Tannins

These are widely distributed in plant flora. They are phenolic compounds of high molecular weight. Tannins are soluble in water and alcohol and are found in the root, bark, stem and outer layers of plant tissue. Tannins have a characteristic feature to tan, i.e. to convert things into leather. The tannins are acidic in reaction and is attributed to the presence of phenolic or carboxylic group. Tannins form complex with proteins, carbohydrates, gelatin and alkaloids.

Tannins are classified in two classes:

1. Hydrolysable tannins.

2. Condensed tannins.

Hydrolysable tannins, upon hydrolysis, produce gallic acid (Fig. 5.212) and ellagic acid (Fig. 5.213). Depending on the type of acid produced, the hydrolysable tannins arecalled gallotannins or egallitannins. On heating, they form pyrogallic acid.

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Fig. 5.212 Structure of Gallic Acid. Fig. 5.213 Structure of Ellagic Acid.

Aceritannin (Fig. 5.214) and hamemellitannin (Fig. 5.215), obtained from Acer sp. and Hamamelis virginiana, respectively, are examples of gallotannins.

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Fig. 5.215 Structure of Hamemellitanin.

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Fig. 5.215 Structure of Hamemellitanin.

Amariin, hydrolysable tannin is found in leaves and seeds of Colutea nepalensis. Oenthein-B is hydrolysable tannin is found in Oenothera erythrosepala. They are also found in Rheum emodi, Aegle marmelos and Quercus infectoria. Ellagic acid isolated from Terminalia arjuna has antimutagenic activity. A dichloromethane-methanol extract of Anisophyllea apetala provided 3'-methyl-3,4-O,O-methylidene-4'-O-p-D-glucopyranosyl ellagic acid, which showed some DNA damaging effect in vitro, and potently inhibited the survival of yeast.

Casuarina stricta has been reported to contain elagitannins including casuarinin, casuriin, stachyurin, casuarictin and strictinin. Casuarinin (Fig. 5.216) isolated from Terminalia arjuna has been studied as antiviral against antiherpes simplex virus type 2 and anticancer activity in non-small cell lung cancerA 549 cell line.

Condensed tannins are formed by condensation of catechin units. These types of tannins are also known as phalbatannins. They are powerful antioxidants and are present in Vitis vinifera, Acacia catechu, Lawsonia inermis and Pinus sylvestris. Some tannins are potential carcinogenic agents.

Acacia catechu (black catechu) contains 10% acacetechin, which on oxidation yields, catechu tannic acid (Fig. 5.217). Uncaria gambier (pale catechu) is good source of tannin. Pterocarpus marsupium contains glucosidal tannin, known as kinotannic acid (70-80%).

Tannins are used as antiseptic and this activity is due to presence of the phenolic group. Tannin-rich medicinal plants are used as healing agents in a number of diseases. In Ayurveda, formulations based on tannin-rich plants have been used for the treatment of diseases like leucorrhoea, rhinnorhoea and diarrhea.

Triphala (combination of fruits of Terminalia chebula, Terminalia belerica and Emblica officinalis) is an example similar to that of tannin containing formulation. Terminalia chebula contains 30% tannins with chebulinic acid (Fig. 5.218) and chebulagic acid (Fig. 5.219), as the principal constituents. Terminalia belerica contains 17% tannins. Recently, elagitannin (peduncugalin) was isolated from the fruits of Emblica officinalis (Fig. 5.220). Neochebulic acid is minor isomer of chebulic acid. Chebulic acid is closely allied to ellagic acid and reported to be hepatoprotective, chebulinic acid is an antioxidant while chebulagic acid is immunosuppressant, respectively.

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Fig. 5.217 Structure of catechu tannic acid.

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Fig. 5.217 Structure of catechu tannic acid.

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Chebulagic Acid Structure
Fig. 5.219 Structure of chebulagic acid.

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Structure Catechutannic

A group which contains useful detoxifying chemicals is the polyphenols, which can also include many of the tannins. There are numerous references to support the use of these materials. The best known, polyphenols probably come from green tea or Thea viridis, a fast growing additive in today's skin care repertoire. Tea contains high levels of tannins or phenolic materials (approx.10-25%) which consist of catechin (flavanol) and gallic acid units and corialgin (Fig. 5.221).

Fig. 5.221 Structure of Corialgin.

Family Melastomataceae elaborate an unusual series of hydrolyzable tannin oligomers such as nobotannin B (Fig. 5.223), which has exhibited anti-Human Immunodeficiency Virus (HIV) property in vitro. Procyanidin B-2 (Fig. 5.224) and castalagin (Fig. 5.225) are ubiquitous in the family and are known to lower blood pressure in spontaneously hypertensive rats dose-dependently through decrease of sympathetic tone.

Tannin Containing Leaves

Fig. 5.223 Structure of Nobotannin B.

Fig. 5.225 Structure of Castalagin.

Green tea infusion contains intact catechin polyphenols, which give rise to its bitterness and astringency. Six catechin polyphenols have been isolated from green tea; (-)-epigallocatechin, (-)-epicatechin, (-)-epigallocatechin-3-O-gallate (EGCG), gallocatechin-3-O-gallate (GCG), methyl-epigallocatechin-3-O-gallate, and (-)-epicatechin-3-O-gallate (ECG). These substances were tested for their antioxidant activity, and the gallic acid esters EGCG and EGC were found to be the strongest antioxidants, with EGCG being over 200 times more active than vitamin E in an in vitro model. In another test, EGCG was more active against fat rancidity (lipid peroxidation) than vitamin C or vitamin E, and also exhibited synergistic action with those vitamins.

Tea contains the polyphenol ECGC, or (-)-epigallocatechin gallate (Fig. 5.225), as major components and it is these in which the researchers are interested. The polyphenols are powerful antioxidants capable of scavenging H2O2 and superoxide anions and thus preventing free radical damage to the body. This is a mechanism that has been associated with cancer and other disorders.

Fig. 5.226 Structure of (-)-epigallocatechin gallate.

ECGC also has an astringent effect and may inhibit cell membrane phosphorylation. The researchers do not know whether the polyphenols inhibit the initiation or the promotion of tumors. Following the oral feeding of a polyphenolic fraction, isolated from green tea, (GTP) in drinking water, an increase in the activities of antioxidant and phase II enzymes in skin, small bowel, liver, and lung of female SKH-1 hairless mice was observed. Prodelphinidin B-2 3,O-gallate (Fig. 5.227), a proanthocyanidin gallate has been demonstrated for anti-proliferative activity in human non-small cell lung cancer A549 cells by G0/G1 arrest and apoptosis induction.

Prodelphinidin

Prodelphinidin B-2 3,3'-di-O-gallate (Fig. 5.228), a proanthocyanidin gallate from Myrica rubra has been demonstrated to arrest cell cycle at

G0/G1 phase and induce apoptosis in MCF-7 and A549 cells by p21/ WAF1 and Fas/APO-1/Fas ligand pathway, respectively.

Moldes Cajitas Forma Mariposa

Putranjivain A (Fig. 5.229), isolated from whole plant of Euphorbia jolkini, has been demonstrated for anti-proliferative activity in human breast adenocarcinoma by blocking cells by G0/G1 arrest and apoptosis induction.

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Punicalagin are tannins, large polyphenol compounds found in Punica granatum. They are the largest molecules found intact in rat plasma after oral ingestion. They were found to show no toxic effects in rats that were given a 6 diet of punicalagin for 37 d. The compound has in vitro antiproliferative, antioxidant and apoptotic activities.

Methyl gallate (Fig. 5.230) from Toona sinensis protects DNA in canine Cocker Spaniel kidney cell line (MDCK) cells against hydrogen peroxide-induced oxidative stress.

Fig. 5.230 Structure of methyl gallate.

Fig. 5.230 Structure of methyl gallate.

Ellagitannins oenothein A and eonothein B from Epilobium species has inhibitory activity on a-reductase and aromatase.

Table 5.12 Tannins from medicinal plants.

S.No

Name of the tannins

Source

1.

Chebulic acid

Terminalia chebula

2.

Chebulinic acid

Terminalia chebula

3.

Chebulagic acid

Terminalia chebula

4.

Corialgin

Terminalia chebula

5.

Ellagic acid

Widely distributed

6.

Gallic acid

Widely distributed

7.

Neochebulinic acid

Terminalia chebula

8.

Oenothein-B

Oenothera erythrosepala

9.

Punicalagin

Terminalia chebula

10.

Tannic acid

Widely distributed

11.

Terchebin

Emblica officinalis, Terminalia chebula

12.

Terflavin-A

Terminalia cattapa T.chebula, T.macroptera

13.

Termigrandin

Terminalia chebula

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Responses

  • donald
    What plant has 80% tannins?
    2 years ago
  • marie
    What are tannins for medicinal plant?
    1 year ago

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