Anthocyanin gives these
pansies their dark purple pigmentation.
'Anthocyanins' (from
Greek: '' (anthos)'' = flower + '' (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
bacteria, and have been observed to occur in all
tissues of higher plants, providing color in
leaves,
stems,
roots,
flowers, and
fruits.
Function
Anthocyanin pigments seem to help many pollinators to locate flowers that contain them, and in fruits, the colorful skins may be recognized by animals which will eat the fruits and disperse the seeds. In photosynthetic tissues (such as leaves), anthocyanins have been shown to act as a "sunscreen", protecting cells from photo-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 is also thought that red coloration of leaves may camouflage leaves from herbivores blind to red wavelengths, or signal unpalatability to herbivores, 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, helping to protect the plant from radicals formed by
UV light and during metabolic processes. This antioxidant property is conserved even after consumption by another organism, which is another reason why fruits and vegetables with red skins and tissues are a nutritious food source.
Occurrence
Juvenile anthocyanin in new
rose growth. The reddish hue disappears as the new leaves mature.
Not all land plants contain anthocyanin and in the
Caryophyllales,
Cactus and
Galium mollugo they are replaced by
betacyanins.
Anatomically, anthocyanins are found 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 10
9 tons per year.
In plants anthocyanins are present together with other natural pigments like the closely chemically related
flavonoids,
carotenoids,
anthoxanthins and
betacyanins.
In still relatively young plants or new growth, where
chlorophyll and wax production has not yet begun and which would be unprotected from
UV light, anthocyanin production increases. Parts or even the whole plant are colored by these "juvenile anthocyanins," and thereby protected from damage. As soon as chlorophyll production begins, the production of the anthocyanin dye is reduced. The build-up of anthocyanin in plants is specific to the plant type, since it depends on the soil conditions, light, warmth and plant type and/or sort. Plants that have only a single anthocyanin as pigment are extremely rare, but not unheard of. The absence or particularly strong prevalence of a certain anthocyanin in a plant is due to genetic circumstances.
Structure
Anthocyanidins: Flavylium cation derivatives
Benzopyrylium (chromenylium) salts with chloride as the counterion
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
Anthocyanins are responsible for the distinctive color of
blood oranges.
#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).
#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.
#The chalcone is subsequently isomerized by the enzyme 'chalcone isomerase' (CHI) to the prototype pigment
naringenin.
#Naringenin is subsequently oxidized by enzymes such as 'flavanone hydroxylase' (FHT or F3H), 'flavonoid 3' hydroxylase' and 'flavonoid 3' 5'-hydroxylase'.
#These oxidation products are further reduced by the enzyme 'dihydroflavonol 4-reductase' (DFR) to the corresponding leucoanthocyanidins.
#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.
#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.
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.
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
# stalling growth of pre-
malignant cells
# accelerating the rate of cell turnover, called
apoptosis, effectively making the cancer cells die faster
# reducing inflammatory mediators that initiate tumor onset
# inhibiting growth of new blood vessels that nourish tumors, a process called
angiogenesis
# 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 witin 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. Analysis and biological activities of anthocyanins., Kong J. M., Chia L. S., Goh N. K., Chia T. F., Brouillard R., , , Phytochemistry, 2003
2. Scientists zero in on health benefits of berry pigments Gross PM
3. Antioxidant activity and phenolic content of Oregon caneberries. Wada L, Ou B
4. Cancer prevention with freeze-dried berries and berry components. Stoner GD, Wang LS, Zikri N, Chen T, Hecht SS, Huang C, Sardo C, Lechner JF
5. Metabolic engineering of anthocyanin biosynthesis in Escherichia coli.
6.
Fighting cancer by the bramble
# Andersen, O.M. ''Flavonoids: Chemistry, Biochemistry and Applications''. CRC Press, Boca Raton FL 2006.
#
A survey of anthocyanins. I, G. M. Robinson, Robert Robinson, , , Biochem J., 1931
External links
★
Anthocyanin Biosynthesis
★
Red leaves - Catalyst ABC
★
Super Blackcurrants With Boosted Vitamin C
★
Quantification of anthocyanins in commercial black currant juices by simple high-performance liquid chromatography. Investigation of their pH stability and antioxidative potency.
★
Chemicals Found in Cherries May Help Fight Diabetes
★
Biochemicals found in dark raspberries may help fight Diabetes and Cancer (in German)
★
A discussion of the role of anthocyanins in hydrangea coloration
★
Anthocyanins FAQ ''MadSci Network'' Functions and uses as pH indicators or for pigment chromatography.
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