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Retinol, the dietary form of vitamin A, is a fat-soluble, antioxidant vitamin important in vision and bone growth. It belongs to the family of chemical compounds known as retinoids. Retinol is ingested in a precursor form; animal sources (milk and eggs) contain retinyl esters, whereas plants (carrots, spinach) contain carotenoids. Tissue cells convert these precursors to retinol, and then to either retinal or retinoic acid . Many of the non-vision functions of vitamin A are mediated by retinoic acid, which acts at intracellular retinoic acid receptors . Retinoic acid is used medicinally as a topical treatment for acne and keratosis pilaris. The other main metabolite, retinal, is used in vision.
As can be seen from the structure, retinol is derived from isoprene, and has an alcohol functional group. The first full synthesis route for the compound was found by David Adriaan van Dorp and Jozef Ferdinand Arens in 1947.
During the absorption process in the intestines, retinol is incorporated into chylomicrons as the ester form, and it is these particles that mediate transport to the liver. Liver cells (hepatocytes) store vitamin A as the ester, and when retinol is needed in other tissues, it is de-esterifed and released into the blood as the alcohol. Retinol then attaches to a serum carrier, retinol binding protein, for transport to target tissues. A binding protein inside cells, cellular retinoic acid binding protein , serves to store and move retinoic acid intracellularly.
Either deficiency or overdose of vitamin A can be harmful or fatal. Deficiency of vitamin A can cause night-blindness, and pale, dry skin. The body converts the dimerized form, carotene, into vitamin A as it is needed, therefore high levels of carotene are not toxic compared to the ester (animal) forms. The livers of certain animals, especially those adapted to polar environments, often contain amounts of vitamin A that would be toxic to humans. The first documented death due to vitamin A poisoning was Xavier Mertz , a Swiss scientist who died in January 1913 on an Antarctic expedition that had lost its food supplies and fell to eating its sled dogs. Mertz consumed lethal amounts of vitamin A by eating the dogs' livers. The liver of the polar bear also has enough vitamin A to kill a human being. Excess vitamin A has also been suspected to be a contributor to osteoporosis.
George Wald won the 1967 Nobel Prize in Physiology or Medicine for his work with retina pigments, including the role of vitamin A.
Retinol can also be used in the treatment of acne in a topical cream. A form of retinoic acid, all-trans retinoic acid (ATRA) is currently used as chemotherapy for acute myeloid leukemia subtype M3. This is because this subtype in most cases involves the retinoic acid receptor (RAR).
Genetically engineered Vitamin A rich rice
Given the large prevalence of Vitamin A deficiency in rice eating societies efforts are on to produce rice rich in Vitamin A. Golden rice project is one such effort that is already undergoing trials.
Last updated: 05-07-2005 12:43:23
Last updated: 05-07-2005 18:09:53