitamin d3)
what is step 1 of the biosynethesis of vitamin D
ergosterol and 7-dehydrocholestrol move to skin
UV light cleaves ring
ergosterol is converted to ergocalciferol in plants
7-dehydrocholestrol is converted to cholecalciferol in humans
what is produced at the end of stage 1 biosynthesis of vitamin D in plants
ergocalciferol
what is produced at the end of stage 1 biosynthesis of vitamin D in the liver
cholecalciferol (inactive)
what is step 2 of the biosynethesis of vitamin D
double hydroxylation
where cholecalciferol turns into 1,25 dihydroxyvitamin D3 (active) via enzymes in the kidney and liver
why is the molecule called 1,25 dihydroxyvitamin D3
what is another name for it
due to the position of hydroxylation
1st hydroxylation is position 25 in the liver
2nd hydroxylation is position 1 in the kidney
calcitriol
where does the first hydroxylation the active form of vitamin D3 occur
position 25 in the liver
where does the second hydroxylation the active form of vitamin D3 occur
position one in the kidney
how is cholesterol transported
in blood bound vitamin D binding protein (DBP)
what is the half life of 1,25 dihydroxycalciferol
what is the half life of 25-hydroxycholecalciferol
only a few hours
several weeks
is vitamin d hyprophobic or hydrophilic
hydrophobic
what should the calcium serum levels be in homeostasis
2.2-2.7 mmol/litre
what can calicum imbalance lead to
hyper and hypocalcaemia- can have life threatening consequences eg in muscular contractions
how is low calcium levels detected
- in the blood
- parathyroid hormone released from parathyroid glands
- effects bone- enhanced bone resorption and increased efflux of calcium ions
- effects kidney- PTH decreases loss of calcium in from the urine (to maintain serum calcium levels). Also releases active vitamin d3 which increases absorption of Ca from dietary sources in the intestines
describe the PTH hormone
short half life rapid response triggers the PTH receptor 1 in bone upregulate RANK ligand down-regulate osteoprotegerin activates osteoclastogenesis
how is calcitonin produced
by parafollicular cells in the thyroid
inhibits osteoclasts and therefore bone is not resorbed so reduces calcium ions serum
how is calcitriol produced
stimulated by the PTH in kidney
increases the calcium absorption in intestine
what can help regulate gene expression
vitamin d responsive elements
give examples of vitamin d responsive elements
osteocalcin
how does the hydroxylation of vitamin d3 occur
by the 25 hydroxylase enzyme
how is the hydroxylation of the 1,25 dihydroxyvitamin D3 regulated
the inhibition of the 25 hydroxylase enzyme by 25 hydroxyvitamin D
where are steroid hormones derived from
cholesterol secreted by steroid glands
how does vitamin D signalling occur
steroid receptor bind to cognate steroid hormone become activated and activate transcription of target genes
which gene is involved in bone matrix
human osteocalcin gene
what is human osteocalcin produced by
osteoblasts and odontoblasts
what does human osteocalcin do
binds hydroxyapatite and calcium crystals
what can deficinecy of vitamin d3 LEAD TO
Skeletal mineralisation deficiency
what can deficinecy of vitamin d3 LEAD TO in children
rickets and osteomalacia
how might vitamin d deficiency affect the oral cavity
increase risk of developing dental caries and perio disease
