vitamins A & E (b2- CVS)

Question 1

retinol vs retinal vs retinoic acid

Answer 1

Retinol: primary alcohol form and storage form, found as retinyl esters

Retinal: an aldehyde derived from the oxidation of retinol
- can be interconverted with retinol

Retinoic Acid: formed by the oxidation of retinal, but it cannot be reduced back to retinal or retinol

Question 1

β-Carotene

Answer 1

pro-vitamin A (a precursor) that can be symmetrically cleaved by carotene dioxygenase to yield 2 molecules of retinal

Question 2

sources of vitamin A

Answer 2

Preformed Vitamin A (Retinoids): come from animal origins and are found in good sources like liver, kidney, cream, butter, and egg yolk

Pro-Vitamin A (Carotenoids, primarily β-carotene): come from plant origins, such as dark leafy green, yellow-orange vegetables, and fruits

Question 3

absorption, transport, & storage of vitamin A

Answer 3

• Dietary retinol transported as retinyl esters in chylomicrons from small intestine to liver
• In liver, retinol is primarily stored as retinyl palmitate (retinyl esters), mainly in the liver and adipose tissue
• When needed, retinol is secreted from the liver in association with plasma retinol binding proteins (RBP) and transthyretin (TTR) for transport to target tissues

Question 4

role of vitamin A in the visual cycle (rhodopsin) aka Wald’s visual cycle

Answer 4

• retina of eye have 2 types of cells: rods (for dim light vision) & cones (for bright light and color vision)

rhodopsin: conjugated protein found in rods thats crucial for visions
- consists of 11-cis-retinal and protein called opsin

light falls on retina
↓
11-cis-retinal in rhodopsin is isomerized to all-trans-retinal by enzyme retinal isomerase
↓
all-trans-retinal separates from opsin
↓
opsin triggers cascade of events leading to a visual signal in brain
↓
the all-trans-retinal is enzymatically converted to all-trans-retinol and then 11-cis-retinal
↓
opsin and 11-cis-retinal combine to regenerate rhodopsin

Question 5

dark adaptation mechanism

Answer 5

When a person shifts from a bright light to a dim light, rhodopsin stores are depleted = vision is impaired

After few minutes, rhodopsin is resynthesized = vision is improved

this process is impaired in Vitamin-A deficiency

Question 6

maintenance of epithelial cells mechanism for vitamin A

Answer 6

retinol & esp retinoic acid function like steroid hormones - essential for normal growth & differentiation of epithelial tissues & mucus secretion

retinoic acid binds to intranuclear receptors (RARs)
↓
forms complex that binds to chromatin
↓
activates transcription of specific genes
↓
leading to the synthesis of specific proteins and cellular differentiation

Question 7

therapeutic uses of vitamin A

Answer 7

Tretinoin (all-trans retinoic acid): for mild cases of acne (local application) and acute promyelocytic leukemia

Isotretinoin (13-cis retinoic acid): administered orally for severe cystic acne unresponsive to conventional therapies

Oral Leucoplakia: Retinoic acid can help revert this precancerous condition to normal epithelium

Question 8

A researcher is studying gene regulation in epithelial cells. She finds that a molecule enters the nucleus, binds to an intranuclear receptor, and activates transcription of specific mRNAs. This molecule is most likely:

Answer 8

Retinoic acid

Question 9

vitamin E is regenerated by which vitamin?

Answer 9

vitamin C

Question 10

absorption, transport, & storage of vitamin E

Answer 10

absorption: like other fat-soluble vitamins, absorbed along w/ lipids in small intestine

transport: via chylomicrons, VLDL, and LDL

storage: stored in adipose tissue & liver

Question 11

causes of oxidative stress in body

Answer 11

• Oxidative phosphorylation
• Ionizing Radiation (act on water forming OH radicals)
• Inflammation (Oxygen dependent )
• Metals (by Fenton reaction)
• Drugs (Detoxification in liver by P450system)

Question 12

mechanism of vitamin E

Answer 12

oxidation reactions produce free radicals which can start chain reactions & cause oxidative damage

vit. E accumulates in circulating lipoproteins, cellular membranes, and fat deposits
↓
it acts as scavenger for molecular oxygen & free radicals reacting very rapidly w/ them
↓
it donates e- to lipid radicals to terminate chain reactions
↓
this protects polyunsaturated fatty acids (esp those in membranes) from peroxidation reactions

Question 13

vitamin E & selenium

Answer 13

Selenium complements the antioxidant effects of vitamin E and reduces the requirement of vitamin E