Vitamin Classification and Structure Quiz

Study Notes

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).

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.

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.

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.

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.

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.

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.

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.