Vitamin A

Question 1

dietary sources of Vitamin A?

Answer 1

liver, eggs, whole milk and milk products, butter, carrots, pumpkin, sweet potatoes and other yellow vegetables

Question 2

ß-carotene is the precursor of Vitamin A. In the intestines ß-carotene is cleaved to all-trans retinal by the enzyme ____(a)____. All-trans retinal is converted to all-trans retinol by the enzyme ____(b)____.

If Vitamin A is required immediately, it goes to the site of utilization. If not needed for use, it is stored as ____(c)____ in the liver. This is through esterification of retinol by ____(d)____ acid. If Vitamin A1 is in excess, it is excreted.

Retinol (A1) —> Retinal —> ____(e)____ (which is excreted)

Answer 2

(a) dioxygenase
(b) retinol dehydrogenase
(c) retinyl palmitate
(d) palmitic acid
(e) retinoic acid
[Diagram 1]

Question 3

State the roles of Vitamin A.

Answer 3

1. Formation of visual pigments i.e. rhodopsin and iodopsin
2. Retinal, a Vitamin A derivative, is the pigment that initiates the response of rod and cone cells of the retina to light, producing a neuronal signal to the brain.
3. Acting through receptor proteins in the cell nucleus, the Vitamin A derivative retinoic acid regulates gene expression in the development of epithelial tissue, including skin.
4. Retinoic acid activates transcription of specific genes that mediate growth and development.

Question 4

Role of Vitamin A in vision
Visual transduction begins when light falls on rhodopsin, many thousands of molecules of which are present in each disk of the outer segments of rod and cone cells (which are two distinct receptor systems in the eye).
Retinal combines with the protein opsin to form rhodopsin.
The light-absorbing pigment ____(a)____ is covalently attached to opsin, the protein component of rhodopsin. When a rhodopsin molecule is excited by visible light, the ____(a)____ undergoes a series of photochemical reactions that convert it to all-trans retinal forcing a change in the shape of the entire rhodopsin molecule. This transformation in the rod cell of the vertebrate retina sends an electrical signal to the brain that is the basis of visual transduction.
____(a)____ is regenerated through enzymatic reactions so that it can be available to combine with opsin to regenerate rhodopsin.

All-trans retinal –(b)–> All-trans retinol –(c)–> 11-cis-retinol –(d)–> (a)
Identify the enzymes labelled (b), (c) and (d).

Answer 4

(a) 11-cis-retinal
(b) retinal reductase
(c) retinol isomerase
(d) retinal reductase
[Diagram]
NB: 11-cis retinol is oxidized to 11-cis retinal.

Question 5

effects of deficiency of Vitamin A

Answer 5

1. xerophthalmia: condition whereby one’s eyes are dry. The cornea is affected and this can lead to blindness. Lack of Vitamin A leads to a lack of mucous which contributes to moisturization of the eye.
2. keratomalacia: clouding of the cornea due to xerophthalmia
3. dry and scaly skin
4. night blindness
5. glare blindness: debilitating loss of visual acuity in bright lighting

Question 6

Choose the correct answer.
The vitamin A derivative that plays a role in vision is
A. all trans-retinal
B. all trans-retinol
C. 11-cis retinal
D. retinoic acid
E. ß-carotene

Answer 6

Choice C.