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26 Cards in this Set

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  • Back
What are the three main forms of Vitamin A?
Retinoic Acid
What form is the storage form?
Storage: Retinol is esterified
Retinyl Ester
What is the "fourth" form of Vitamin A? Conversion rate?
But only about 6 or 8 are active
1 ug of carotenoid per 12 ug of active vitamin/ low rate because cleaving on carotenoid non-specific; limited absorption of carotenoids; low enzyme activity
What is the enzyme that cleaves carotenoids?
B-carotene dioxygenase
What forms of Vit A are in diet?
Retinol, Retinyl-Esters, and Carotenes
Describe the interconversion of the vitamin A
Retinyl-Ester <--> Retinol<--> Retinal-->Rentinoic Acid
Where is Vit A stored?
Liver/ Form = retinyl ester
Retinyl Ester <--> Retinol (both ways)
Retinol <--> Retinal (to nal form)
Retinol Ester Synthetase
Retinol Ester Hydrolase

Retinol Dehydrogenase
Where does the Vit A interconversion occur?
Liver: hepatocytes
What are the intracellular binding proteins of VitA?
4 of them:
CRBPI & II - I in most cells & responsible for main. of retinyl ester form; II in intestinal mucosa
These bind retinal and retinol

CRABPI & II - II is only in skin
What is the purpose of the intracellular binding proteins?
To guide Vitamin A onto its next metabolic step
How is Vit A transported in body?
Excreted from liver as RetiNOL. Binds to RBP (retinol binding protein) which then binds to transthyretin (1:1)
Why does Retinol•RBP bind with transthyretin?
To prevent renal catabolism
What happens to RBP levels with low Vit A?
RBP in blood drops, but liver still produces: just doesn't secrete.
What is they key to vision?
The isomerization of 11-cis-retinal to all-trans retinal; this causes a conformational change in Opsin, which leads to the closing of a sodium channel, and action potential, and then signal.
Where is Vit A found in the eye?
It (as 11-cis-retinal) combines with Opsin to make Rhodopsin which is embedded in discs that comprise the rods.
How does vitamin A get into the eye?
Retinol first diffuses into the pigmented epithelial cell; it undergoes a series of reactions that transform it into 11-cis-retinal; this form
What happens when a photon of light hits the eye?
11-cis isomerized to AT and rhodopsin (opsin) undergoes conformational change; metarhodopson I->II. II is active form and causes cascade of events leading to signal reaching brain. MRII->III-> opsin + retinol

Retinol must diffuse back into pigmented epithelium cell to get converted back into usable form.
What is the space between the pigmented epithelial cell/ rod called?
interphotoreceptor matrix
How are opsin and 11-cis-R linked?
they form a schiff's base (imine)
What form of Vitamin A is involved in gene expression/cell proliferation?
ONLY retinoic acid!!
Describe Vit A's role in gene expression.
Vit A (in the RA form) forms a dimer with two nuclear receptors, RAR and RXR. This complex then binds with DNA ("zinc fingers") and modulates production of specific proteins.

Cell differentiation/proliferation is mediated by increase/decrease of specific proteins synthesized.
What if I don't consume enough retinoic acid? Symptoms?
Issues are going to revolve around gene expression/cell differentiation,proliferation:
*Increased susceptibility to infection
What is Xeropthalmia?
Drying out of the eyeball due to lack of sufficient secretions.

Early sign: Bitot's spot. Keratanized cells. Bacterial grow. Eventual blindness.

#1 cause of childhood blindness in developing countries
What is Hyperkeratosis?
Secretion of excess keratin (v. tough protein): results in bumpies all over skin
What if I don't consume enough Retinol/Al/Retinyl Esters?
Nightblindness/ affects on vision