Vitamin A: Structure, Properties, Functions, and Deficiency

Vitamins are the organic compounds occurring in small quantities in different natural foods and necessary for growth and maintenance of good health in human beings and in experimental animals. Basically, vitamins can be divided into two types based on solubility. Vitamins mean Vital Amines

Vitamin A structure functions 2020

They are 

  1. Fat-soluble (Vitamin A, D, E, and K)
  2. Water-soluble (B-Complex vitamin and Vitamin C)

In this article, we are adding the details of Vitamin A. 

Historical aspects of Vitamin A

  • Vitamin A was first isolated by Mc Collum, Davis Simmonds, and Kennedy in 1913. 
  • Richard Kuhn identified Carotenes. Received Nobel prize in 1937.
  • Vitamin A structure is elucidated by Paul Karrer in 1931.

Common names for Vitamin A

  • Anti-xerophthalmic factor
  • Bright eyes Vitamin

Other Names

  • 3-Dehydroretinol,
  • 3-Déhydrorétinol,
  • Acétate de Rétinol,
  • Antixerophthalmic Vitamin,
  • Axerophtholum,
  • Dehydroretinol,
  • Déhydrorétinol,
  • Fat-Soluble Vitamin,
  • Oleovitamin A,
  • Palmitate de Rétinol,
  • Retinoid, Retinoids,
  • Rétinoïdes, Retinol,
  • Rétinol,
  • Retinol Acetate,
  • Retinol Palmitate,
  • Retinyl Acetate,
  • Rétinyl Acétate,
  • Retinyl Palmitate,
  • Vitaminum A
  • Rétinyl Palmitate,
  • Vitamin A Acetate,
  • Vitamin A Palmitate,
  • Vitamin A1,
  • Vitamin A2,
  • Vitamina A,
  • Vitamine A,
  • Vitamine A1,
  • Vitamine A2,
  • Vitamine Liposoluble,

Sources of Vitamin A

  • Rich sources of Vitamin A foods are “Cod liver oil” and “Shark liver oil”.
  • Fish liver oils, butter, Milk, Eggs, and Kidney.
  • Pigmented vegetables and fruits
  • Carotene is the provitamin of Vitamin A and is highly found in Carrot and yellow vegetables.
vitamin A food sources

Structure of Vitamin A

Vitamin A structure

It contains a beta-ionine ring and side chain. The side chain consists of Two Isoprene units, four double bonds, and one alcoholic group.

Chemical names

Retinol (A1) C20H30O
3-dehydroretinol (A2) C20H28O

Chemical Properties

  • It is a colorless oily substance. It forms a transient blue color with saturated “Antimony trichloride” (AnCl3). (It is an important principle in the “Cataract” method used to determine Vitamin A.
  • Vitamin A absorbs UV light. IT shows maximum absorption at 325nm.
  • It is quite stable in the absence of air and light. It forms esters with higher fatty acids.
  • It is soluble in Fat solvents and fats. But insoluble in water. 

Biochemical Functions

1. Visual cycle

Rhodopsin plays a pivotal role in vision. It is a membrane protein found in the photoreceptor cells of the retina. The proteins are oriented in such a way that its N-terminal end faces the intradiscal space on the receptor matrix.

Rhodopsin is made up of the protein Opsin and 11-cis-retinal attached through a Schiff baser to a lysine residue of the protein. The 11-cis-retinal is photosensitive.

When the falls on the retina, the 11-cis-retinal isomerases to all-trans-retinal. This creates a series of the unstable molecule and the Schiff base linkage is finally broken. Then all-trans-retinal is released from the protein. Opsin remains in the retina. but trans-retinal enters the blood circulation.

The main role of Vitamin A is Vision. The retina contains two types of light receptors i.e., “Rods” and “Cones”.

  • Rods function in dim light but do not recognize color.
  • Cones function in bright light and are responsible for color vision. Rods contain a protein “Opsin” and “11-cis retinol”, which is a prosthetic group.

11-cis retinol is synthesized from trans retinol.  All trans-retinol are converted into 11-cis-trans retinal by several steps. Rods absorbs light within picoseconds. It undergoes several changes i.e., 11-cis retinol is converted to trans retinol. 

Thus trans retinol is dissociated from Opsin and transported to the liver. Where it is converted to 11-cis retinal. It is ready for another Visual cycle.

visual cycle

Cycle diagram

2. Other Biochemical Functions

  • Vitamin A is necessary for the maintenance of normal Epithelium and skin. 
  • Retinoic acid is found to have an important role in Glycoprotein synthesis
  • Retinoyl phosphate acts as a donor of oligosaccharide units across the lipid bilayers of the cell.
  • Retinol acts as a steroid hormone in controlling the expression of certain genes. This may account for the requirement of Vitamin A for normal reproduction
  • Vitamin A is required for “Sulfation” of the mucopolysaccharides in the matrix of bone.

Deficiency Disorders

Here are the vitamin a deficiency symptoms

  1. Night Blindness: Inn the early stage of Vitamin A deficiency, the individuals can not see well in dim light. This condition is called “Night Blindness” (or) “Nyctalopia”. This is due to an inadequate amount of Vitamin A required for the regeneration o “Rhodopsin”.
  2. Xerophthalmia: In this condition, lacrymal glads get keratinized and stop their secretions. Thus eyes become dry. This is due to the deficiency of Vitamin A.
  3. Keratomalacia: If Xerosis of the conjunctiva and cornea is severe that leads to the condition called “Keratomalacia”. Symptoms are “metaplasia” and “degeneration of the corneal epithelium”

Overdose of Vitamin A

The side effects of too much vitamin A gives serious problems in pregnant women. They are fatigue, brittle bones, joint pain, vomiting, and severe gastrointestinal distress.

If pregnant women take an excessive amount of vitamin A, it leads to a serious problem for the unborn child including birth defects. The recommended RDA values are given below

What are the benefits of vitamin A?

  • Here are the few benefits of Vitamin A. They are
  • It protects your eyes from age-related issues in vision and night blindness
  • It protects from some types of cancers
  • It gives good strength to the immune system
  • It gives good support for your bone health
  • It promoted healthy growth and reproduction

The FAQ on Vitamin A

Which foods are high in vitamin A?

vitamin A food sources

The best sources of vitamin A are Cod liver oil, Eggs, Fortified breakfast cereals, Fortified skim milk, orange, Yellow vegetables, fruits, Other sources of beta-carotene such as broccoli, spinach, the darkest green, and leafy vegetables.

What does vitamin A do in the human body?

Vitamin A is insoluble in water and soluble in Fats (So it is called “Fat-soluble vitamin”). It is very important for normal vision, immune system development and for reproduction. It also helps sensitive organ developments like Lungs, Kidneys and other organs. 

How do we get vitamin A?

To get a good amount of Vitamin A, we have to take the good sources of beta-carotene in our diet. It will convert into Vitamin A in our body. The good sources are Yellow, red and green leafy vegetables – spinach, sweet potatoes, carrots, and red pepper. Yello fruits like papaya, apricots, and mango are other good sources of Vitamin A rich foods.

What is the recommended dose of vitamin A?

According to RDA (Recommended Dietary Allowance) Values, here are the vitamin a dosage per day
1. For men – 900 mcg per day,
2. For Women – 700 mcg per day,
3. For Pregnant Women – 700 mcg per day,
4. For breast-feeding women – 1300 mcg daily

Focus Points

  1. What is the storage form of Vitamin A is Retinyl palmitate. It’s overdose causes injury to lysosomes.
  2. Retinal is a Visual pigment
  3. Which is the carrier for oligosaccharides in glycoprotein synthesis? Retinoic acid

Additional readings

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