Vitamin A

Question 1

What are the four forms of vitamin A?

Answer 1

All-trans-retinol (active form)
Retinal
Retinoic acid
Retinyl ester

Question 2

What are the vitamin A forms found in food? (2)

Answer 2

Retinyl palmitate
B-carotene

• carotenoids are often bound to protein
• retinol (as all-trans) is typically bound to fatty acid esters
• retinyl palmitate often is bound to protein

Question 3

What is the difference in the structure of vitamin A vs. B-carotene?

Answer 3

B-carotene is a combination of two vitamin A structures with one of the structures flipped upside down.

Question 4

How is B-carotene converted to vitamin A?

Answer 4

B-carotene is first cleaved to 2 molecules of retinal

Then the two retinal molecules are reduced to 2 molecules of retinol

Question 5

What does RAE stand for?

Answer 5

RAE = retinol activity equivalent

Question 6

What are the main acronyms that are important for the function of vitamin A? What are their functions?

Answer 6

CRBPII - (cellular retinol-binding protein II)
Function: cellular transport and storage

LRAT -
Function: cellular transport and storage

CRABP -

Question 7

What is the role of CRBPII?

Answer 7

Cellular retinol-binding protein II

• Abundant in the small intestine
• Binds retinol and retinal, making them soluble
• Directs reduction of retinal to retinol
• Escorts retinol to LRAT

Question 8

What is the role of LRAT?

Answer 8

Lecithin retinol acyltransferase

• adds FAs (mainly palmitate) to retinol to form retinyl esters
• retinol must be esterified to exit the intestinal cell

Question 9

Describe the steps in Vitamin A absorption.

Answer 9

1. CRBPII binds to both retinol and retinal inside the cell
2. Retinal (attached to CRBPII) is converted to retinol to form retinol CRBPII retinol
3. LRAT esterifies palmitic acid (FA) onto the CRBPII-bound retinol to form CRBPII-retinylpalmitate
4. Retinyl esters are incorporated along w/ phospholipids, triacylglycerol, cholesterol esters, carotenoids, and apoproteins to form a chylomicron
5. chylomicrons leave the intestinal cell and enter the lymph
6. retinoic acid can directly enter the blood where it attaches to albumin for transport to the liver

Question 10

What are the metabolic functions of vitamin A?

Answer 10

• vision
• cellular differentiation and gene expression
• growth
• immune system
• bone development
• reproduction

Question 11

What is the role of vision for vitamin A?

Answer 11

• serves as the photosensitive chromophoric group of rhodopsin found in rod cells of the retina

rhodopsin detects small amount of light important for night vision

Question 12

How does night blindness occur in relation to vitamin A?

Answer 12

all-trans-retinal (active form) must be converted back to 11-cis-retinal to regenerate rhodopsin

storage retinyl esters can be hydrolyzed, isomerized, oxidized to 11-cis-retinal, and shuttled to the rod cell

Failure to regenerate 11-cis-retinal results in night blindness