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Anthocyanins

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Anthocyanins

Not to be confused with anthocyanidins, the sugar-free counterparts of anthocyanins.


Anthocyanins (also anthocyans; from Greek: ἀνθός (anthos) = flower + κυανός (kyanos) = blue) are water-soluble vacuolar pigments that may appear red, purple, or blue depending on the pH. They belong to a parent class of molecules called flavonoids synthesized via the phenylpropanoid pathway; they are odorless and nearly flavorless, contributing to taste as a moderately astringent sensation. Anthocyanins occur in all tissues of higher plants, including leaves, stems, roots, flowers, and fruits. Anthoxanthins are clear, white to yellow counterparts of anthocyanins occurring in plants. Anthocyanins are derived from anthocyanidins by adding pendant sugars.[1]

Function

In flowers, bright-reds and -purples are adaptive for attracting pollinators. In fruits, the colorful skins also attract the attention of animals, which may eat the fruits and disperse the seeds. In photosynthetic tissues (such as leaves and sometimes stems), anthocyanins have been shown to act as a "sunscreen", protecting cells from high-light damage by absorbing blue-green and ultraviolet light, thereby protecting the tissues from photoinhibition, or high-light stress. This has been shown to occur in red juvenile leaves, autumn leaves, and broad-leaf evergreen leaves that turn red during the winter. The red coloration of leaves has been proposed to possibly camouflage leaves from herbivores blind to red wavelengths, or signal unpalatability, since anthocyanin synthesis often coincides with synthesis of unpalatable phenolic compounds.[2]

In addition to their role as light-attenuators, anthocyanins also act as powerful antioxidants. However, it is not clear whether anthocyanins can significantly contribute to scavenging of free radicals produced through metabolic processes in leaves, since they are located in the vacuole and, thus, spatially separated from metabolic reactive oxygen species. Some studies have shown hydrogen peroxide produced in other organelles can be neutralized by vacuolar anthocyanin.

Light absorbance

The absorbance pattern responsible for the red color of anthocyanins may be complementary to that of green chlorophyll in photosynthetically active tissues such as young Quercus coccifera leaves. It may protect the leaves from attacks by plant eaters that may be attracted by green color.[3]

pH indicator

Anthocyanins can be used as pH indicators because their color changes with pH; they are pink in acidic solutions (pH < 7), purple in neutral solutions (pH ~ 7), greenish-yellow in alkaline solutions (pH > 7), and colourless in very alkaline solutions, where the pigment is completely reduced.[4]

Occurrence

Anthocyanins are found in the cell vacuole, mostly in flowers and fruits but also in leaves, stems, and roots. In these parts, they are found predominantly in outer cell layers such as the epidermis and peripheral mesophyll cells.

Most frequently occurring in nature are the glycosides of cyanidin, delphinidin, malvidin, pelargonidin, peonidin, and petunidin. Roughly 2% of all hydrocarbons fixed in photosynthesis are converted into flavonoids and their derivatives such as the anthocyanins. No fewer than 109 tons of anthocyanins are produced in nature per year. Not all land plants contain anthocyanin; in the Caryophyllales (including cactus, beets, and amaranth), they are replaced by betalains. Anthocyanins and betalains have never been found in the same plant.[5][6]

Plants with abnormally high anthocyanin quantities are popular as ornamental plants.

In food

Food source Anthocyanin content
in mg per 100 g
Açaí 320
Blackcurrant 190-270
Chokeberry 1,480[7]
Eggplant 750
Blood orange ~200
Marion blackberry 317[8]
Black raspberry 589[9]
Raspberry 365
Wild blueberry 558[10]
Cherry 350-400
Redcurrant 80-420
Purple corn 1,642[11]

Plants rich in anthocyanins are Vaccinium species, such as blueberry, cranberry, and bilberry; Rubus berries, including black raspberry, red raspberry, and blackberry; blackcurrant, cherry, eggplant peel, black rice, Concord grape, muscadine grape, red cabbage, and violet petals. Anthocyanins are less abundant in banana, asparagus, pea, fennel, pear, and potato, and may be totally absent in certain cultivars of green gooseberries.[7] Red-fleshed peaches are rich in anthocyanins.[12]

The highest recorded amount appears to be specifically in the seed coat of black soybean (Glycine max L. Merr.) containing some 2,000 mg per 100 g[13] and in skins and pulp of black chokeberry (Aronia melanocarpa L.) (table). However, the Amazonian palmberry, açaí, having about 320 mg per 100 g[14] of which cyanidin-3-glucoside is the most prevalent individual anthocyanin (approximately 10 mg per 100 g),[15] is also a high-content source for which only a small fraction of total anthocyanins has been determined to date. Due to critical differences in sample origin, preparation and extraction methods determining anthocyanin content,[16][17] the values presented in the adjoining table are not directly comparable.

Nature, traditional agriculture, and plant breeding have produced various uncommon crops containing anthocyanins, including blue- or red-flesh potatoes and purple or red broccoli, cabbage, cauliflower, carrots, and corn. Tomatoes have been bred conventionally for high anthocyanin content by crossing wild relatives with the common tomato to transfer a gene called the anthocyanin fruit tomato (aft) gene into a larger and more palatable fruit.[18]

Tomatoes have also been genetically modified with transcription factors from snapdragons to produce high levels of anthocyanins in the fruits.[19][20][21] Anthocyanins can also be found in naturally ripened olives,[22][23] and are partly responsible for the red and purple colors of some olives.[22]

Autumn leaf color


The reds, the purples, and their blended combinations that decorate autumn foliage come from anthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.[24] They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences, both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced.[25]

Anthocyanins are present in about 10% of tree species in temperate regions, although in certain areas such as New England, up to 70% of tree species may produce the pigment.[24]

Many science textbooks incompletely state that autumn coloration (including red) is the result of breakdown of green chlorophyll, which unmasks the already-present orange, yellow, and red pigments (carotenoids, xanthophylls, and anthocyanins, respectively). While this is indeed the case for the carotenoids and xanthophylls (orange and yellow pigments), anthocyanins are not synthesized until the plant has begun breaking down the chlorophyll.[24]

Structure

Anthocyanidins: Flavylium cation derivatives

See Anthocyanidins article.

Selected anthocyanidins and their substitutions
Basic structure Anthocyanidin R3 R4 R5 R3 R5 R6 R7
Aurantinidin −H −OH −H −OH −OH −OH −OH
Cyanidin −OH −OH −H −OH −OH −H −OH
Delphinidin −OH −OH −OH −OH −OH −H −OH
Europinidin −OCH3 −OH −OH −OH −OCH3 −H −OH
Pelargonidin −H −OH −H −OH −OH −H −OH
Malvidin −OCH3 −OH −OCH3 −OH −OH −H −OH
Peonidin −OCH3 −OH −H −OH −OH −H −OH
Petunidin −OH −OH −OCH3 −OH −OH −H −OH
Rosinidin −OCH3 −OH −H −OH −OH −H −OCH3

Anthocyanins: Glycosides of anthocyanidins

The anthocyanins, anthocyanidins with sugar group(s), are mostly 3-glucosides of the anthocyanidins. The anthocyanins are subdivided into the sugar-free anthocyanidin aglycones and the anthocyanin glycosides. As of 2003, more than 400 anthocyanins had been reported[26] while more recent literature (early 2006), puts the number at more than 550 different anthocyanins. The difference in chemical structure that occurs in response to changes in pH is the reason why anthocyanins are often used as pH indicators, as they change from red in acids to blue in bases.

Anthocyanins: Stability

Anthocyanins are thought to be subject to physiochemical degradation in vivo and in vitro. Structure, pH, temperature, light, oxygen, metal ions, intramolecular association, and intermolecular association with other compounds (copigments, sugars, proteins, degradation products, etc.) are generally known to affect the color and stability of anthocyanins.[27] B-ring hydroxylation status and pH have been shown to mediate the degradation of anthocyanins to their phenolic acid and aldehyde constituents.[28] Indeed, significant portions of ingested anthocyanins are likely to degrade to phenolic acids and aldehyde in vivo, following consumption. This characteristic confounds scientific isolation of specific anthocyanin mechanisms in vivo.

Biosynthesis

  1. Anthocyanin pigments are assembled like all other flavonoids from two different streams of chemical raw materials in the cell:
  2. These streams meet and are coupled together by the enzyme chalcone synthase, which forms an intermediate chalcone-like compound via a polyketide folding mechanism that is commonly found in plants.
  3. The chalcone is subsequently isomerized by the enzyme chalcone isomerase to the prototype pigment naringenin.
  4. Naringenin is subsequently oxidized by enzymes such as flavanone hydroxylase, flavonoid 3' hydroxylase and flavonoid 3' 5'-hydroxylase.
  5. These oxidation products are further reduced by the enzyme dihydroflavonol 4-reductase to the corresponding colorless[29] leucoanthocyanidins.
  6. Leucoanthocyanidins were once believed to be the immediate precursors of the next enzyme, a dioxygenase referred to as anthocyanidin synthase or leucoanthocyanidin dioxygenase. Flavan-3-ols, the products of leucoanthocyanidin reductase (LAR), have been recently shown to be their true substrates.
  7. The resulting unstable anthocyanidins are further coupled to sugar molecules by enzymes such as UDP-3-O-glucosyltransferase[30] to yield the final relatively stable anthocyanins.

More than five enzymes are thus required to synthesize these pigments, each working in concert. Even a minor disruption in any of the mechanism of these enzymes by either genetic or environmental factors would halt anthocyanin production. While the biological burden of producing anthocyanins is relatively high, plants benefit significantly from environmental adaptation, disease tolerance, and pest tolerance provided by anthocyanins.

In anthocyanin biosynthetic pathway, L-phenylalanine is converted to naringenin by phenylalanine ammonialyase (PAL), cinnamate 4-hydroxylase (C4H), 4-coumarate CoA ligase (4CL), chalcone synthase (CHS) and chalcone isomerase (CHI). And then, the next pathway is catalyzed the formation of complex aglycone and anthocyanin composition by flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), UDP-glucoside: flavonoid glucosyltransferase (UFGT) and methyl transferase (MT). Among those, UFGT is divided into UF3GT and UF5GT, which are responsible for the glucosylation of anthocyanin to produce stable molecules.[31]

In Arabidopsis thaliana, two glycosyltransferases, UGT79B1 and UGT84A2, are involved in the anthocyanin biosynthetic pathway. The UGT79B1 protein converts cyanidin 3-O-glucoside to cyanidin 3-O-xylosyl(1→2)glucoside. UGT84A2 encodes sinapic acid: UDP-glucosyltransferase.[32]

Genetic analysis

The phenolic metabolic pathways and enzymes can be studied by mean of transgenesis of genes. The Arabidopsis regulatory gene in the production of anthocyanin pigment 1 (AtPAP1) can be expressed in other plant species.[33]

Potential food value

Anthocyanins are considered secondary metabolites as a food additive with E number E163 (INS number 163); they are approved for use as a food additive in the EU,[34] Australia and New Zealand.[35]

Although anthocyanins are powerful antioxidants in vitro,[36] this antioxidant property is unlikely to be conserved after the plant is consumed. As interpreted by the Linus Pauling Institute and European Food Safety Authority, dietary anthocyanins and other flavonoids have little or no direct antioxidant food value following digestion.[37][38][39] Unlike controlled test-tube conditions, the fate of anthocyanins in vivo shows they are poorly conserved (less than 5%), with most of what is absorbed existing as chemically modified metabolites that are rapidly excreted.[40]

The increase in antioxidant capacity of blood seen after the consumption of anthocyanin-rich foods may not be caused directly by the anthocyanins, but instead may result from increased uric acid levels derived from metabolism of flavonoids.[40]

Dye-sensitized solar cells

Anthocyanins have been used in organic solar cells because of their ability to convert light energy into electrical energy.[41] The many benefits to using dye-sensitized solar cells instead of traditional pn junction silicon cells include lower purity requirements and abundance of component materials, such as titania, as well as the fact they can be produced on flexible substrates, making them amenable to roll-to-roll printing processes.[42]

Research on health benefits

General research

Richly concentrated as pigments in berries, anthocyanins were the topics of research presented at a 2007 symposium on health benefits that may result from berry consumption.[43]

According to a 2009 study: "A growing body of evidence suggests anthocyanins and anthocyanidins may possess analgesic properties in addition to neuroprotective and anti-inflammatory activities".[44]

In vitro, anthocyanins possess MAO inhibitory activity for both MAO-A and MAO-B; MAO function is connected to neurodegenerative diseases, depression, and anxiety. The relevance to humans of anthocyanins and MAO activity requires further research, however.[45]

Anthocyanins also fluoresce; combined with their antioxidant properties, this can be a powerful tool for plant cell research, allowing live cell imaging for extended periods of time without a requirement for other fluorophores.[46]

Cancer research

According to the American Cancer Society, researchers are investigating the positive effects phytochemicals such as anthocyanin may have in the human diet, by researching the possible effect of specific compounds. However, there have been no strong studies in humans showing that any phytochemical supplement can prevent or treat cancer.[47]

Use as visual markers to mark genetically modified materials

Anthocyanin production can be engineered into genetically modified materials to enable their visual identification.[48]

See also

  • Phenolic compounds in wine
  • p-Coumaroylated anthocyanin

References

Further reading

  • Andersen, O.M. Flavonoids: Chemistry, Biochemistry and Applications. CRC Press, Boca Raton FL 2006. ISBN 978-0-8493-2021-7.
  • Gould, K. / Davies, K. / Winefield, C. (eds.) Anthocyanins: Biosynthesis, Functions, and Applications. Springer, 2008. ISBN 978-0-387-77334-6

External links

  • Catalyst.
  • Discussion of carnivorous plants concerning the evolution, biosynthesis and function of anthocyanins
  • Also here is a modified draft.
  • The Washington Post.

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