|Molar mass||302.236 g/mol|
|Appearance||yellow crystalline powder|
|Melting point||316 °C (601 °F; 589 K)|
|Practically insoluble in water; soluble in aqueous alkaline solutions|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
Quercetin is a flavonoid widely distributed in nature. The name has been used since 1857, and is derived from quercetum (oak forest), after Quercus. It is a naturally occurring polar auxin transport inhibitor.
|Foods containing quercetin||Quercetin (mg/100g of edible portion)|
|dock like sorrel||86|
|Hungarian wax pepper||51|
|sea buckthorn berry||8|
|prickly pear cactus fruits||5|
|apples, Red Delicious||4|
|tea, black or green Camellia sinensis||2|
In red onions, higher concentrations of quercetin occur in the outermost rings and in the part closest to the root, the latter being the part of the plant with the highest concentration. One study found that organically grown tomatoes had 79% more quercetin than chemically grown fruit. Quercetin is present in various kinds of honey from different plant sources.
Phenylalanine is converted to 4-coumaroyl-CoA in a series of steps known as the general phenylpropanoid pathway using phenylalanine ammonia-lyase, cinnamate-4-hydroxylase, and 4-coumaroyl-CoA-ligase. 4-Coumaroyl-CoA is added to three molecules of malonyl-CoA to form tetrahydroxychalcone using 7,2’-dihydroxy-4’-methoxyisoflavanol synthase. Tetrahydroxychalcone is then converted into naringenin using chalcone isomerase. Naringenin is then converted into eriodictyol using flavanoid 3’-hydroxylase. Eriodictyol is then converted into dihydroquercetin with flavanone 3-hydroxylase, which is then converted into quercetin using flavonol synthase.
Quercetin release in the rutin degradation pathway
Quercetin 3-O-sulfate is a human plasma quercetin metabolite.
Quercetin is the aglycone form of a number 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. Likewise guaijaverin is the 3-O-arabinoside, hyperoside is the 3-O-galactoside, isoquercitin is the 3-O-glucoside and spiraeoside is the 4'-O-glucoside. CTN-986 is a quercetin derivative found in cottonseeds and cottonseed oil. Miquelianin is the quercetin 3-o-β-d-glucuronopyranoside.
Fate in vivo
Effects of consumption by humans and other animals
Quercetin itself (aglycone quercetin), as opposed to quercetin glycosides, is not a normal dietary component. In a bioavailability study in rats, radiolabelled quercetin-4'-glucoside was converted to phenolic acids as it passed through the gastrointestinal tract, producing compounds not monitored in previous animal studies of aglycone quercetin. All but 4% was recovered within 72 hours (69% in urine), indicating low retention and high excretion, a characteristic of ingested polyphenols. Quercetin may also induce insulin secretion by activation of L-type calcium channels in the pancreatic β-cells.
Quercetin has not been confirmed scientifically as a specific therapeutic for any condition nor approved by any regulatory agency. The European Food Safety Authority evaluated possible health claims associated with consumption of quercetin, finding that no cause-and-effect relationship has been established for any physiological effect.
Although quercetin is under basic and early-stage clinical research for a variety of disease conditions, there exists no sufficient evidence that it has any beneficial effect in the human body. The US FDA has issued warning letters, e.g., to emphasize that quercetin is not a defined nutrient, cannot be assigned a dietary content level and is not regulated as a drug to treat any human disease.
Quercetin is contraindicated with some antibiotics; it may interact with fluoroquinolones (an antibiotic), as quercetin competitively binds to bacterial DNA gyrase. Whether this inhibits or enhances the effect of fluoroquinolones is not certain.
AHFS Drug Information (2010) identifies quercetin as an inhibitor of CYP2C8, and specifically names it as a drug with potential to have harmful interactions with taxol/paclitaxel. As paclitaxel is metabolized primarily by CYP2C8, its bioavailability may be increased unpredictably, potentially leading to harmful side-effects.
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