Vitamin A

Question 1

I RAE =

Answer 1

1 g all trans retinol
2 g all trans carotene in oil
12 g food all trans carotene (b- carotene)
24 g all other (a-carotene, etc)

Question 2

most prevalent Vit A forms

Answer 2

a, b-carotene, lycopene, lutein, zeaxanthin, b-cryptoxanthin

Question 3

Provitamin A carotenoids

Answer 3

a,b- carotene, b-cryptoxanthin

Question 4

Transport

Answer 4

intestinal cell –> re-esterification of retinol, catalyzed by LRAT, packaged into CM

carried by lymph, absorbed retinyl esters taken up by liver w/ CM remnant by receptor mediated endocytosis –> transport from intestinal cell to liver

Question 5

Cell Metabolism

LRAT/ ARAT

Answer 5

liver: location of hydrolysis and esterification
Storage as RE in stellate cells

LRAT: acyl groups from phosphatidylcholine, involves retinol complexed w CRBPII

ARAT: acyl groups from CoA

Question 6

Conversion enzymes

Answer 6

cleavage enzyme 15,15’ dioxygenase

reducing enzyme retinaldehyde reductase

Question 7

Vitamin A Binding Proteins

Answer 7

Plasma: RAP solubilizes and delivers retinol to cells
Cytosol: limits “free” retinoid + channel retinoids to specific enzymes responsible for metabolic transformations
Nuclei: specific retinoid receptor proteins regulate activity of retinoid responsive genes

Question 8

Vision

Answer 8

11- cis- retinal bound to opsin
photon absorption –> isomerization –> all trans retinal –> release from opsin –> neuronal signaling cascade

*regeneration at Retinal Pigment Epithelium (RPE)

Question 9

Gene Expression

Answer 9

RAR binds retinoid acid and 9 cis retinoic
RXR only binds 9 cis retinoic

RARE on DNA.
RA + RAR –> (+) receptor –> RARE- DNA –> regulation of expression of genes downstream

Question 10

Deficiency

Answer 10

night blindness –> low rhodopsin
xerophthalmia: dry/ ulcers on cornea. bilots spots, xerosis
immunity: inc m/m d/t infection. low lymphoid cellularity, low antibodies, low cell mediated immunity, low innate immunity
mucous secreting epithelial tissue: keratinizing –> follicular hyperkeratosis

Question 11

Nutritional Status Assessment

Answer 11

Relative dose response. 5hr-0hr / 5hr
>20% = inadequate hepatic stores
retinyl palmitate,
MRDR is dehydroretinol, single sample @ 5 hrs

Question 12

Toxicity

Answer 12

marginal discoloration of gingiva

3rd trimester –> microtia, anotia, craniofacial defects

Question 13

At Risk

Answer 13

premature, infants, pregnant/ lactating, cystic fibrosis

Question 14

Osteoporosis

Answer 14

inc Vit A –> low BMD –> inc fracture risk?