Anthocyanin

Anthocyanins (from Greek: ἀνθός (anthos) = flower + κυανός (kyanos) = blue) are water-soluble vacuolar flavonoid pigments that appear red to blue, according to pH. They are synthesized by organisms of the plant kingdom, and have been observed to occur in all tissues of higher plants, providing color in leaves, stems, roots, flowers, and fruits.

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Function

    In flowers, bright reds and purples due to anthocyanin pigments help attract 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 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-leaved evergreen leaves that turn red during the winter. It has also been proposed that red coloration of leaves may camouflage leaves from herbivores blind to red wavelengths, or signal unpalatability, since anthocyanin synthesis often coincides with synthesis of unpalatable phenolic compounds.

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, since they are located in the vacuole, and thus, spatially separated from most free-radicals. However, this antioxidant property is conserved after consumption by another organism, which is one reason why fruits and vegetables with red skins and tissues are a nutritious food source.

Occurrence

Not all land plants contain anthocyanin; in the Caryophyllales (including cactus, beets, and amaranth) they are replaced by betalains.

Anatomically, 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. The amounts are relatively large: one kilogram of blackberry for example contains approximately 1.15 gram, and red and black legumes can contain 20 gram per 1 kg. Other plants rich in anthocyanins are blackcurrant, chokeberry, cherry, eggplant, blue grape, Vaccinium and red cabbage and also the Usambara-violet. Anthocyanins are less abundant in banana, asparagus, pea, fennel, pear and potato. Most frequent in nature are the glycosides of cyanidin, delphinidin, malvidin, pelargonidin, peonidin and petunidin. Roughly 2% of all hydrocarbons fixated in photosynthesis are converted into flavonoids and their derivatives such as the anthocyanins. This is no less than 10⁹ tons per year.

Autumn leaf color

  Many science text books incorrectly state that all autumn coloration (including red) is simply 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 present until the leaf begins breaking down the chlorophyll, during which time the plant begins to synthesize the anthocyanin, presumably for photoprotection during nitrogen translocation.

Structure

Anthocyanidins: Flavylium cation derivatives

The pigment components of anthocyanidins, the sugar-free anthocyanins, can be identified based on the structure of a large group of polymethine dye, the benzopyrylium (chromenylium) ion. In particular anthocyanidins are salt derivatives of the 2-phenylchromenylium cation also known as flavylium cation. As shown in the figure below, the phenyl group at the 2-position, can carry different substituents. The counterion of the flavylium cation is mostly chloride. With this positive charge the anthocyanins differ from other flavonoids.

Anthocyanins: Glucosides of anthocyanidins

  The anthocyanins, anthocyanidins with sugar group, 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^[1] 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 indicator, as they change from red in acids to blue in bases.

Biosynthesis

1. Anthocyanin pigments are assembled like all other flavonoids from two different streams of chemical raw materials in the cell:
   • One stream involves the shikimate pathway to produce the amino acid phenylalanine. (see phenylpropanoids)
   • The other stream produces 3 molecules of malonyl-CoA, a C3 unit from a C2 unit (acetyl-CoA).
2. These streams meet and are coupled together by the enzyme chalcone synthase (CHS), which forms an intermediate chalcone via a polyketide folding mechanism that is commonly found in plants.
3. The chalcone is subsequently isomerized by the enzyme chalcone isomerase (CHI) to the prototype pigment naringenin.
4. Naringenin is subsequently oxidized by enzymes such as flavanone hydroxylase (FHT or F3H), flavonoid 3' hydroxylase and flavonoid 3' 5'-hydroxylase.
5. These oxidation products are further reduced by the enzyme dihydroflavonol 4-reductase (DFR) to the corresponding leucoanthocyanidins.
6. It was believed that leucoanthocyanidins are the immediate precursors of the next enzyme, a dioxygenase referred to as anthocyanidin synthase (ANS) or leucoanthocyanidin dioxygenase (LDOX). It was recently shown however that flavan-3-ols, the products of leucoanthocyanidin reductase (LAR), are the true substrates of ANS/LDOX.
7. The resulting, unstable anthocyanidins are further coupled to sugar molecules by enzymes like UDP-3-O-glucosyl transferase to yield the final relatively stable anthocyanins.

More than five enzymes are thus required to synthesize these pigments, each working in concert. Any even minor disruption in any of the mechanism of these enzymes by either genetic or environmental factors would halt anthocyanin production.

Usage

Anthocyanins are considered secondary metabolites and allowed as a food additive with E number 163.

Anthocyanins also act as powerful antioxidants. This antioxidant property is conserved even after the plant which produced the Anthocyanin is consumed by another organism, which is another reason why fruits and vegetables with red skins and tissues are a nutritious food source.

Recent 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^[2]. Scientists provided laboratory evidence for potential health effects against

• cancer
• aging and neurological diseases
• inflammation
• diabetes
• bacterial infections

Cancer research on anthocyanins is the most advanced, where black raspberry (Rubus occidentalis L.) preparations were first used to inhibit chemically induced cancer of the rat esophagus by 30-60% and of the colon by up to 80%. Effective at both the initiation and promotion/progression stages of tumor development, black raspberries are a practical research tool and a promising therapeutic source, as they contain the richest contents of anthocyanins among native North American berries^[3].

Work on laboratory cancer models has shown that black raspberry anthocyanins inhibit promotion and progression of tumor cells by

1. stalling growth of pre-malignant cells
2. accelerating the rate of cell turnover, called apoptosis, effectively making the cancer cells die faster
3. reducing inflammatory mediators that initiate tumor onset
4. inhibiting growth of new blood vessels that nourish tumors, a process called angiogenesis
5. minimizing cancer-induced DNA damage.

On a molecular level, berry anthocyanins were shown to turn off genes involved with proliferation, apoptosis, inflammation and angiogenesis. In 2007, black raspberry studies entered the next pivotal level of research – the human clinical trial – for which several approved studies are underway to examine anti-cancer effects of black raspberries and cranberries on tumors in the esophagus, prostate and colon^[4].

In December 2004 a peer-reviewed study at Michigan State University published by the American Chemical Society noted that anthocyanins could boost insulin production by up to 50%. However the study leader noted that despite the initial excitement, more study would be needed. Also in 2005, an article published in Applied and Environmental Microbiology demonstrated for the first time the biosynthesis of anthocyanins in bacteria ^[5].

In 2007 a study at the University of Pittsburgh discovered that anthocyanins kills human cancer cells while not affecting healthy cells. At low doses of cyanidin-3-rutinoside (C-3-R), half of the cancer cells in all lines of the test human leukemia and lymphoma cells died within 18 hours. When the amount of C-3-R was more than doubled, all of the cancer cells died within 18 hours. The mechanism seems to be that cancereous cells respond to C-3-R by releasing peroxides which kill the cancer cells. Normal cells do not release peroxides when C-3-R is administered. ^[6]

References

1. ^ Kong J. M., Chia L. S., Goh N. K., Chia T. F., Brouillard R. (2003). "Analysis and biological activities of anthocyanins.". Phytochemistry 64 (5): 923-33. doi:10.1016/S0031-9422(03)00438-2.
2. ^ Gross PM (2007). Scientists zero in on health benefits of berry pigments. Natural Products Information Center. Retrieved on 2007-07-27.
3. ^ Wada L, Ou B (2002). Antioxidant activity and phenolic content of Oregon caneberries.. J Agric Food Chem. Jun 5;50(12):3495-500.. Retrieved on 2007-07-27.
4. ^ Stoner GD, Wang LS, Zikri N, Chen T, Hecht SS, Huang C, Sardo C, Lechner JF (2007). Cancer prevention with freeze-dried berries and berry components.. 1: Semin Cancer Biol. May 10; [Epub ahead of print]. Retrieved on 2007-07-27.
5. ^ Metabolic engineering of anthocyanin biosynthesis in Escherichia coli..
6. ^ Fighting cancer by the bramble.

1. Andersen, O.M. Flavonoids: Chemistry, Biochemistry and Applications. CRC Press, Boca Raton FL 2006.
2. G. M. Robinson, Robert Robinson (1931). "A survey of anthocyanins. I". Biochem J. 25 (5): 1687–1705.