Lecture

Medicinal plants and medical plant material that contain tannic substances.

Tannins are astringent, bitter plant polyphenols that either bind and precipitate or shrink proteins. The astringency from the tannins is what causes the dry and puckery feeling in the mouth following the consumption of unripened fruit or red wine. Likewise, the destruction or modification of tannins with time plays an important role in the ripening of fruit

The term tannin (from tanna, an Old High German word for oak or for tree) refers to the use of wood tannins from oak in tanning animal hides into leather; however, the term is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong complexes with proteins and other macromolecules.

The above tannin-protein co-precipitation is important not only in the leather industry but also in relation to the physiological activity of herbal medicines, taste of foodstuffs and beverages, and in the nutritional value of feeds for herbivores.

Two main groups of tannins are usually recognized; these are the hydrolysable tannins and the condensed tannins (proanthocyanidins).

Hydrolysable tannins

These may be hydrolysed by acids or enzymes such as tannase. They are formed from several molecules of phenolic acids such as gallic and hexahydroxydiphenic acids which are united by ester linkages to a central glucose molecule.

Fig.1 Gallic acid

Three principal types of hydrolysable tannins are:

- gallitannins (ethers of gallic acid and sugars);

- ellagitannins (ethers of hexahydroxydiphenic (ellagic) acid and sugars);

- non-sugar ethers of phenolcarbolic acids.

Examples of drugs containing hydrolysable tannins are:

Gallitannins: rhubarb, cloves, bearberry leaves, Chinese galls, Turkish galls, hamamelis, chestnut and maple.

Ellagitannins: pomegranate rind, pomegranate bark, myrobalans, eucalyptus leaves and oak bark.

Condensed tannins (proanthocyanidins)

Unlike hydrolysable tannins, these are not readily hydrolysed to simpler molecules and they do not contain a sugar moiety. They are related to the flavonoid pigments and have polymeric flavan-3-ol structures.

Fig.2 Flavone

They are divided into 3 groups as well:

- derivatives of flavanols-3;

- derivatives of flavandiols-3,4;

- derivatives of oxystilbens (diphenilethylene).

Some drugs (e.g. tea, hamamelis leaves and hammamelis bark) contain both hydrolysable and condensed tannins. The following are rich in condensed tannins:

1. Barks: cinnamon, wild cherry, cinchona, willow, acacia (wattle, mimosa), oak and hamamelis

2. Roots and rhizomes: kramelia and male fern

3. Flowers: lime and hawthorn

4. Seeds: cocoa, guarana, kola and areca

5. Fruits: cranberries, grapes, hawthorn

6. Leaves: hamamelis, hawthorn and tea, especially green tea

7. Extracts and dried juices: catechu, acacia, East Indian kino, butea gum and eucalyptus kino.

Occurrence of tannins

T. are of wide occurrence in plants and are usually found in greatest quantity in dead or dying cells. They exert an inhibitory effect on many enzymes due to protein precipitation and, hence, they may contribute a protective function in barks and heartwoods. Commercial tannins, as used in the leather industry, are obtained from quebracho, wattle, chestnut and myrobalans trees. Pharmaceutical tannin is prepared from oak galls and yields glucose and gallic acid on hydrolysis; many commercial samples contain some free gallic acid.

Properties and tests.

T. are soluble in water, dilute alkalis, glycerol and acetone, but generally only sparingly soluble in other organic solvents. Solutions precipitate heavy metals, alkaloids, glycosides and gelatin.

Tannins can be extracted with hot water, and after that extract should be purified by chlorophorm, diethyl ether and ethyl acetate respectively.

Identification.

1. Gelatin test. Solutions of tannins precipitate a solution of gelatin.

2. Tannins also precipitate alkaloids.

3. With iron-ammonium alum, hydrolysable tannins give blue-black colour and condensed T. brownish-green colour.

4. Condensed tannins with vanillin and concentrated hydrochloric acid or 70% sulfuric acid give red colour.

5. With lead acetate in acetic acid hydrolysable tannins precipitate, and condensed stay in solution.

6. Free ellagic acid can be identified with sodium nitrite and 3-4 drops of acetic acid, a red-violet colour develops.

7. Bound ellagic acid can be determined with sodium nitrite and hydrochloric or sulfuric acid. Develops red colour which becomes blue.

8. Chromatography is used to identify low-molecular tannins.

Biological action and uses of tannins.

Tannins may be employed medicinally in antidiarrheal, haemostatic, and antihaemorrhoidal compounds.

The anti-inflammatory effect of tannins helps control all indications of gastritis, esophagitis, enteritis, and irritating bowel disorders. Diarrhea is also treated with an effective astringent medicine that does not stop the flow of the disturbing substance in the stomach; rather, it controls the irritation in the small intestine.

Tannins not only heal burns and stop bleeding, but they also stop infection while they continue to heal the wound internally. The ability of tannins to form a protective layer over the exposed tissue keeps the wound from being infected even more. Tannins are also beneficial when applied to the mucosal lining of the mouth.

Tannins can also be effective in protecting the kidneys. Tannins have been used for immediate relief of sore throats, diarrhea, dysentery, hemorrhaging, fatigue, skin ulcers and as a cicatrizant on gangrenous wounds. Tannins can cause regression of tumors that are already present in tissue, but if used excessively over time, they can cause tumors in healthy tissue. Tannins are used indirectly as molluscicides to interrupt the transmission cycle of schistosomiasis.

They have also been reported to have anti-viral effects. When incubated with red grape juice and red wines with a high content of condensed tannins, the poliovirus, herpes simplex virus, and various enteric viruses are inactivated.

Tannins can also be used to pull out poisons from poison oak or from bee stings, causing instant relief. The tannins help draw out all irritants from the skin because tannin is an astringent that tightens pores and pulls out liquids.

MP and MPM containing tannins

Folia Rhois coriariae

Rhus coriaria- Sumach, Sicilian sumac

Anacardiaceae (cashew or sumac family)

Constituents. Sumac leaves contain 13-25% of tannins, flavonoids, ether oil, ascorbic acid.

Uses. Sumac leaves are used to get tannin, which is part of Galascorbin and Novikov’s liquid.

Folia Cotini coggygriae

Cotinus coggygria- Common Smoketree, Smokebush

Anacardiaceae

Geographical distribution. Native to southern Europe and central China.

Constituents. Gallotannin, flavonoids, volatile oils

Uses. Medicines- Galascorbin has astringent and P-vitaminic action, Flacumin, which contains up to 75% of flavonoids and has cholagogue action.

Rhizomata Bergeniae

Bergenia crassifolia- Elephant’s ears, Siberian tea

Saxifragaceae

Geographical distribution. East Asia and North-West China to Siberia.

MPM description. Horizontal rhizome is ramified, 1.5-3 cm in width and up to 2m long, dark reddish-brown, light-brown inside.

Constituents. MPM contains blend of gallotannins and condensed tannins (21-25%), arbutin (up to 5%), katekhin, gallic acid, traces of rutin and quercetin.

Uses. Decoction is used to treat infectious colitis and enterocolitis, externally- to treat mouth ulcers. In gynaecology- to treat cervical erosion.

Fructus Alni

Alnus incana- Grey or Speckled Alder

Alnus glutinosa- Black Alder, European Alder or Common Alder

Betulaceae

Geographical distribution. Grey or Speckled Alder is a species of alder with a wide range across the cooler parts of the Northern Hemisphere. Common Alder is an alder native to most of Europe, including all of the British Isles and Fennoscandia and locally in southwest Asia.

Constituents. MPM contains ellagotannins, gallotannins, free gallic and ellagic acids up to 15%.

Uses. Fruits are used as astringent, anti-inflammatory and haemostatic agent; they are part of stomach mixes, “Kamilal” (suppositories). “Altan” is used as anti-inflammation and pain-killing drug to treat indigestion.

Cortex Quercus

Quercus robur- Pedunculate Oak or English oak

Fagaceae

Geographical distribution. It is native to most of Europe, and to Asia Minor to the Caucasus, and also to parts of North Africa.

Description

The commercial bark occurs as channelled pieces have a smooth, greyish-green cork with lenticels, older pieces have a greyish-brown rhytidome and show a fracture, granular in the outer part and fibrous and splintery in the inner part. Conspicuous features of the reddish-brown powder include cork cells, lignified fibres with crystal sheaths of calcium oxalate, pitted sclereids and cluster crystals of calcium oxalate in parenchymatous cells.

Constituents. Bark contains tannins, mainly of condensed type (12%), free gallic and ellagic acids, flavonoids, saponins, carbohydrates.

Uses. Oak bark is used medicinally for its astringent and anti-inflammation properties to treat gingivitis and stomatitis, burns and frostbite. Oak bark is included to anti-haemorrhoidal mixes and complex medicines.

Fructus Myrtilli

Folia Myrtilli

Vaccinium myrtillus- Bilberry

Ericaceae (Heather family)

Geographical distribution. Bilberries are extremely difficult to grow and are thus seldom cultivated. Fruits are mostly collected from wild plants growing on publicly accessible lands, notably Finland, Sweden, Norway, Scotland, Wales, Ireland, parts of England, Alpine countries, Carpathian Mountains in Ukraine, Poland and northern parts of Turkey and Russia.

The edible sweet-tasting berries are collected from July to September.

Constituents. Fruits contain condensed tannins (12%); anthocyanidins, organic acids, carotene, pectin.

Leaves contain condensed tannins (7-20%), phenols- neomyrtillin, myrtillin (2%), arbutin (1%), flavonoids.

Uses. Fruits are used as mild astringent agent to treat diarrhea in children and acute enterocolitis in adults. Myrtillin reduces sugar level in blood and thus is part of antidiabetics collection “Arphazetin”.

Prepared by assist. Dakhym I.S.