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),