Apigenin

Apigenin is found in many fruits and vegetables, but parsley, celery, celeriac, and chamomile tea are the most common sources.^[3] Apigenin is particularly abundant in the flowers of chamomile plants, constituting 68% of total flavonoids.^[4] Dried parsley can contain about 45 mg apigenin/gram of the herb, and dried chamomile flower about 3–5 mg/gram.^[5] 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.^[6]

Apigenin competitively binds to the benzodiazepine site on GABA_A receptors.^[7] There exist conflicting findings regarding how apigenin interacts with this site.^[8][9]

Apigenin is biosynthetically derived from the general phenylpropanoid pathway and the flavone synthesis pathway.^[10] The phenylpropanoid pathway starts from the aromatic amino acids L-phenylalanine or L-tyrosine, both products of the Shikimate pathway.^[11] 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.^[12] 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.^[13] Chalcone isomerase (CHI) then isomerizes the product to close the pyrone ring to make naringenin. Finally, a flavanone synthase (FNS) enzyme oxidizes naringenin to apigenin.^[14] 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.^[15]

The naturally occurring glycosides formed by the combination of apigenin with sugars include:

• Apiin (apigenin 7-O-apioglucoside), isolated from parsley^[16] 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)

Some foods contain relatively high amounts of apigenin:^[17]

• Amentoflavone