lecture 17: vitamin A

Question 1

vitamin A

Answer 1

compounds that exhibit the biological activity of retinol the alcoholic form of vitamin A
fat soluble
vision, immunity, metabolism, cell proliferation

Question 2

carotenoids

Answer 2

plant derived precursors for vitamin A, can be converted to retinol in animals

Question 3

how is vitamin A stored?

Answer 3

stored as retinyl esters in the liver

Question 4

major forms of vitamin A in the diet

Answer 4

retinyl esters: animal sources – hydroyzed in lumen
carotenoids: plants

Question 5

carboxylester lipase

Answer 5

secreted into intestinal lumen from pancreas to hydrolyze short-chain retinyl esters

Question 6

retinyl ester hydrolase

Answer 6

hydrolyzes retinyl esters with long FA chain
brush border membrane of small intestine

Question 7

absorption of vitamin A

Answer 7

retinyl esters are hydrolyzed in lumen
absorbed β-carotene cleaved into two molecules of retinal
Formation of retinyl esters > packaged into chylomicrons, secreted to tissues

Question 8

ARAT and LRAT

Answer 8

retinol > retinyl esters
reformation fo retinyl esters after absorption

Question 9

major site of vitamin A storage

Answer 9

liver

Question 10

how is vitamin A transported

Answer 10

retinyl esters transported in chylomicrons and secreted to tissues

Question 11

retinol binding protein

Answer 11

water soluble proteins that carry retinol through circulation

interactions of retinoids with these proteins retard their degradation, decrease their concentrations in membranes, and increase their concentrations in aqueous spaces

Question 12

hydrophobic B barrel

Answer 12

retinol binding site of RBP

Question 13

The main site of synthesis and secretion of RBP

Answer 13

liver

Question 14

11-cis-isomer of retinal

Answer 14

visual transduction

Question 15

physiological function of RA

Answer 15

regulate transcription of multiple genes

Question 16

rhodopsin

Answer 16

senses light, membrane protein located in photoreceptor cells, uses 11-cis-retinal as chromophore

Question 17

11-cis-retinal and vision process

Answer 17

light sensed by rhodopsin, which uses 11-cis-retinal as its chromophore
absorption of a photon by 11-cis-retinal triggers hyperpolarization of plasma membrane and isomerization to all-trans form
intermediate conformation interacts with transducin, exchange of GDP for GTP
bleached rhodopsin can be regenerated by replenishing 11-cis-retinal supply from RPE cells

Question 18

transducin

Answer 18

G protein, interaction with conformational intermediate of rhodopsin leads to exchange of transducin-bound GDP for a GTP

GTP-bound transducin activates phosphodiesterase > breakdown of cGMP to inactive GMP

Question 19

phosphodiesterase

Answer 19

breaks down cGMP to inactive GMP > closes sodium channels and hyperpolarization of plasma membrane

Question 20

retinoic acid function

Answer 20

regulation of cell proliferation and differentiation

Question 21

all-trans-RA

Answer 21

enhances transcription of target genes by activating RAR and PPAR

Question 22

retinoid nuclear receptors

Answer 22

RAs regulate gene expression by activating these receptors
bind to DNA recognition sequence, facilitate rate of gene transcription

Question 23

9-cis-RA

Answer 23

activates RXRs

Question 24

RAR and PPAR heterodimers

Answer 24

associate with RXR to form heterodimers, homodimers do not readily form
RXR-RAR and RXR-PPAR

Question 25

RXR heterodimers

Answer 25

regulated by more than one type of ligand
RXRs = master regulators of several signaling nutrients and hormones
ex. vitamin D VDR, thyroid hormone TR

Question 26

nonpermissive vs permissive heterodimers

Answer 26

nonpermissive: binding of one ligand does not permit binding of the other
permissive: requires binding of both ligands

Question 27

co-regulatory proteins

Answer 27

function as transcriptional corepressors in the absence of ligands
upon ligand binding, receptors dissociate corepressors and recruit coactivators

Question 28

vitamin A deficiency

Answer 28

mainly plant based diet, little dietary fat (little absorption of fat-soluble vitamins)

drying of cornea > damage to cornea > blindness

keratinization of tracheal epithelium, thinning of intestinal epithelium