ImageFile = Retinol structure.svg
ImageSize = 350px
IUPACName =(2E,4E,6E,8E)-3,7-dimethyl- 9-(2,6,6-trimethylcyclohex-1-enyl)nona-2,4,6,8-tetraen-1-ol
Section1 = Chembox Identifiers
CASNo = 11103-57-4
PubChem = 1071
SMILES = CC(=CC=CC(C)=CCO) C=CC1=C(C)CCCC1(C)C
Section2 = Chembox Properties
Formula = C20H30O
MolarMass = 286.456 g/mol
Section3 = Chembox Hazards
Retinol (Afaxin), the animal form of
vitamin A, is a fat-soluble vitamin important in vision and bonegrowth. It belongs to the family of chemical compounds known as retinoids. It is also a diterpenoid. Retinol is ingested in a precursor form; animal sources ( liverand eggs) contain retinyl esters, whereas plants ( carrots, spinach) contain pro-vitamin A carotenoids. Hydrolysis of retinyl esters results in retinol while pro-vitamin A carotenoids can be cleaved to produce retinal. Retinal, also known as retinaldehyde, can be reversibly reduced to produce retinol or it can be irreversibly oxidized to produce retinoic acid. The best described active retinoid metabolites are 11-"cis"-retinal and the all-trans and 9-cis-isomers of retinoic acid.
Elmer McCollum, a biochemistat the University of Wisconsin-Madison, and colleague Marguerite Davisidentified a fat-soluble nutrient in butterfatand cod liver oil. Their work confirmed that of Thomas Osborne and Lafayette Mendel, at Yale, which suggested a fat-soluble nutrient in butterfat, also in 1913.Semba RD. Vitamin A as "Anti-Infective" Therapy, 1920–1940. Journal of Nutrition. 1999;129:783-791. PMID 10203551. [http://www.nutrition.org/cgi/content/full/129/4/783 Full Text Available] ] Vitamin A was first synthesized in 1947 by two Dutch chemists, David Adriaan van Dorpand Jozef Ferdinand Arens.
Chemical structure and function
Many different geometric isomers of retinol, retinal and retinoic acid are possible as a result of either a "
trans" or " cis" configuration of four of the five double bonds found in the polyenechain. The "cis" isomers are less stable and can readily convert to the all-"trans" configuration (as seen in the structure of all-"trans"-retinol shown here). Nevertheless, some "cis" isomers are found naturally and carry out essential functions. For example, the 11-"cis"-retinal isomer is the chromophoreof rhodopsin, the vertebrate photoreceptormolecule. Rhodopsin is comprised of the 11-cis-retinal covalently linked via a Schiff baseto the opsinprotein (either rod opsin or blue, red or green cone opsins). The process of vision relies on the light-induced isomerisation of the chromophore from 11-"cis" to all-"trans" resulting in a change of the conformation and activation of the photoreceptor molecule. One of the earliest signs of vitamin A deficiency is night-blindness followed by decreased visual acuity. George Waldwon the 1967 Nobel Prize in Physiology or Medicinefor his work with retinapigments (also called visual pigments), which led to the understanding of the role of vitamin A in vision.
Many of the non-visual functions of vitamin A are mediated by retinoic acid, which regulates gene expression by activating intracellular
retinoic acid receptors. The non-visual functions of vitamin A are essential in the immunological function, reproduction and embryonic development of vertebrates as evidenced by the impaired growth, susceptibility to infection and birth defects observed in populations receiving suboptimal vitamin A in their diet.
Role in embryology
Retinoic acid via the retinoic acid receptor influences the process of cell differentiation, hence, the growth and development of embryos. During development there is a concentration gradient of retinoic acid along the anterior-posterior (head-tail) axis. Cells in the embryo respond differently to retinoic acid depending on the amount present. For example, in vertebrates the hindbrain transiently forms eight rhombomers and each rhombomere has a specific pattern of genes being expressed. If retinoic acid is not present the last four rhombomeres do not develop. Instead rhombomeres 1-4 grow to cover the same amount of space as all eight would normally occupy. Retinoic acid has its effects by turning on a differential pattern of Hox genes which encode different homeodomain transcription factors which in turn can turn on cell type specific genes. Deletion of the Hox-1 gene from rhombomere 4 makes the neurons growing in that region behave like neurons from rhombomere 2. The retina is also patterned by retinoic acid, with a concentration gradient that is high on the ventral side of the retina and low on the dorsal side.
tem cell biology
Retinoic acid is an influential factor used in
differentiationof stem cells to more committed fates, echoing retinoic acid's importance in natural embryonic developmental pathways. It is thought to initiate differentiation into a number of different cell lineages by unsequestering certain sequences in the genome.
It has numerous applications in a plethora of stem cell differentiation protocols; amongst these are the differentiation of human
embryonic stem cells to posterior foregut lineages and also to functional motor neurons.
Vitamin A is required in the production of
rhodopsin, the visual pigmentused in low light levels. This is why eating foods rich in vitamin A is said to allow an individual to see in the dark.
Vitamin A is essential for the correct functioning of
epithelial cells. In Vitamin A deficiency, mucus-secreting cellsare replaced by keratinproducing cells, leading to xerosis.
Glycoproteinsynthesis requires adequate Vitamin A status. In severe Vitamin A deficiency, lack of glycoproteins may lead to corneal ulcers or liquefaction.
Vitamin A is essential to maintain intact
epithelial tissues as a physical barrier to infection; it is also involved in maintaining a number of immune cell types from both the innate and acquired immune systems. These include the lymphocytes( B-cells, T-cells, and natural killer cells), as well as many myelocytes ( neutrophils, macrophages, and myeloid dendritic cells).
Formation of red blood cells (Haematopoiesis)
Vitamin A may be needed for normal
haematopoiesis; deficiency causes abnormalities in iron metabolism.
Vitamin A affects the production of human growth hormone.
All retinoid forms of vitamin A are used in cosmetic and medical applications applied to the skin. Retinoic acid, termed
Tretinoinin clinical usage, is used in the treatment of acne and keratosis pilarisin a topical cream. An isomer of tretinoin, isotretinoinis also used orally (under the trade names " Accutane" and "Roaccutane"), generally for severe or recalcitrant acne.
In cosmetics, vitamin A derivatives are used as anti-aging chemicals- vitamin A is absorbed through the skin and increases the rate of skin turnover, and gives an increase in
collagengiving a more youthful appearance [ [http://archderm.ama-assn.org/cgi/content/abstract/143/5/606 Archives of Dermatology] ]
Tretinoin, under the alternative name of all-trans retinoic acid (ATRA), is used as chemotherapy for
acute promyelocytic leukemia, a subtype of acute myelogenous leukemia. This is because cells of this subtype of leukemia are sensitive to agonists of the retinoic acid receptors (RARs).
Units of measurement
When referring to dietary allowances or
nutritional science, retinol is usually measured in international units (IU). IU refers to biological activity and therefore is unique to each individual compound, however 1 IU of retinol is equivalent to approximately 0.3 micrograms (300 nanograms).
This vitamin plays an essential role in vision, particularly night vision, normal bone and tooth development, reproduction, and the health of skin and mucous membranes (the mucus-secreting layer that lines body regions such as the respiratory tract). Vitamin A also acts in the body as an antioxidant, a protective chemical that may reduce the risk of certain cancers.
There are two sources of dietary vitamin A. Active forms, which are immediately available to the body are obtained from animal products. These are known as retinoids and include retinal and retinol. Precursors, also known as provitamins, which must be converted to active forms by the body, are obtained from fruits and vegetables containing yellow, orange and dark green pigments, known as carotenoids, the most well-known being beta-carotene. For this reason, amounts of vitamin A are measured in Retinal Equivalents (RE). One RE is equivalent to 0.001 mg of retinal, or 0.006 mg of beta-carotene, or 3.3 International Units of vitamin A.
In the intestine, vitamin A is protected from being chemically changed by vitamin E. Vitamin A is fat-soluble and can be stored in the body. Most of the vitamin A you eat is stored in the liver. When required by a particular part of the body, the liver releases some vitamin A, which is carried by the blood and delivered to the target cells and tissues.
Dietary Reference Intake(DRI) Recommended Daily Amount (RDA) for Vitamin A for a 25-year old male is 900 micrograms/day, or 3000 IU.
Food Standards Agencystates that an average adult should not consume more than 1500 micrograms (5000 IU) per day, because this increases the chance of osteoporosis.
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. Carotenoid bioavailabilityranges between 1/5 to 1/10 of retinol's. Carotenoids are better absorbed when ingested as part of a fatty meal. Also, the carotenoids in vegetables, especially those with tough cell walls (e.g. carrots), are better absorbed when these cell walls are broken up by cooking or mincing.
Vitamin A deficiencyfor details"Vitamin A deficiency is common in developing countries but rarely seen in developed countries. Approximately 250,000 to 500,000 malnourished children in the developing world go blind each year from a deficiency of vitamin A. Night blindnessis one of the first signs of vitamin A deficiency. Vitamin A deficiency contributes to blindness by making the corneavery dry and damaging the retinaand cornea.
Retinoid overdose (toxicity)
Hypervitaminosis Afor details"The Tolerable Upper Intake Level (UL) for vitamin A, for a 25-year old male, is 3,000 micrograms/day, or about 10,000 IU.
Too much vitamin A in retinoid form can be harmful or fatal, resulting in what is known as
hypervitaminosis A. 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. Thus, vitamin A toxicity is typically reported in Arctic explorers and people taking large doses of synthetic vitamin A. 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. [ [http://www.studentbmj.com/back_issues/0502/life/158.html An account of Mertz's illness] , retrieved 20 June 2007.]
If eaten in one meal, 30 to 90 grams of
polar bearliver is enough to kill a human being, or to make even sled dogs very ill. [ [http://prof_anil_aggrawal.tripod.com/poiso032.html A. Aggrawal, Death by Vitamin A] ]
Excess vitamin A has also been suspected to be a contributor to
osteoporosis. This seems to happen at much lower doses than those required to induce acute intoxication. Only preformed vitamin A can cause these problems, because the conversion of carotenoids into vitamin A is downregulated when physiological requirements are met. An excessive uptake of carotenoids can, however, cause carotenosis.
beta carotenewas interestingly associated with an increase in lung cancerwhen it was studied in a lung cancer prevention trial in male smokers. In non-smokers, the opposite effect has been noted.
Excess preformed vitamin A during early pregnancy has also been associated with a significant increase in birth defects. [Challem, Jack, [http://www.thenutritionreporter.com/A-vitamins.html Caution Urged With Vitamin A in Pregnancy: But Beta-Carotene is Safe] , "The Nutrition Reporter Newsletter", 1995, retrieved 20 June 2007.] These defects may be severe, even life-threatening. Even twice the daily recommended amount can cause severe birth defects. [Stone, Brad. [http://www.fda.gov/bbs/topics/ANSWERS/ANS00689.html Vitamin A and Birth Defects] , 6 October 1995, retrieved 20 June 2007.] The FDA currently recommends that pregnant women get their Vitamin A from foods containing beta carotene and that they should ensure that they consume no more than 5,000 IU of preformed Vitamin A (if any) per day. Although Vitamin A is necessary for fetal development, most women carry stores of Vitamin A in their fat cells, so oversupplementation should be strictly avoided.
A review of all randomized controlled trials in the scientific literature by the
Cochrane Collaborationpublished in JAMA in 2007 found that vitamin A significantly "increased" mortality by 16% (Relative Risk 1.16, 95% confidence interval 1.10-1.24). [Bjelakovic et al., [http://jama.ama-assn.org/cgi/content/abstract/297/8/842 Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: Systematic review and meta-analysis] , "JAMA", 297:842; February 28, 2007.]
All sources of vitamin A can provide retinol, but retinoids are found naturally in some foods of animal origin. Each of the following contains at least 0.15 mg of retinoids per 1.75-7 oz. (50-200 g):
Cod Liver Oil
Liver(beef, pork, chicken, turkey, fish)
Pumpkin seed oil
Synthetic retinol is marketed under the following trade names: Acon, Afaxin, Agiolan, Alphalin, Anatola, Aoral, Apexol, Apostavit, Atav, Avibon, Avita, Avitol, Axerol, Dohyfral A, Epiteliol, Nio-A-Let, Prepalin, Testavol, Vaflol, Vi-Alpha, Vitpex, Vogan, and Vogan-Neu.
Night blindness—the inability to see well in dim light—is associated with a deficiency of
vitamin A. At first, the most light sensitive (containing more retinal) protein rhodopsinis influenced. Less pigmented by retinal iodopsins (three forms/colors in humans), responsible for color visionand sensing relatively high light intensities (day vision), are less impaired at early stages of the vitamin Adeficiency. All these protein-pigment complexes are located in the light-sensing cells in eye's retina.
When stimulated by light, rhodopsin splits into a protein and a cofactor: opsin and all-"trans"-retinal (a form of vitamin A). The regeneration of active rhodopsin requires opsin and 11-"cis"-retinal. The regeneration of 11-"cis"-retinal occurs in vertebrates via a sequence of chemical transformations that constitute "the visual cycle" and which occurs primarily in the retinal pigmented epithelial cells.
Without adequate amounts of retinal, regeneration of rhodopsin is incomplete and night blindness occurs.
Closely related chemicals
Isotretinoin(Tradename: Accutane(US), Roaccutane)
*Retinyl palmitate ("vitamin A" or "pro-vitamin A")
Genetically engineered vitamin A enriched rice
Due to the high prevalence of vitamin A deficiency in developing countries, there are efforts to produce
genetically modifiedrice rich in beta carotene. The idea is that this would help poor people, who can not afford a varied diet containing sufficient natural sources of vitamin A, meet their dietary needs. The golden riceproject is one such effort, and is already undergoing trials.
* [http://ods.od.nih.gov/factsheets/cc/vita.html NIH Office of Dietary Supplements - Vitamin A]
* [http://www.biochemj.org/bj/348/0481/bj3480481.htm Retinol binding protein]
* [http://www.merck.com/mrkshared/mmanual/section1/chapter3/3b.jsp Merck Manual of Diagnosis and Therapy: Vitamin A Deficiency]
* [http://www.who.int/nutrition/publications/vitamin_a_pub/en/index.html WHO publications on Vitamin A Deficiency]
* [http://news.bbc.co.uk/2/hi/health/6677631.stm BBC News article on Retinol's use as an anti-wrinkle cream]
Wikimedia Foundation. 2010.