Apigenin (4′,5,7-trihydroxyflavone), found in many plants, is a

natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical sy ...

belonging to the

flavone Flavone is an organic compound with the formula . A white solid, flavone is a derivative of chromone with a phenyl (Ph) substituent adjacent to the ether group. The compound is of little direct practical importance, but susbstituted derivatives, ...

class that is the aglycone of several naturally occurring

glycosides In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...

. It is a yellow crystalline solid that has been used to dye wool.

Sources in nature

Apigenin is found in many fruits and vegetables, but

parsley Parsley, or garden parsley (''Petroselinum crispum'') is a species of flowering plant in the family Apiaceae that is native to the central and eastern Mediterranean region (Sardinia, Lebanon, Israel, Cyprus, Turkey, southern Italy, Greece, ...

celery Celery (''Apium graveolens'') is a marshland plant in the family Apiaceae that has been cultivated as a vegetable since antiquity. Celery has a long fibrous stalk tapering into leaves. Depending on location and cultivar, either its stalks, ...

celeriac Celeriac (''Apium graveolens'' var. ''rapaceum''), also called celery root, knob celery, and turnip-rooted celery (although it is not a close relative of the turnip), is a variety of celery cultivated for its edible stem or hypocotyl, and sh ...

, and

chamomile Chamomile (American English) or camomile (British English; see spelling differences) ( or ) is the common name for several plants of the family Asteraceae. Two of the species, ''Matricaria recutita'' and ''Anthemis nobilis'', are commonly us ...

tea are the most common sources. Apigenin is particularly abundant in the flowers of chamomile plants, constituting 68% of total

flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans. Chemically, flavonoids ...

s. Dried parsley can contain about 45 mg apigenin/gram of the herb, and dried chamomile flower about 3-5 mg/gram. The apigenin content of fresh parsley is reportedly 215.5 mg/100 grams, which is much higher than the next highest food source, green celery hearts providing 19.1 mg/100 grams.

Biosynthesis

Apigenin is biosynthetically derived from the general phenylpropanoid pathway and the flavone synthesis pathway. The phenylpropanoid pathway starts from the aromatic amino acids L-phenylalanine or L-tyrosine, both products of the Shikimate pathway. When starting from L-phenylalanine, first the amino acid is non-oxidatively deaminated by phenylalanine ammonia lyase (PAL) to make cinnamate, followed by oxidation at the ''para'' position by cinnamate 4-hydroxylase (C4H) to produce ''p''-coumarate. As L-tyrosine is already oxidized at the ''para'' position, it skips this oxidation and is simply deaminated by tyrosine ammonia lyase (TAL) to arrive at ''p''-coumarate. To complete the general phenylpropanoid pathway, 4-coumarate CoA ligase (4CL) substitutes coenzyme A (CoA) at the carboxy group of ''p''-coumarate. Entering the flavone synthesis pathway, the type III polyketide synthase enzyme chalcone synthase (CHS) uses consecutive condensations of three equivalents of malonyl CoA followed by aromatization to convert ''p''-coumaroyl-CoA to chalcone.

Chalcone isomerase In enzymology, a chalcone isomerase () is an enzyme that catalyzes the chemical reaction :a chalcone \rightleftharpoons a flavanone Hence, this enzyme has one substrate, a chalcone, and one product, a flavanone. This enzyme belongs to the fa ...

(CHI) then isomerizes the product to close the pyrone ring to make naringenin. Finally, a flavanone synthase (FNS) enzyme oxidizes naringenin to apigenin. Two types of FNS have previously been described; FNS I, a soluble enzyme that uses 2-oxogluturate, Fe²⁺, and ascorbate as cofactors and FNS II, a membrane bound, NADPH dependent cytochrome p450 monooxygenase.

Glycosides

The naturally occurring

glycoside In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...

s formed by the combination of apigenin with sugars include: * Apiin (apigenin 7-''O''-apioglucoside), isolated from

and celery * Apigetrin (apigenin 7-glucoside), found in dandelion coffee * Vitexin (apigenin 8-''C''-glucoside) * Isovitexin (apigenin 6-''C''-glucoside) * Rhoifolin (apigenin 7-''O''- neohesperidoside) * Schaftoside (apigenin 6-''C''-glucoside 8-''C''-arabinoside)

References

{{Progestogenics Aromatase inhibitors Delta-opioid receptor antagonists Flavones GABAA receptor positive allosteric modulators Resorcinols Kappa-opioid receptor antagonists Mu-opioid receptor antagonists NMDA receptor antagonists Phytoestrogens Progestogens Vinylogous carboxylic acids

Sources in nature

Biosynthesis

Glycosides

See also

References