Bioflavonoids

A flavonoid is identical to a bioflavonoid; furthermore, a flavonoid by definition is a polyphenol, since it contains more than one benzene ring in its structure. However, many regard polyphenols to be tannins. As seen above, green tea has an abundance of flavonoids and flavonals as well as tannin. These compounds have been well characterized in the herb and numerous papers support their use as antioxidants or free radical scavengers.

Flavonoids are an important group of polyphenols, widely distributed in plant flora. Four thousand flavonoids are known to exist and some of them are pigments in higher plants. Quercetin, kaempferol and quercitrin are common flavonoids present in nearly 70% of plants.

Flavonoids are derived from parent compounds known as flavans (Fig. 5.110).

Diagrams Medicinal Plants
Fig. 5.110 Structure of Flavan. The following diagram represents the flavonoid molecule.

Fig. 5.111 Structure of Flavonoid Molecule.

Fig. 5.111 Structure of Flavonoid Molecule.

Flavonoids are classified into five groups: A. Flavones and flavonols (Fig. 5.112)

They are yellow and are usually found in a majority of plants. Leaves of Calycopteris floribunda contain flavanol calycopterin, which is considered to be anthelmentic.

OH O

Apigenin

OH O

Kampferol oh

OH O

Apigenin

OH O

OH O

Rhamnetin oh o

Kampferol oh

^chs

OH O

Rhamnetin oh o

Isorhamnetin oh oh

oh oh o Quercetin oh o Quercetin

Isorhamnetin

Querctirin

Querctirin

OH

OH O

Vitexin

Fig. 5.112 Structure of Flavones and Flavonols.

Tetrahydroamentoflavone, from Semecarpus anacardium, inhibits the enzymatic activity of cyclooxygenase, with an IC50 value of 29.5 ^M (COX-1). Lanaroflavone (Fig. 5.113), from Campnosperma panamaense, inhibited Plasmodium falciparum K1 chloroquine-resistant strain and Leishmania donovani cultured in vitro with IC values of 0.2g/mL and 3.9g/ mL, respectively.

Semecarpus Anacardium

oh o

Fig. 5.113 Structure of Lanaroflavone.

oh o

Fig. 5.113 Structure of Lanaroflavone.

5,3-dihydroxy-3,6,7,8,4-pentamethoxyflavone (Fig. 5.114) from Polanisia dodecandra inhibits a broad panel of cancer cells: central nervous system cancer (SF-268, SF-539, SNB-75, U-251), nonsmall- cell lung cancer (HOP-62, NCI-H266, NCI-H460, NCI-H522), small-cell lung cancer (DMS-114), ovarian cancer (OVCAR-3, SKOV- 3), colon cancer (HCT-116), renal cancer (UO-31), a melanoma cell line (SK-MEL-5), and leukemia cell lines (HL-60, SR), cultured in vitro.

OCH3

oh o

Fig. 5.114 Structure of 5,4'-Dihydroxy-3,6,7,8,4'-pentamethoxyflavone.

oh o

Fig. 5.114 Structure of 5,4'-Dihydroxy-3,6,7,8,4'-pentamethoxyflavone.

Acacetin (5,7-dihydroxy-4'methoxy-flavone), present in Cirisium rhinoceros inhibits the proliferation of the human liver and lung cancer cells, HepG2 and A549 cells, respectively by blocking apoptosis and cell cycle progression. (Fig. 5.115).

ho o

Fig. 5.115 Structure of 5,7-dihydroxy-4'methoxy-flavone.

ho o

Fig. 5.115 Structure of 5,7-dihydroxy-4'methoxy-flavone.

The rhizomes of Iris nepalensis is reported to characteterize several isoflavones, including irisolone and irisolidone (Figs. 5.116, 5.117).

och3

och3

och3 o Fig. 5.116 Structure of Irisolone.

och3 o Fig. 5.116 Structure of Irisolone.

h3co

och3 o

Fig. 5.117 Structure of Irisolidone.

h3co och3

och3 o

Fig. 5.117 Structure of Irisolidone.

B. Flavonols

These are found in plants of Rutaceae.

Fig. 5.118 Structure of Flavonol (Naringin).

Fig. 5.118 Structure of Flavonol (Naringin).

C. Anthocyanidins

Anthocyanidins are derived from flavonols. In nature, they are found as glycosides and are called anthocyanins. They have characteristic colors ranging from red to blue and are responsible for the color of fruits. Cyanidin and delphinidin are similar examples (Fig. 5.119).

Cyanidin
Structure Silymarin
Delphinidin Fig. 5.119 Structure of Anthocyanidins.

D. Proanthocyanidins

Proanthocyanidins on hydrolysis yield anthocyanidins. Procyanidin and prodelphinidin are common examples. Prodelphinidin isolated from Rhynchosia minima has demonstrated antibiotic activity.

E. Catechin and leucoanthocyanidins (Fig. 5.120)

Catechins are also derived from flavones. Leucoanthocyanidins have an additional hydroxy group.

Fig. 5.120 Structure of Catechin.

Flavonolignans

Silymarin is a flavonol-lignan mixture obtained from seeds of Silybum marianum (Fig. 5.121). Silymarin is a mixture of silybin, isosilybin, silychristin and silydianin. Silybin A and B are collectively known as silibinin. Silymarin is a reputed hepatoprotective drug.

oh o

Fig. 5.121 . Structure of Silymarin.

oh o

Fig. 5.121 . Structure of Silymarin.

Flavonolignans (hydnowightin, hydnocarpin, and neohydnocarpin) have been isolated from the seeds of Hydnocarpus wightiana. They are reported to be hypolipidemic, anti-inflammatory and antineoplastic (Figs. 5. 122,5.123).

och3

"O

O OH Fig. 5.123 Structure of Hydnowightin.

More about bioflavonoids

Catechin reported from Artocarpus integra has gastro protective activity. Cyanidanol, a stereoisomer of catechin from the seed coat of Anacardium occidentale has an inhibiting action on histidine decarboxylase. Flavones, oroxylin-a, baicalein and chrysin have been isolated from Oroxylum indicum. Acacia auriculaeformis is reported to contain flavone glycoside, auriculoside which is a CNS depressant.

Ononis spinosa contains flavonoids including formononetin (Fig. 5. 124), and biochanin A. (Fig. 5.125). Flavonoids, orientin (Fig. 5.126), and vicenin, isolated from the leaves of Ocimum sanctum have significant antioxidant activity. They increase the survival time in lethally irradiated mice.

Fig. 5.124 Structure of Formononetin.

Fig. 5.125 Structure of Biochanin.

OH O Fig. 5.126 Structure of Orientin.

Isoliquiritigenin, a flavonoid, present in licorice inhibits the proliferation of the human liver and lung cancer cells, HepG2 and A549 cels, respectively by blocking apoptosis and cell cycle progression (Fig. 5.127).

"OH

Fig. 5.127 Structure of Isoliquiritigenin.

Aspalathin is monomeric flavonoid found in Aspalathus linearis (Rooibos tea). It has significant antioxidant property. Chrysoeriol (3-methoxylutein) is a flavonoid found in snapdragons has anti-inflammatory and antioxidant properties (Fig. 5.128). Tricin is a flavone present in rice bran. It has an antioxidant property (Fig. 5.129).

-ch3

oh o

Fig. 5.128 Structure of Chrysoeriol.

CH2OH HOH2C-C-NH-CH 2-cooh ch2oh

Fig. 5.129 Structure of Tricin.

Certain flavonoids of Cananbis sativa (flavocannabiside andflavosativaside) are weak inhibitors of lens aldose reductase, an enzyme implicated in the pathogenesis of cataracts in people suffering from diabetes and galactosemia (Figs. 5.130, 5.131).

OH OH

hoh2c

OH OH

OH O Fig. 5.130 Structure of Flavocannabiside.

Fig. 5.131 Structure of Flavosativaside.

Fig. 5.131 Structure of Flavosativaside.

ho o

Flavonoids are widely distributed in the plant kingdom and have long been under investigation for antiparasitic activity without significant prospects for therapeutic value. With intensive studies on Artemisia annua the situation has changed dramatically. Flavonoids isolated from A. annua were not found active against P. falciparum, but demonstrated a marked and selective potentiating effect on the antiplasmodial activity of artemisinin. Artemetin (Fig. 5.132) and casticin (Fig. 5.133) act synergistically with artemisinin. Both are methoxylated flavones and are described as inhibitors of L-glutamine uptake by infected macrophages.

From Artemisia indica, the flavonoids sakuranetin and 7-methoxyaromadendrin (Fig. 5.134) also show high antiprotozoal activity. The mode of action of antiprotozoal flavonoids remains unclear. The effect on the generation of reactive oxygen species has been discussed, and so has sophisticated biochemical mechanisms like the inhibition of the P-glycoprotein-like transporter or modulation of protein phosphorylation on the SPK89 protein kinase in trypanosomes. Sakuranetin is also present in Dodonaea viscosa.

Fig. 5.132 Structure of Artemetin.

Fig. 5.133 Structure of Casticin.

Fig. 5.132 Structure of Artemetin.

Fig. 5.133 Structure of Casticin.

H3CO

H3CO

oh o

7-methoxyaromadendrin

Sakuranetin

Fig. 5.134 Structure of flavonoids of Artemisia indica.

oh o

7-methoxyaromadendrin

Sakuranetin

Fig. 5.134 Structure of flavonoids of Artemisia indica.

Pinitol, present in Bougenvillia spectabilis has potent hypoglycemic activity (Fig. 5.135).

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