Apigenin

Apigenin (4′,5,7-trihydroxyflavone), found in many plants, is a flavone compound that is the aglycone of several naturally occurring glycosides. It is a yellow crystalline solid that has been used to dye wool.

Apigenin is abundant in parsley, celery, celeriac, and chamomile flowers. It occurs in many fruits and vegetables, with the highest concentrations in dried and fresh parsley.

Sources in nature

Apigenin is found in many fruits and vegetables, but parsley, celery, celeriac, and chamomile tea are the most common sources. Apigenin is particularly abundant in the flowers of chamomile plants, constituting 68% of total flavonoids. Dried parsley can contain about 45 mg apigenin per gram. The apigenin content of fresh parsley is reportedly 215 mg per 100 grams, which is much higher than the next highest food source.

Pharmacology

In vitro, apigenin binds competitively to the benzodiazepine site on GABA_A receptors. There exist conflicting findings regarding how apigenin interacts with this site.

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 to make cinnamate, followed by oxidation at the para position by cinnamate 4-hydroxylase 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 to arrive at p-coumarate. To complete the general phenylpropanoid pathway, 4-coumarate CoA ligase substitutes coenzyme A at the carboxy group of p-coumarate. Entering the flavone synthesis pathway, the type III polyketide synthase enzyme chalcone synthase uses consecutive condensations of three equivalents of malonyl-CoA followed by aromatization to convert p-coumaroyl-CoA to chalcone. Chalcone isomerase then isomerizes the product to close the pyrone ring to make naringenin. Finally, a flavanone synthase enzyme oxidizes naringenin to apigenin. Two types of flavone synthase (FNS) have 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 glycosides formed by the combination of apigenin with sugars include:

• Apiin (apigenin 7-O-apioglucoside), isolated from parsley 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)

In diet

Some foods contain relatively high amounts of apigenin:

Product	Apigenin (milligrams per 100 grams)
Parsley, dried	4503.5
Parsley, fresh	215.5
Celery hearts, green	19.1
Rutabagas, raw	4

See also

• Amentoflavone