Apigenin (4′,5,7-trihydroxyflavone), found in many plants, is a
natural product belonging to the
flavone class that is the
aglycone of several naturally occurring
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,
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]
Pharmacology
Apigenin competitively binds to the benzodiazepine site on
GABAA receptors.[7] There exist conflicting findings regarding how apigenin interacts with this site.[8][9]
Biosynthesis
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]
Glycosides
The naturally occurring
glycosides formed by the combination of apigenin with sugars include:
Apiin (apigenin 7-O-apioglucoside), isolated from
parsley[16] and celery
Apigenin (4′,5,7-trihydroxyflavone), found in many plants, is a
natural product belonging to the
flavone class that is the
aglycone of several naturally occurring
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,
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]
Pharmacology
Apigenin competitively binds to the benzodiazepine site on
GABAA receptors.[7] There exist conflicting findings regarding how apigenin interacts with this site.[8][9]
Biosynthesis
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]
Glycosides
The naturally occurring
glycosides formed by the combination of apigenin with sugars include:
Apiin (apigenin 7-O-apioglucoside), isolated from
parsley[16] and celery