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 Flower. 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
.
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
GABAA receptor 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, Fe
2+, 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:
In diet
Some foods contain relatively high amounts of apigenin:
[ USDA Database for the Flavonoid Content of Selected Foods, Release 3 (2011)]
|
|
| Parsley, dried | 4503.5 |
| Parsley, fresh | 215.5 |
| Celery hearts, green | 19.1 |
| Rutabaga | 4 |
See also
