Retinol
Encyclopedia
Retinol is one of the animal forms of vitamin A
. It is a diterpenoid
and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester
derivative of the alcohol serves as the storage form of the vitamin in animals.
When converted to the retinal (retinaldehyde) form, vitamin A is essential for vision
, and when converted to retinoic acid
, is essential for skin health and bone
growth. These chemical compounds are collectively known as retinoid
s, and possess the structural motif of all-trans retinol as a common feature in their structure. Structurally, all retinoids also possess a β-ionone ring and a polyunsaturated
side chain, with either an alcohol
, aldehyde
, a carboxylic acid
group or an ester group. The side chain is composed of four isoprenoid units, with a series of conjugated double bonds which may exist in trans- or cis-configuration.
Retinol is produced in the body from the hydrolysis of retinyl esters, and from the reduction of retinal. Retinol in turn is ingested in a precursor form; animal sources (liver
and egg
s) contain retinyl ester
s, whereas plant
s (carrot
s, spinach
) contain pro-vitamin A carotenoid
s (these may also be considered simply vitamin A). 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, which then cannot function as the vitamin in the eye.
Commercial production of retinol typically requires retinal synthesis through reduction of a pentadiene derivative and subsequent acidification/hydrolysis of the resulting isomer to produce retinol. Pure retinol is extremely sensitive to oxidization and is prepared and transported at low temperatures and oxygen free atmospheres. When prepared as a dietary supplement, retinol is stabilized as the ester
derivatives retinyl acetate or retinyl palmitate
.
, a biochemist
at the University of Wisconsin–Madison
, and colleague Marguerite Davis
identified a fat-soluble nutrient in butterfat
and 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. Vitamin A was first synthesized in 1947 by two Dutch chemists, David Adriaan van Dorp
and Jozef Ferdinand Arens.
Although the vitamin A was not identified until the 20th century, written observations of conditions created by deficiency of this nutrient appeared much earlier in history. Sommer (2008) classified historical accounts related to vitamin A and/or manifestations of deficiency as follows: "Ancient" accounts; 18th- to 19th-century clinical descriptions (and their purported etiologic associations); early 20th-century laboratory animal experiments, and clinical and epidiomologic observations that identified the existence of this unique nutrient and manifestations of its deficiency.
s found in the polyene
chain. 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 chromophore
of rhodopsin
, the vertebrate
photoreceptor
molecule. Rhodopsin is composed of the 11-cis-retinal covalently linked via a Schiff base
to the opsin
protein (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 Wald
won the 1967 Nobel Prize in Physiology or Medicine
for his work with retina
pigments (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 nuclear retinoic acid receptor
s. 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.
of 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 the experimental induction of stem cell differentiation; amongst these are the differentiation of human embryonic stem cell
s to posterior foregut lineages and also to functional motor neuron
s.
, the visual pigment used in low light levels. This is why eating foods rich in vitamin A is often said to allow an individual to see in the dark.
producing cells, leading to xerosis
.
synthesis requires adequate vitamin A status. In severe vitamin A deficiency, lack of glycoproteins may lead to corneal ulcers or liquefaction.
; deficiency causes abnormalities in iron metabolism.
in clinical usage, is used in the treatment of acne
and keratosis pilaris
in a topical cream. An isomer of tretinoin, isotretinoin
is also used orally (under the trade names Accutane and Roaccutane), generally for severe or recalcitrant acne.
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).
al science, retinol is usually measured in international unit
s (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 retinaldehyde 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 consumed 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.
(DRI) Recommended Daily Amount (RDA) for vitamin A for a 25-year-old male is 900 micrograms/day, or 3000 IU.
Estimates have changed over time of the rate at which β-carotene is converted to vitamin A in the human body. An early estimate of 6:1 was revised to 12:1 and from recent studies and experimental trials carried out in developing nations it was revised again to 21:1. The implication of the reduced estimate is that larger quantities of β-carotene are needed to yield the necessary dietary requirement of vitamin A. This means that more continents are affected by the deficiency of vitamin A than was previously thought. Changing dietary choices in Africa, Asia, and South America will not be sufficient, and agricultural practices on those continents will need to change.
The Food Standards Agency
states 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 intestine
s, retinol is incorporated into chylomicron
s as the ester form, and it is these particles that mediate transport to the liver
. Liver cells (hepatocyte
s) 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 intracellular
ly. Carotenoid bioavailability
ranges 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 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 blindness is one of the first signs of vitamin A deficiency. Vitamin A deficiency contributes to blindness by making the cornea
very dry and damaging the retina
and cornea.
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.
Vitamin A toxicity occurs when an individual ingests vitamin A in large amounts more than the daily recommended value in the threshold of 25,000 IU/Kg or more. Often, the individual consumes about 3-4 times the RDA's specification Toxicity of vitamin A is believed to be associated with the intervention methods used to upgrade vitamin A levels in the body such as food modification, fortification and supplementation, all of which are employed to combat vitamin A deficiency Toxicity is classified into two categories: acute and chronic toxicities. The former occurs few hours or days after ingestion of large amounts of vitamin A accidentally or via inappropriate therapy. The later toxicity (Chronic) takes place when about 25,000 IU/Kg or more of vitamin A is consumed for a prolonged period of time. Symptoms associated with both toxicities include, but not limited to nausea, blurred vision, fatigue, weight-loss, menstrual abnormalities etc.
If eaten in one meal, 30 to 90 grams of polar bear
liver is enough to kill a human being, or to make even sled dogs very ill.
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
.
The carotenoid synthetic form of beta-carotene
was interestingly associated with an increase in lung cancer
when 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. These defects may be severe, even life-threatening. Even twice the daily recommended amount can cause severe birth defects. 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 Collaboration
published in JAMA
in 2007 found that supplementation with beta-carotene or vitamin increased mortality by 5% and 16%, respectively.
Contrary to earlier observations, recent studies emerging from some developing countries (India, Bangladesh and Indonesia) have strongly suggested that dosing expectant mothers in the population in which vitamin A deficiency is common and maternal mortality is high can greatly reduce maternal mortality ratio Similarly, dosing newborn infants with 50,000 IU (15 mg) of vitamin A within 2 days of birth, can significantly reduce neonatal mortality
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.
There are three known routes to retinol that are used industrially, all of which start with beta-ionone.
. At first, the most light sensitive (containing more retinal) protein rhodopsin
is influenced. Less pigmented by retinal iodopsins (three forms/colors in humans), responsible for color vision
and sensing relatively high light intensities (day vision), are less impaired at early stages of the vitamin A
deficiency. 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.
, there are efforts to produce genetically modified rice 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 rice
project is one such effort, and is already undergoing trials.
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...
. It is a diterpenoid
Diterpene
Diterpenes are a type of terpenes composed of four isoprene units. They derive from geranylgeranyl pyrophosphate. Diterpenes form the basis for biologically important compounds such as retinol, retinal, and phytol...
and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
derivative of the alcohol serves as the storage form of the vitamin in animals.
When converted to the retinal (retinaldehyde) form, vitamin A is essential for vision
Visual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
, and when converted to retinoic acid
Retinoic acid
Retinoic acid is a metabolite of vitamin A that mediates the functions of vitamin A required for growth and development. Retinoic acid is required in chordate animals which includes all higher animals from fishes to humans...
, is essential for skin health and bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
growth. These chemical compounds are collectively known as retinoid
Retinoid
The retinoids are a class of chemical compounds that are related chemically to vitamin A. Retinoids are used in medicine, primarily due to the way they regulate epithelial cell growth....
s, and possess the structural motif of all-trans retinol as a common feature in their structure. Structurally, all retinoids also possess a β-ionone ring and a polyunsaturated
Saturation (chemistry)
In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
side chain, with either an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
, aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
, a carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
group or an ester group. The side chain is composed of four isoprenoid units, with a series of conjugated double bonds which may exist in trans- or cis-configuration.
Retinol is produced in the body from the hydrolysis of retinyl esters, and from the reduction of retinal. Retinol in turn is ingested in a precursor form; animal sources (liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and egg
Egg (food)
Eggs are laid by females of many different species, including birds, reptiles, amphibians, and fish, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell, albumen , and vitellus , contained within various thin membranes...
s) contain retinyl ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s, whereas plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s (carrot
Carrot
The carrot is a root vegetable, usually orange in colour, though purple, red, white, and yellow varieties exist. It has a crisp texture when fresh...
s, spinach
Spinach
Spinach is an edible flowering plant in the family of Amaranthaceae. It is native to central and southwestern Asia. It is an annual plant , which grows to a height of up to 30 cm. Spinach may survive over winter in temperate regions...
) contain pro-vitamin A carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...
s (these may also be considered simply vitamin A). Hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of retinyl esters results in retinol, while pro-vitamin A carotenoids can be cleaved to produce retinal
Retinal
Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision...
. Retinal, also known as retinaldehyde, can be reversibly reduced to produce retinol or it can be irreversibly oxidized to produce retinoic acid, which then cannot function as the vitamin in the eye.
Commercial production of retinol typically requires retinal synthesis through reduction of a pentadiene derivative and subsequent acidification/hydrolysis of the resulting isomer to produce retinol. Pure retinol is extremely sensitive to oxidization and is prepared and transported at low temperatures and oxygen free atmospheres. When prepared as a dietary supplement, retinol is stabilized as the ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
derivatives retinyl acetate or retinyl palmitate
Retinyl palmitate
Retinyl palmitate, or vitamin A palmitate, is the ester of retinol and palmitic acid, with formula C36H60O2.Palmitate is the major component of palm oil...
.
Discovery
In 1913, Elmer McCollumElmer McCollum
Elmer Verner McCollum was an American biochemist known for his work on the influence of diet on health.-Life and education:McCollum was born on a farm near Fort Scott, Kansas, where he spent his first seventeen years...
, a biochemist
Biochemist
Biochemists are scientists who are trained in biochemistry. Typical biochemists study chemical processes and chemical transformations in living organisms. The prefix of "bio" in "biochemist" can be understood as a fusion of "biological chemist."-Role:...
at the University of Wisconsin–Madison
University of Wisconsin–Madison
The University of Wisconsin–Madison is a public research university located in Madison, Wisconsin, United States. Founded in 1848, UW–Madison is the flagship campus of the University of Wisconsin System. It became a land-grant institution in 1866...
, and colleague Marguerite Davis
Marguerite Davis
Marguerite Davis was the co-discoverer of Vitamin A and Vitamin B with Elmer Verner McCollum in 1913..-External links:*...
identified a fat-soluble nutrient in butterfat
Butterfat
Butterfat or milkfat is the fatty portion of milk. Milk and cream are often sold according to the amount of butterfat they contain.- Composition :The fatty acids of butterfat are typically composed as follows :...
and cod liver oil
Cod liver oil
Cod liver oil is a nutritional supplement derived from liver of cod fish. It has high levels of the omega-3 fatty acids, EPA and DHA, and very high levels of vitamin A and vitamin D. It is widely taken to ease the symptoms of arthritis and for other health benefits...
. Their work confirmed that of Thomas Osborne and Lafayette Mendel
Lafayette Mendel
Lafayette Benedict Mendel was an American biochemist known for his work in nutrition including the study of Vitamin A, Vitamin B, lysine and tryptophan....
, at Yale
Yale University
Yale University is a private, Ivy League university located in New Haven, Connecticut, United States. Founded in 1701 in the Colony of Connecticut, the university is the third-oldest institution of higher education in the United States...
, which suggested a fat-soluble nutrient in butterfat, also in 1913. Vitamin A was first synthesized in 1947 by two Dutch chemists, David Adriaan van Dorp
David Adriaan van Dorp
David 'Davy' Adriaan van Dorp was a Dutch chemist.Van Dorp was born as the son of Hendrik van Dorp and Maria van Dorp, and studied chemistry in Amsterdam where he received a PhD for his thesis Aneurine en gistphosphatase in 1941.In 1946, while employed by the Dutch Organon company in Oss, Van...
and Jozef Ferdinand Arens.
Although the vitamin A was not identified until the 20th century, written observations of conditions created by deficiency of this nutrient appeared much earlier in history. Sommer (2008) classified historical accounts related to vitamin A and/or manifestations of deficiency as follows: "Ancient" accounts; 18th- to 19th-century clinical descriptions (and their purported etiologic associations); early 20th-century laboratory animal experiments, and clinical and epidiomologic observations that identified the existence of this unique nutrient and manifestations of its deficiency.
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 bondDouble bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
s found in the polyene
Polyene
Polyenes are poly-unsaturated organic compounds that contain one or more sequences of alternating double and single carbon-carbon bonds. These double carbon-carbon bonds interact in a process known as conjugation, which results in an overall lower energy state of the molecule.Organic compounds with...
chain. 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 chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...
of rhodopsin
Rhodopsin
Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...
, the vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
photoreceptor
Photoreceptor protein
Photoreceptors are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some bacteria...
molecule. Rhodopsin is composed of the 11-cis-retinal covalently linked via a Schiff base
Schiff base
A Schiff base, named after Hugo Schiff, is a compound with a functional group that contains a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group, not hydrogen....
to the opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
protein (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 Wald
George Wald
George Wald was an American scientist who is best known for his work with pigments in the retina. He won a share of the 1967 Nobel Prize in Physiology or Medicine with Haldan Keffer Hartline and Ragnar Granit.- Research :...
won the 1967 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...
for his work with retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
pigments (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 nuclear retinoic acid receptor
Retinoic acid receptor
The retinoic acid receptor is a type of nuclear receptor that is activated by both all-trans retinoic acid and 9-cis retinoic acid. There are three retinoic acid receptors , RAR-alpha, RAR-beta, and RAR-gamma, encoded by the , , genes, respectively...
s. 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. Retinoic acid is not required for patterning of the retina as originally proposed, but retinoic acid synthesized in the retina is secreted into surrounding mesenchyme where it is required to prevent overgrowth of perioptic mesenchyme which can cause microphthalmia, defects in the cornea and eyelid, and rotation of the optic cup.Stem cell biology
Retinoic acid is an influential factor used in differentiationCellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
of 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 the experimental induction of stem cell differentiation; amongst these are the differentiation of human embryonic stem cell
Embryonic stem cell
Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of the blastocyst, an early-stage embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consist of 50–150 cells...
s to posterior foregut lineages and also to functional motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
s.
Vision
Vitamin A is required in the production of rhodopsinRhodopsin
Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...
, the visual pigment used in low light levels. This is why eating foods rich in vitamin A is often said to allow an individual to see in the dark.
Epithelial cells
Vitamin A is essential for the correct functioning of epithelial cells. In vitamin A deficiency, mucus-secreting cells are replaced by keratinKeratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
producing cells, leading to xerosis
Xerosis
Xerosis cutis is the medical term for dry skin.It can have many different causes, including general dehydration, atopic dermatitis, Vitamin A deficiency, and maybe diabetes. Treatment is primarily symptomatic. "Xero", meaning dry or dehydrated, "osis" usually referring to a medical disease or...
.
Glycoprotein synthesis
GlycoproteinGlycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
synthesis requires adequate vitamin A status. In severe vitamin A deficiency, lack of glycoproteins may lead to corneal ulcers or liquefaction.
Immune system
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 haematopoiesisHaematopoiesis
Haematopoiesis is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells...
; deficiency causes abnormalities in iron metabolism.
Clinical use
All retinoid forms of vitamin A are used in cosmetic and medical applications applied to the skin. Retinoic acid, termed TretinoinTretinoin
Tretinoin is the acid form of vitamin A and is also known as all-trans retinoic acid or ATRA. It is a drug commonly used to treat acne vulgaris and keratosis pilaris. It is available as a cream or gel...
in clinical usage, is used in the treatment of acne
Acne vulgaris
Acne vulgaris is a common human skin disease, characterized by areas of skin with seborrhea , comedones , papules , pustules , Nodules and possibly scarring...
and keratosis pilaris
Keratosis pilaris
Keratosis pilaris is a common, autosomal dominant, genetic follicular condition that is manifested by the appearance of rough bumps on the skin...
in a topical cream. An isomer of tretinoin, isotretinoin
Isotretinoin
Isotretinoin, INN, is a medication used mostly for cystic acne. It was first developed for brain, pancreatic and other cancers. It is used to treat harlequin-type ichthyosis, a usually lethal skin disease, and lamellar ichthyosis. Its effects are systemic and nonselective...
is also used orally (under the trade names Accutane and Roaccutane), generally for severe or recalcitrant acne.
Tretinoin, under the alternative name of all-trans retinoic acid (ATRA), is used as chemotherapy for acute promyelocytic leukemia
Acute promyelocytic leukemia
Acute promyelocytic leukemia is a subtype of acute myelogenous leukemia , a cancer of the blood and bone marrow. It is also known as acute progranulocytic leukemia; APL; AML with t, PML-RARA and variants; FAB subtype M3 and M3 variant.In APL, there is an abnormal accumulation of immature...
, 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 nutritionNutrition
Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....
al science, retinol is usually measured in international unit
International unit
In pharmacology, the International Unit is a unit of measurement for the amount of a substance, based on biological activity or effect. It is abbreviated as IU, as UI , or as IE...
s (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).
Nutrition
| Vitamin properties | |
|---|---|
| Solubility Solubility Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on... |
Fat |
| RDA (adult male) | 900 µg Microgram In the metric system, a microgram is a unit of mass equal to one millionth of a gram , or 1/1000 of a milligram. It is one of the smallest units of mass commonly used... /day |
| RDA (adult female) | 700 µg Microgram In the metric system, a microgram is a unit of mass equal to one millionth of a gram , or 1/1000 of a milligram. It is one of the smallest units of mass commonly used... /day |
| RDA upper limit (adult male) | 3,000 µg Microgram In the metric system, a microgram is a unit of mass equal to one millionth of a gram , or 1/1000 of a milligram. It is one of the smallest units of mass commonly used... /day |
| RDA upper limit (adult female) | 3,000 µg Microgram In the metric system, a microgram is a unit of mass equal to one millionth of a gram , or 1/1000 of a milligram. It is one of the smallest units of mass commonly used... /day |
| Deficiency symptoms | |
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| Excess symptoms | |
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Osteoporosis Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered... (suspected, long-term) |
|
| Common sources | |
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Liver The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion... and other organs Fruit In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,... s |
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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 retinaldehyde 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 consumed 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 intake
The Dietary Reference IntakeDietary Reference Intake
The Dietary Reference Intake is a system of nutrition recommendations from the Institute of Medicine of the U.S. National Academy of Sciences. The DRI system is used by both the United States and Canada and is intended for the general public and health professionals...
(DRI) Recommended Daily Amount (RDA) for vitamin A for a 25-year-old male is 900 micrograms/day, or 3000 IU.
Estimates have changed over time of the rate at which β-carotene is converted to vitamin A in the human body. An early estimate of 6:1 was revised to 12:1 and from recent studies and experimental trials carried out in developing nations it was revised again to 21:1. The implication of the reduced estimate is that larger quantities of β-carotene are needed to yield the necessary dietary requirement of vitamin A. This means that more continents are affected by the deficiency of vitamin A than was previously thought. Changing dietary choices in Africa, Asia, and South America will not be sufficient, and agricultural practices on those continents will need to change.
The Food Standards Agency
Food Standards Agency
The Food Standards Agency is a non-ministerial government department of the Government of the United Kingdom. It is responsible for protecting public health in relation to food throughout the United Kingdom and is led by a board appointed to act in the public interest...
states that an average adult should not consume more than 1500 micrograms (5000 IU) per day, because this increases the chance of osteoporosis
Osteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
.
During the absorption process in the intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
s, retinol is incorporated into chylomicron
Chylomicron
Chylomicrons are lipoprotein particles that consist of triglycerides , phospholipids , cholesterol and proteins .They transport dietary lipids from the intestines to other locations in the body...
s as the ester form, and it is these particles that mediate transport to the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
. Liver cells (hepatocyte
Hepatocyte
A hepatocyte is a cell of the main tissue of the liver. Hepatocytes make up 70-80% of the liver's cytoplasmic mass.These cells are involved in:* Protein synthesis* Protein storage* Transformation of carbohydrates...
s) 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
Retinol binding protein
Retinol-binding proteins are a family of proteins with diverse functions. They are carrier proteins that bind retinol.Assessment of retinol-binding protein is used to determine visceral protein mass in nutritional studies related to health.-Genes:...
, for transport to target tissues. A binding protein inside cells, cellular retinoic acid binding protein, serves to store and move retinoic acid intracellular
Intracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...
ly. Carotenoid bioavailability
Bioavailability
In pharmacology, bioavailability is a subcategory of absorption and is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered...
ranges 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.
Deficiency
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
very dry and damaging the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
and cornea.
Interventions/remedies
Interventions or remedies in vitamin A deficiency in a deficient population may be enforced using three approaches:(A) through dietary modification involving the adjustment of menu choices of affected persons from available food sources to optimize vitamin A content, (B) enriching commonly eaten and affordable foods with vitamin A, a process called fortification. It involves addition of synthetic vitamin A to staple foods like margarine, bread, flours, cereals and other infant formulae during processing and (C) giving high-doses of vitamin A to the targeted deficient population, a method known as supplementation. Caution should however be exercised when using supplementation as a method of replenishing vitamin A in the body so that upper harmful limits are not attained.Retinoid overdose (toxicity)
- see Hypervitaminosis AHypervitaminosis AHypervitaminosis A refers to the effects of excessive vitamin A intake.-Presentation:Effects include* Birth defects* Liver problems* Reduced bone mineral density that may result in osteoporosis* Coarse bone growths...
for details
The Tolerable Upper Intake Level
Dietary Reference Intake
The Dietary Reference Intake is a system of nutrition recommendations from the Institute of Medicine of the U.S. National Academy of Sciences. The DRI system is used by both the United States and Canada and is intended for the general public and health professionals...
(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
Hypervitaminosis A
Hypervitaminosis A refers to the effects of excessive vitamin A intake.-Presentation:Effects include* Birth defects* Liver problems* Reduced bone mineral density that may result in osteoporosis* Coarse bone growths...
. The body converts the dimerized form, carotene
Carotene
The term carotene is used for several related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are all coloured to the human eye...
, 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
Xavier Mertz
Xavier Mertz was a Swiss explorer, mountaineer and skier, from Basel. He took part in the Far Eastern Party, a 1912–13 component of the Australasian Antarctic Expedition, which claimed his life...
, a Swiss
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
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.
Vitamin A toxicity occurs when an individual ingests vitamin A in large amounts more than the daily recommended value in the threshold of 25,000 IU/Kg or more. Often, the individual consumes about 3-4 times the RDA's specification Toxicity of vitamin A is believed to be associated with the intervention methods used to upgrade vitamin A levels in the body such as food modification, fortification and supplementation, all of which are employed to combat vitamin A deficiency Toxicity is classified into two categories: acute and chronic toxicities. The former occurs few hours or days after ingestion of large amounts of vitamin A accidentally or via inappropriate therapy. The later toxicity (Chronic) takes place when about 25,000 IU/Kg or more of vitamin A is consumed for a prolonged period of time. Symptoms associated with both toxicities include, but not limited to nausea, blurred vision, fatigue, weight-loss, menstrual abnormalities etc.
If eaten in one meal, 30 to 90 grams of polar bear
Polar Bear
The polar bear is a bear native largely within the Arctic Circle encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the world's largest land carnivore and also the largest bear, together with the omnivorous Kodiak Bear, which is approximately the same size...
liver is enough to kill a human being, or to make even sled dogs very ill.
Excess vitamin A has also been suspected to be a contributor to osteoporosis
Osteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
. 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
Carotenosis
Carotenaemia is the presence in blood of the yellow pigment carotene from excessive intake of carrots or other vegetables containing the pigment resulting in increased serum carotenoids. It can lead to subsequent yellow-orange discoloration and their subsequent deposition in the outermost layer...
.
The carotenoid synthetic form of beta-carotene
Beta-carotene
β-Carotene is a strongly-coloured red-orange pigment abundant in plants and fruits. It is an organic compound and chemically is classified as a hydrocarbon and specifically as a terpenoid , reflecting its derivation from isoprene units...
was interestingly associated with an increase in lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...
when 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. These defects may be severe, even life-threatening. Even twice the daily recommended amount can cause severe birth defects. 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 Collaboration
Cochrane Collaboration
The Cochrane Collaboration is a group of over 28,000 volunteers in more than 100 countries who review the effects of health care interventions tested in biomedical randomized controlled trials. A few more recent reviews have also studied the results of non-randomized, observational studies...
published in JAMA
Journal of the American Medical Association
The Journal of the American Medical Association is a weekly, peer-reviewed, medical journal, published by the American Medical Association. Beginning in July 2011, the editor in chief will be Howard C. Bauchner, vice chairman of pediatrics at Boston University’s School of Medicine, replacing ...
in 2007 found that supplementation with beta-carotene or vitamin increased mortality by 5% and 16%, respectively.
Contrary to earlier observations, recent studies emerging from some developing countries (India, Bangladesh and Indonesia) have strongly suggested that dosing expectant mothers in the population in which vitamin A deficiency is common and maternal mortality is high can greatly reduce maternal mortality ratio Similarly, dosing newborn infants with 50,000 IU (15 mg) of vitamin A within 2 days of birth, can significantly reduce neonatal mortality
Sources
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 (49.6–198.4 g):- Cod liver oilCod liver oilCod liver oil is a nutritional supplement derived from liver of cod fish. It has high levels of the omega-3 fatty acids, EPA and DHA, and very high levels of vitamin A and vitamin D. It is widely taken to ease the symptoms of arthritis and for other health benefits...
- ButterButterButter is a dairy product made by churning fresh or fermented cream or milk. It is generally used as a spread and a condiment, as well as in cooking applications, such as baking, sauce making, and pan frying...
, MargarineMargarineMargarine , as a generic term, can indicate any of a wide range of butter substitutes, typically composed of vegetable oils. In many parts of the world, the market share of margarine and spreads has overtaken that of butter... - LiverLiverThe liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
(beef, pork, chicken, turkey, fish) - EggsEgg (food)Eggs are laid by females of many different species, including birds, reptiles, amphibians, and fish, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell, albumen , and vitellus , contained within various thin membranes...
- CheeseCheeseCheese is a generic term for a diverse group of milk-based food products. Cheese is produced throughout the world in wide-ranging flavors, textures, and forms....
, MilkMilkMilk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
Synthetic sources
SyntheticChemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
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.
There are three known routes to retinol that are used industrially, all of which start with beta-ionone.
Night vision
Night blindness—the inability to see well in dim light—is associated with a deficiency of vitamin AVitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...
. At first, the most light sensitive (containing more retinal) protein rhodopsin
Rhodopsin
Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...
is influenced. Less pigmented by retinal iodopsins (three forms/colors in humans), responsible for color vision
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...
and sensing relatively high light intensities (day vision), are less impaired at early stages of the vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...
deficiency. All these protein-pigment complexes are located in the light-sensing cells in eye's retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
.
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
- TretinoinTretinoinTretinoin is the acid form of vitamin A and is also known as all-trans retinoic acid or ATRA. It is a drug commonly used to treat acne vulgaris and keratosis pilaris. It is available as a cream or gel...
(Tradename: Retin-A) - IsotretinoinIsotretinoinIsotretinoin, INN, is a medication used mostly for cystic acne. It was first developed for brain, pancreatic and other cancers. It is used to treat harlequin-type ichthyosis, a usually lethal skin disease, and lamellar ichthyosis. Its effects are systemic and nonselective...
(Tradename: Accutane(US), Roaccutane) - Retinyl palmitateRetinyl palmitateRetinyl palmitate, or vitamin A palmitate, is the ester of retinol and palmitic acid, with formula C36H60O2.Palmitate is the major component of palm oil...
("vitamin A palmitate")
Genetically engineered vitamin A enriched rice
Due to the high prevalence of vitamin A deficiency in developing countriesDeveloping country
A developing country, also known as a less-developed country, is a nation with a low level of material well-being. Since no single definition of the term developing country is recognized internationally, the levels of development may vary widely within so-called developing countries...
, there are efforts to produce genetically modified rice 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 rice
Golden rice
Golden rice is a variety of Oryza sativa rice produced through genetic engineering to biosynthesize beta-carotene, a precursor of pro-vitamin A in the edible parts of rice...
project is one such effort, and is already undergoing trials.