Vitamin A (Retinol); Function, Transport, Metabolism, Deficiency, Toxicity, Sources



  • Biologically active forms – retinoids: retinol, retinal, retinoic acid.

    Cyclohexane ring and isoprenoid chain
  • Major vit. A precursor (provitamins) → plants carotenoids.
  • The foodstuff of animals origin contains most of vit. A in the form of esters (retinylpalmitates) – retinol and long fatty acid

Vitamin A transport and Metabolism

  • Retinol esters → hydrolysis by pancreatic enzymes to retinol.
  • b-carotene is cleaved to retinal by b-carotene 15,15´ dioxygenase (cofactors iron and bile salts).
  • Intestinal cells → esterification of retinol → transported in chylomicrons.
  • Remnants of chylomicrons → liver→ esterification (if the concentration exceeds 100 mg, esters are stored ).
  • Transport of retinol to target organs tightly bound to retinol-binding protein, RBP.

Cone cells vs Rod Cells

  • A derivative of Vitamin A plays a crucial role in vision when it is bound to a protein called opsin.
  • The cone cells in the retina of the eye contain several types of opsin and are responsible for vision in bright light and for the color vision.
  • The rod cells in the retina contain only one type of opsin; they are responsible for vision in dim light.
  • The aldehyde group of retinal forms a mine with the side chain amino acid group of a lysine residue in the rod cell opsin.
  • The product is called rhodopsin which occupies 60% of the membrane in rod cells

Vitamin A and Vision

  • A is necessary to form rhodopsin (in rodes, night vision) and iodopsins (photopsins, in cones – color vision) – visual pigment.
  • Retinaldehyde is a prosthetic group of light-sensitive opsin protein.
  • In the retina, all-trans-retinol is isomerized to 11-cis-retinol → oxidized to 11-cis-retinaldehyde, this reacts with opsin (Lys) → to form the holoprotein rhodopsin.
  • Absorption of light → conformation changes of opsin → photorhodopsin.
  • The following is a series of isomerization→ initiation of a nerve impulse.
  • The final step is hydrolysis to release all-trans-retinaldehyde and opsin.
  • Deficiency of vit. A leads to night blindness.
  • Vitamin A is an important antioxidant.

Vitamin A and other Functions

Transcription and cell differentiation

  • Retinoic acid regulates the transcription of genes – acts through nuclear receptors (steroid-like receptors).
  • By binding to various nuclear receptors, vit. A stimulates (RAR – retinoic acid receptor) or inhibits (RXR- retinoid „X“ receptor) transcription of genes transcription. All-trans-retinoic acid binds to RAR and 9-cis-retinoic acid binds to RXR.
  • Retinoic acid is necessary for the function and maintenance of epithelial tissues.

Vitamin A Deficiency

  • The early sign → a loss of sensitivity to green light,
    • prolonged deficiency → impairment to adapt to dim light
    • more prolonged deficiency leads to night blindness
    • Ever escalated deficiency leads to squamous metaplasiacolumnar epithelia are transformed into heavily keratinized squamous epithelia.
  • The conjunctiva loses mucus-secreting cells → glycoprotein content of the tears is reduced → xerophthalmia (“dry eyes”)
    • Often complication – a bacterial or chlamydial infection which results in perforation of the cornea and blindness
  • Transformation of respiratory epithelium – loss of protective airway function (antibacterial properties) → bronchitis.
  • Conversion of the urinary tract epithelium → higher frequency of urinary stone formation
  • Immunosuppression
  • Impairment of reproductive function (both in men and women).
  • Worldwide deficiency of vit. A
  • 3 – 10 mil. children become xerophthalmic every year
    • 250 000 to 500 000 go to blindness
    • 1 million dies from infections

Vitamin A Toxicity

  • Toxic dose:
    • a single dose of more than 200 mg
    • more than 40 mg per day
    • Acute symptoms – headache, vomiting, impaired consciousness.
  • Chronic intoxication – weight loss, vomiting, pain in joints, muscles, blurred vision, hair loss, excessive bone growth.
  • Both vit. A Excess and deficiency in pregnancy are teratogenic – retinoic acid is a gene regulator during early fetal development
  • Carotenoids are nontoxicaccumulation in tissues rich in lipids (the skin of babies overdosed with carrot juice maybe orange).

Metabolic Function of Vitamin A

  • Vision
  • Gene transcription
  • Immune function
  • Embryonic development and reproduction
  • Bone metabolism
  • Hematopoiesis
  • Skin health
  • Antioxidant activity

Sources of Vitamin A

  • Cod liver oil
  • Meat
  • Egg
  • Milk
  • Dairy products
  • Carrot
  • Broccoli
  • Spinach
  • Papaya
  • Apricots

Binod G C

I'm Binod G C (MSc), a PhD candidate in cell and molecular biology who works as a biology educator and enjoys scientific blogging. My proclivity for blogging is intended to make notes and study materials more accessible to students.

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