Flavonoid

Flavonoids (or bioflavonoids) (from the Latin word flavus meaning yellow, their colour in nature) are a class of plant secondary metabolites.

Flavonoids were referred to as Vitamin P (probably due to the effect they had on the permeability of vascular capillaries) from the mid-1930s to early 50s, but the term has since fallen out of use.

According to the IUPAC nomenclature, they can be classified into:
 * flavones(http://en.wikipedia.org/wiki/Flavones), derived from 2-phenylchromen-4-one (2-phenyl-1,4-benzopyrone) structure (examples: quercetin, rutin).
 * isoflavonoids, derived from 3-phenylchromen-4-one (3-phenyl-1,4-benzopyrone) structure
 * neoflavonoids, derived from 4-phenylcoumarine (4-phenyl-1,2-benzopyrone) structure.



The three flavonoid classes above are all ketone-containing compounds, and as such, are flavonoids and flavonols. This class was the first to be termed "bioflavonoids." The terms flavonoid and bioflavonoid have also been more loosely used to describe non-ketone polyhydroxy polyphenol compounds which are more specifically termed flavanoids, flavan-3-ols (or catechins).

Functions of flavonoids in plants
Flavonoids are widely distributed in plants fulfilling many functions.

Flavonoids are the most important plant pigments for flower coloration producing yellow or red/blue pigmentation in petals designed to attract pollinator animals.

In higher plants, Flavonoids are involved in UV filtration, symbiotic nitrogen fixation and floral pigmentation.

They may act as a chemical messenger or physiological regulator, they can also act as cell cycle inhibitors.

Flavonoids secreted by the root of their host plant help Rhizobia in the infection stage of their symbiotic relationship with legumes like peas, beans, clover, and soy. Rhizobia living in soil are able to sense the flavonoids and this triggers the secretion of Nod factors, which in turn are recognized by the host plant and can lead to root hair deformation and several cellular responses such as ion fluxes and the formation of a root nodule.

In addition, some flavonoids have inhibitory activity against organisms that cause plant disease e.g. Fusarium oxysporum.

Potential salutary effects on human health
Flavonoids (specifically flavanoids such as the catechins) are "the most common group of polyphenolic compounds in the human diet and are found ubiquitously in plants". Flavonols, the original bioflavonoids such as quercetin, are also found ubiquitously, but in lesser quantities.

The widespread distribution of flavonoids, their variety and their relatively low toxicity compared to other active plant compounds (for instance alkaloids) mean that many animals, including humans, ingest significant quantities in their diet. Preliminary research indicates that flavonoids may modify allergens, viruses, and carcinogens, and so may be biological "response modifiers". In vitro studies show that flavonoids also have anti-allergic, anti-inflammatory, anti-microbial, anti-cancer, and anti-diarrheal activities.

Antioxidant activity in vitro
Flavonoids (both flavonols and flavanols) are most commonly known for their antioxidant activity in vitro. At high experimental concentrations that would not exist in vivo, the antioxidant abilities of flavonoids in vitro may be stronger than those of vitamin C and E, depending on concentrations tested.

Consumers and food manufacturers have become interested in flavonoids for their possible medicinal properties, especially their putative role in inhibiting cancer or cardiovascular disease. Although physiological evidence is not yet established, the beneficial effects of fruits, vegetables, tea, and red wine have sometimes been attributed to flavonoid compounds.

Negligible antioxidant properties of flavonoids in vivo
A research team at the Linus Pauling Institute and the European Food Safety Authority state that flavonoids, inside the human body, are of little or no direct antioxidant value. Body conditions are unlike controlled test tube conditions, and the flavonoids are poorly absorbed (less than 5%), with most of what is absorbed being quickly metabolized and excreted.

The increase in antioxidant capacity of blood seen after the consumption of flavonoid-rich foods may not be caused directly by the flavonoids themselves, but is probably due to increased production of uric acid resulting from excretion of flavonoids from the body. According to Frei, "we can now follow the activity of flavonoids in the body, and one thing that is clear is that the body sees them as foreign compounds and is trying to get rid of them."

Potential anti-cancer activity
Flavonoids might induce mechanisms that affect cancer cells and inhibit tumor invasion. In preliminary studies, UCLA cancer researchers proposed that smokers who ate foods containing certain flavonoids, such as the flavan-3-ols (catechins) found in strawberries and green and black teas, kaempferol from brussel sprouts and apples, and quercetin from beans, onions and apples, may have reduced risk of developing lung cancer.

Carcinogenic potential
Flavonoids were found to be strong topoisomerase inhibitors and induce DNA mutations in the MLL gene, which are common findings in neonatal acute leukemia. The DNA changes were increased by treatment with flavonoids in cultured blood stem cells. In one study, a high flavonoid-content diet in mothers seemed to increase risk of MLL+ acute myeloid leukemia in neonates. This result was not statistically significant though, and when the data on all types of leukiama in the study were taken together, a beneficial effect of the high-flavonoid diet was seen.

Natural phenols (flavonoids in one set of experiments and delphinidin in another ) were found to be strong topoisomerase inhibitors, similar to some chemotherapeutic anticancer drugs including etoposide and doxorubicin. This property may be responsible for both an anticarcinogenic-proapoptotic effect and a carcinogenic, DNA damaging potential of the substances.

<!-- Many groups of polyphenols were found to be strong topoisomerase inhibitors, similarly to some chemotherapeutic anticancer drugs including etoposide and doxorubicin and induce DNA mutations in MLL gene, which are common findings in neonatal acute leukemia. The DNA changes were highly increased by treatment with flavonoids in cultured blood stem cells. Maternal high polyphenol\flavonoid content diet is suspected to increase risk of particularry acute myeloid leukemia in neonates. Polyphenols have both anticarcinogenic - proapoptotic effect and a carcinogenic, DNA damaging, mutagenic potential. Adults seem to rapidly metabolize most of polyphenols, so a toxic mutagenic effect may not be pronounced. Some polyphenols, such as EGCG, were found to rapidly induce detoxifying Nrf2 transcription factor activity which seems to be responsible for observed beneficial effects of the substances. Therefore "bioflavonoid" supplements should be not used by pregnant women. -->

Quercetin
Quercetin, a flavonoid and more specifically a flavonol, is the aglycone form of other flavonoid glycosides, such as rutin and quercitrin, found in citrus fruit, buckwheat and onions. Quercetin forms the glycosides, quercitrin and rutin, together with rhamnose and rutinose, respectively.

Although there is preliminary evidence that asthma, lung cancer and breast cancer are lower among people consuming higher dietary levels of quercetin, the U.S. Food and Drug Administration (FDA), EFSA and the American Cancer Society have concluded that no physiological role exists. The American Cancer Society states that dietary quercetin "is unlikely to cause any major problems or benefits."

Epicatechin


Epicatechin may improve blood flow and has potential for cardiac health. Cocoa, the major ingredient of dark chocolate, contains relatively high amounts of epicatechin and has been found to have nearly twice the antioxidant content of red wine and up to three times that of green tea in vitro. In the test outlined above, it appears the potential antioxidant effects in vivo are minimal as the antioxidants are rapidly excreted from the body.

Dietary sources
Good sources of flavonoids include all citrus fruits, berries, ginkgo biloba, onions (particularly red onion ), parsley, pulses, tea (especially white and green tea), red wine, seabuckthorn, and dark chocolate (with a cocoa content of seventy percent or greater).

Citrus
The citrus bioflavonoids include hesperidin (a glycoside of the flavanone hesperetin), quercitrin, rutin (two glycosides of the flavonol quercetin), and the flavone tangeritin. In addition to possessing in vitro antioxidant activity and an ability to increase intracellular levels of vitamin C, rutin and hesperidin may have beneficial effects on capillary permeability and blood flow. They also exhibit anti-allergy and anti-inflammatory benefits of quercetin from in vitro studies. Quercetin can also inhibit reverse transcriptase, part of the replication process of retroviruses. The therapeutic relevance of this inhibition has not been established. Hydroxyethylrutosides (HER) have potential for use in the treatment of abnormal capillary permeability, bruising, hemorrhoids, and varicose veins.

Dark chocolate
Flavonoids exist naturally in cacao, but because they can be bitter, they are often removed from chocolate, even dark chocolate. Although flavonoids are present in milk chocolate, milk may interfere with their absorption.

Subgroups
Over 5000 naturally occurring flavonoids have been characterized from various plants. They have been classified according to their chemical structure, and are usually subdivided into the following subgroups (for further reading see ):

Flavones
Flavones are divided into four groups:

Isoflavones

 * Isoflavones
 * Isoflavones use the 3-phenylchromen-4-one skeleton (with no hydroxyl group substitution on carbon at position 2).
 * Examples: Genistein, Daidzein, Glycitein

Flavan-3-ols, Flavan-4-ols, Flavan-3,4-diols, and proanthocyanidins
Derivatives of flavan.


 * Flavan-3-ols (also known as flavanols) and Proanthocyanidins
 * Flav an -3-ols use the 2-phenyl- 3,4-dihydro -2H-chromen-3-ol skeleton.
 * Catechin (C), Gallocatechin (GC), Catechin 3-gallate (Cg), Gallocatechin 3-gallate (GCg)), Epicatechins (Epicatechin (EC)), Epigallocatechin (EGC), Epicatechin 3-gallate (ECg), Epigallocatechin 3-gallate (EGCg)
 * Proanthocyanidins are dimers, trimers, oligomers, or polymers of the flavanols.

Anthocyanidins

 * Anthocyanidins
 * Anthocyanidins are the aglycones of anthocyanins. Anthocyanidins use the flavylium (2-phenylchromenylium) ion skeleton
 * Examples: Cyanidin, Delphinidin, Malvidin, Pelargonidin, Peonidin, Petunidin

Availability through microorganisms
Several recent research articles have demonstrated the efficient production of flavonoid molecules from genetically-engineered microorganisms.

Scientists who worked on flavonoids
Jeffrey Harborne investigated the role of flavonoids in interactions between plants and insects. He also investigated the relationship between anthocyanins and the ecology of pollination. He has published on chemotaxonomy as in his research articles on the prevention of anthocyanins, flavones and auron in the primrose family (Primulaceae), in snapdragons (Antirrhinum) and a number of other plants. He also published on isoflavones and chemical ecology.

Databases

 * FlavonoidViewer.jp (Japanese, English), a database on flavonoids by Arita Group (Univ of Tokyo, RIKEN Plant Science Center, and Keio Univ), Nishioka Group (Kyoto and Keio Univ) and Kanaya Group (NAIST)
 * USDA Database of Flavonoid content of food (pdf)

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