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Questions and Answers
Which of the following vitamins is classified as fat soluble?
Which of the following vitamins is classified as fat soluble?
What is the primary function of retinol in the body?
What is the primary function of retinol in the body?
Which compound is the precursor of vitamin A found in plant foods?
Which compound is the precursor of vitamin A found in plant foods?
What is the conversion efficiency of β-Carotene to Vitamin A in humans?
What is the conversion efficiency of β-Carotene to Vitamin A in humans?
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Which form of vitamin A cannot be reduced back in the body once formed?
Which form of vitamin A cannot be reduced back in the body once formed?
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Which of the following is a source of preformed vitamin A?
Which of the following is a source of preformed vitamin A?
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What molecules are formed by the oxidative cleavage of β-Carotene in the intestine?
What molecules are formed by the oxidative cleavage of β-Carotene in the intestine?
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What complex does retinol form for transport in the blood?
What complex does retinol form for transport in the blood?
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What is the primary function of retinoic acid in the context of vitamin A action?
What is the primary function of retinoic acid in the context of vitamin A action?
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Which of the following forms of vitamin A is involved in the visual cycle?
Which of the following forms of vitamin A is involved in the visual cycle?
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What is the daily requirement of retinol activity equivalents (RAE) for adult females?
What is the daily requirement of retinol activity equivalents (RAE) for adult females?
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Which of the following is NOT a therapeutic use of vitamin A?
Which of the following is NOT a therapeutic use of vitamin A?
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Which vitamin A compound is specifically linked to epithelial tissue differentiation?
Which vitamin A compound is specifically linked to epithelial tissue differentiation?
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How many milligrams of β-carotene is equivalent to 1 RAE?
How many milligrams of β-carotene is equivalent to 1 RAE?
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What is the role of the retinol-RBP complex in vitamin A metabolism?
What is the role of the retinol-RBP complex in vitamin A metabolism?
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Which of the following is NOT a form of vitamin A?
Which of the following is NOT a form of vitamin A?
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Study Notes
Vitamin Classification
- Vitamins are classified as either water soluble or fat soluble.
- Water soluble vitamins include vitamin C and B complex vitamins, which include thiamin (B1), riboflavin (B2), niacin (B3), biotin, pantothenic acid, folic acid, and vitamin B12.
- Fat soluble vitamins include vitamins A, D, E, and K, as well as other vitamins like pyridoxine (vitamin B6).
Structure of Vitamin A
- Vitamin A is a fat-soluble vitamin.
- Retinoids include both natural and synthetic forms of vitamin A, which may or may not show Vitamin A activity.
- The 4 forms of vitamin A are retinol, retinal, retinoic acid, and β-carotene.
- Retinol is found in animal tissue as retinyl esters, a primary alcohol with a β-ionone ring and an unsaturated side chain.
- β-carotene is found in plant food, can be oxidatively cleaved in the intestine to yield two molecules of retinal, and has 1/12 the vitamin A activity of retinol.
- Retinal is an aldehyde derived from the oxidation of retinol, can be readily interconverted with retinol, and is further oxidized to retinoic acid.
- Retinoic acid is an acid form derived from the oxidation of retinal and cannot be reduced in the body.
Sources of Vitamin A
- Preformed vitamin A is found in animal tissues such as liver, kidney, cream, butter, and egg yolk.
- β-carotene, which is a precursor to vitamin A, can be found in yellow and dark green vegetables and fruits.
Absorption and Transport of Vitamin A
- Retinyl esters are hydrolyzed into fatty acids and retinol in the intestine, along with β-carotene and retinal.
- Retinol is then re-esterified to a long-chain fatty acid in enterocytes, forming retinyl esters, which are secreted as a component of chylomicrons in lymph.
- Chylomicron remnants deliver retinyl palmitate, all-trans-retinol, and retinol to the liver, where retinol is bound to retinol-binding protein (RBP).
- The retinol-RBP complex is transported in the plasma, where it can be taken up by various tissues.
Mechanism of Action of Vitamin A
- Retinol is oxidized to retinoic acid, which controls the production of specific proteins by regulating retinoid-specific RNA synthesis.
- The specific receptor proteins bind to retinoic acid and form a complex that interacts with nuclear chromatin.
- Retinoids control the expression of the gene that produces keratin in most epithelial tissues of the body.
- RBP, a plasma retinol-binding protein, delivers retinol to cells, where it is oxidized to retinoic acid.
- Retinoic acid inactivates a receptor in the cytosol of an epithelial cell, creating an activated receptor complex, which then travels to the nucleus and interacts with genes to produce mRNA that codes for specific proteins involved in cellular differentiation.
Functions of Vitamin A
- Vitamin A, in its various forms (retinol, retinal, retinoic acid), and carotenes, is important for maintaining vision, growth, reproduction, and epithelial tissue differentiation and serves as a therapeutic agent for psoriasis, promyelocytic leukemia, and severe acne.
- The retinol-RBP complex delivers all-trans-retinol to retinal rod and cone cells, where it is esterified to all-trans-retinyl esters.
- Isomerization of all-trans-retinyl esters to 11-cis retinol followed by reduction to 11-cis retinal and oxidation to opsin produces rhodopsin.
- When exposed to light, rhodopsin bleaches, causing a conformational change in opsin and the release of all-trans-retinal, ultimately triggering the visual signal transduction pathway.
Requirement of Vitamin A
- The requirement of vitamin A is measured in retinol activity equivalents (RAE).
- Adults should aim to consume 900 RAE daily for males and 700 RAE daily for females.
- 1 RAE is equivalent to 1 mg of retinol, 12 mg of β carotene, or 24 mg of other carotenoids.
Summary
- Vitamin A is a fat-soluble vitamin with 4 forms: retinol, retinal, retinoic acid, and β-carotene.
- These forms act by controlling the production of specific proteins.
- Vitamin A is vital for maintaining vision, growth, reproduction, and epithelial tissue differentiation.
- Vitamin A also has therapeutic applications in managing conditions such as psoriasis, promyelocytic leukemia, and severe acne.
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Description
Test your knowledge on the classification of vitamins and the structure of Vitamin A. This quiz covers both water-soluble and fat-soluble vitamins, as well as the various forms of Vitamin A and their properties. Determine how well you understand these essential nutrients and their biological significance.
