The vitamin exists in three major forms: retinal (the aldehyde isoform), retinol (the alcohol isoform), and retinoic acid (RA), which is the irreversibly oxidized form of retinol.
Retinol empirical formula: C20H30O with molecular weight of 286.45
Retinol is soluble in fats and oils and practically insoluble in water and glycerol. Vitamin A esters are readily soluble in fats, oils, ether, acetone and chloroform. They are soluble in alcohol but insoluble in water.
Dietary vitamin A is absorbed in the small intestine in the form of retinol and transported in blood attached to retinol-binding protein (RBP). Inside the cells, retinol is oxidized into its main biologically active derivatives, first retinaldehyde (retinal), which plays a role in vision, and then retinoic acid (RA), which regulates the expression of multiple target genes.
Among other functions of vitamin A:
* Vitamin A appears to facilitate the mobilization of iron from storage sites to the developing red blood cell for incorporation into hemoglobin, the oxygen carrier in red blood cells.
*Together with the protein opsin, the 11-cis isomer of retinol forms the light -sensitive visual pigment rhodopsin located in the rod of the retina. Rod cells with rhodopsin can detect very small amounts of light, making them important for night vision.
*Animal studies shown that vitamin A is required for normal growth and development. Retinol and retinoic acid (RA) are essential for embryonic development. During fetal development, RA functions in limb development and formation of the heart, eyes, and ears.
*Another major function of vitamin A is its role in cell differentiation.
*Vitamin A often called the antireflective vitamin, is protection against infections. The skin and mucosal cells (cells that line the airways, digestive tract, and urinary tract) function as a barrier and form the body's first line of defense against infection. Retinol and its metabolites are required to maintain the integrity and function of these cells.
Properties and functions of vitamin A
