The biological role of vitamin A and its properties are discussed here.
Vitamin A was recognized by Elmer McCollum in 1915 as an essential nutritional factor. In 1917 Holmes isolated this vitamin from fish liver oil. Vit A is considered to be the antixerophthalmic factor or bright eyes vitamin. In 1946, Milas synthesized this vitamin in the lab.
Characteristics of Vitamin A
- The richest source of vitamin A is fish liver oils, especially from sharks and halibut.
- But it is least found in cod liver oils.
- The polar bear’s liver has a high concentration of this vitamin.
- Dairy products are excellent natural sources of vitamin A.
- Certain fruits and vegetables, especially the yellow varieties contain vitamin A in its provitamin form or as ascarotenes.
Structure of Vitamin A
- Vit A occurs in A1 and A2 forms.
- They are produced from precursor carotenoids called provitamins such as carotenes and cryptoxanthin. Carotenes are the most potent of the two.
- Carotene is an orange-red hydrocarbon formed of eight 5-carbon isoprenoid units. They form long chains of up to 40 carbon with an ionone ring at the ends.
- Each carotene produces two molecules of vitamin A1 which is commonly seen in salt-water fishes.
- Vit A2 is seen in freshwater fishes.
- It has a distinctive conjugate double bond between C3 -C4 of the ionone ring.
Properties of Vitamin A
- Retinol of Vit A is a colorless oil but the isolated needles are yellowish.
- This fat-soluble vitamin provides the absorption band at 328 nm.
- This vitamin is not much lost in cooking but oxidation under UV light destroys it.
- Unless protected by vitamin E, retinol is unstable in air.
Metabolism of Vitamin A
The metabolic pathway of Vitamin A is as follows.
- Dietary carotene is split into two molecules of retinal with the help of intestinal enzymes.
- Retinal is enzymatically reduced to retinol.
- Retinol is circulated in the blood through Retinol Binding Protein (RBP)
- Retinol is esterified by palmitic acid in the liver to form retinyl ester and is stored there.
- When required, these retinyl esters are enzymatically hydrolyzed into retinol.
- To excretion, the liver can convert retinol into retinal and into retinoic acid which will be sent to the intestine through bile.
Biological Role of Vitamin A
- Vitamin A helps in the maintenance of epithelial cells of the skin, digestive tract, respiratory system, and urinogenital system.
- Vit A protects the cell walls from oxidation by carcinogens and free radicals.
- Its deficiency could cause diseases such as
- Night blindness or Nyctalopia
- Xerophthalmia (the delicate membrane of the eye becomes scaly),
- Keratomalacia or softening of the cornea
- Phrynoderma (hard toad-like skin)
- Stunted growth in children
