In Chemistry, Glycosides Are Certain Molecules in Which a Sugar Part Is

GLYCOSIDES Glycosides may be defined as the organic compounds from plants or animal sources, which on enzymatic or acid hydrolysis give one or more sugar moieties along with non- sugar moiety. Glycosides play numerous important roles in living organisms. Many plants store important chemicals in the form of inactive glycosides; if these chemicals are needed, the glycosides are brought in contact with water and an enzyme, and the sugar part is broken off, making the chemical available for use. Many such plant glycosides are used as medications. In animals (including humans), poisons are often bound to sugar molecules in order to remove them from the body. Formally, a glycoside is any molecule in which a sugar group is bonded through its carbon atom to another group via an O-glycosidic bond or an S-glycosidic bond; glycosides involving the latter are also called thioglycosides. The sugar group is then known as the glycone and the non-sugar group as the aglycone or genin part of the glycoside. The glycone can consist of a single sugar group (monosaccharide) or several sugar groups (oligosaccharide). Classification Classification based on linkages Based on the linkage of sugar moiety to aglycone part 1. O-Glycoside:-Here the sugar is combined with alcoholic or phenolic hydroxyl function of aglycone.eg:-digitalis. 2. N-glycosides:-Here nitrogen of amino group is condensed with a sugar ,eg- Nucleoside 3. S-glycoside:-Here sugar is combined with sulphur of aglycone,eg- isothiocyanate glycosides. 4. C-glycosides:-By condensation of a sugar with a cabon atom, eg-Cascaroside, aloin. Glycosides can be classified by the glycone, by the type of glycosidic bond, and by the aglycone. By glycone If the glycone group of a glycoside is glucose, then the molecule is a glucoside; if it is fructose, then the molecule is a fructoside; if it is glucuronic acid, then the molecule is a glucuronide; etc. In the body, toxic substances are often bonded to glucuronic acid to increase their water solubility; the resulting glucuronides are then excreted. By type of glycosidic bond Depending on whether the glycosidic bond lies "above" or "below" the plane of the cyclic sugar molecule, glycosides are classified as α-glycosides or β-glycosides. Some enzymes such as α-amylase can only hydrolize α-linkages; others, such as emulsin, can only affect β-linkages. By aglycone Glycosides are also classified according to the chemical nature of the aglycone. For purposes of biochemistry and pharmacology, this is the most useful classification. Alcoholic glycosides An example of an alcoholic glycoside is salicin which is found in the genus salix. Salicin is converted in the body into salicylic acid, which is closely related to aspirin and has analgesic, antipyretic and antiinflammatory effects. Anthraquinone glycosides These glycosides contain an aglycone group that is a derivative of anthraquinone. They are present in senna, rhubarb and aloes; they have a laxative effect. Coumarin glycosides Here the aglycone is coumarin. An example is apterin which is reported to dilate the coronary arteries as well as block calcium channels.those obtained from dried leaves of Psoralia corylifolia have main glycosides psoralin and corylifolin. Cyanogenic glycosides In this case, the aglycone contains a cyanide group, and the glycoside can release the poisonous hydrogen cyanide if acted upon by some enzyme. An example of these is amygdalin from almonds. Cyanogenic glycosides can be found in the fruits (and wilting leaves) of the rose family (including cherries, apples, plums, almonds, peaches, apricots, raspberries, and crabapples). Cassava, an important food plant in Africa and South America, contains cyanogenic glycosides and therefore has to be washed and ground under running water prior to consumption. Sorghum (Sorghum bicolor) expresses cyanogenic glycosides in its roots and thus is resistant to pests such as rootworms (Diabrotica spp.) that plague its cousin maize (Zea mays L.). Flavonoid glycosides Here the aglycone is a flavonoid. This is a large group of flavonoid glycosides. Examples include: • Hesperidin (aglycone: Hesperetin, glycone: Rutinose) • Naringin (aglycone: Naringenin, glycone: Rutinose) • Rutin (aglycone: Quercetin, glycone: Rutinose) • Quercitrin (aglycone: Quercetin, glycone: Rhamnose) Among the important effects of flavonoids are their antioxidant effect. They are also known to decrease capillary fragility. Phenolic glycosides (simple) Here the aglycone is a simple phenolic structure. An example is arbutin found in the Common Bearberry Arctostaphylos uva-ursi. It has a urinary antiseptic effect. Rutin found in rooibos tea. Saponin glycosides These compounds give a permanent froth when shaken with water. They also cause hemolysis of red blood cells. Saponin glycosides are found in liquorice. Their medicinal value is due to their expectorant effect. Steroidal glycosides or cardiac glycosides Here the aglycone part is a steroidal nucleus. These glycosides are found in the plant genera Digitalis, Scilla, and Strophanthus. They are used in the treatment of heart diseases e.g. congestive heart failure and arrhythmia. Steviol glycosides These sweet glycosides found in the stevia plant Stevia rebaudiana bertoni have 40-300 times the sweetness of sucrose. The two primary glycosides, stevioside and rebaudioside A, are used as natural sweeteners in many countries. These glycosides have steviol as the aglycone part. Glucose or rhamnose-glucose combinations are bound to the ends of the aglycone to form the different compounds. Isothiocyanate glycosides As the name implies, these compounds contain sulfur. Examples include sinigrin, found in black mustard, and sinalbin, found in white mustard. 1 . Steroidal glycosides or cardiac glycosides Many of the plants known to contain cardiac or cardiotonic glycosides have long been used as arrow poisons (e.g. Strophanthus) or as heart drugs (e.g. Digitalis). They are used to strengthen a weakened heart and allow it to function more efficiently, though the dosage must be controlled very carefully since the therapeutic dose is so close to the toxic dose. In plants, cardiac glycosides are confined to the Angiosperms, but are found in both monocotyledons and dicotyledons. The therapeutic action of cardioactive glycosides depends on the structure of the aglycone, and on the type and number of sugar units attached. Two types of aglycone are recognized, cardenolides, e.g. digitoxigenin from Digitalis purpurea and bufadienolides, e.g. hellebrigenin from Helleborus niger The cardenolides are more common, and the plant families the Apocynaceae (e.g. Strophanthus), Liliaceae (e.g. Convallaria), and Scrophulariaceae (e.g. Digitalis) yield medicinal agents. The rarer bufadienolides are found in some members of the Liliaceae (e.g. Urginea) and Ranunculaceae (e.g. Helleborus). Biosynthesis: Formed through mevalonate and deoxyxylulose pathway Test for cardiac glycosides (Keller-Killani test): Five ml of extract was treated with 2 ml of glacial acetic acid containing one drop of ferric chloride solution. This was underlayed with 1 ml of concentrated sulphuric acid. A brown ring of the interface indicates a deoxysugar characteristic of cardenolides. A violet ring may appear below the brown ring, while in the acetic acid layer, a greenish ring may form just gradually throughout thin layer Digitalis purpurea Source:- Digitalis leaf consists of the dried leaf of the red foxglove Digitalis purpurea (Scrophulariaceae). Geographical source and collection :- The plant is a biennial herb, common in Europe and North America, which forms a low rosette of leaves in the first year, and its characteristic spike of purple (occasionally white) bellshaped flowers in the second year. It is potentially very toxic, but is unlikely to be ingested by humans. Digitalis purpurea is cultivated for drug production, principally in Europe, the first year leaves being harvested then rapidly dried at 60°C as soon as possible after collection. This procedure is necessary to inactivate hydrolytic enzymes which would hydrolyse glycoside linkages in the cardioactive glycosides giving rise to less active derivatives. Chemical constituents:- The cardioactive glycoside content of Digitalis purpurea leaf is 0.15–0.4%, consisting of about 30 different structures. The major components are based on the aglycones digitoxigenin, gitoxigenin, and gitaloxigenin, the latter being a formate ester. The glycosides comprise two series of compounds, those with a tetrasaccharide glucose– (digitoxose)s – unit and those with a trisaccharide (digitoxose)3 – unit. The latter group (the secondary glycosides) are produced by partial hydrolysis from the former group (the primary glycosides) during drying by the enzymic action of a β-glucosidase, which removes the terminal glucose. Thus the principal glycosides in the fresh leaves, namely purpureaglycoside A and purpureaglycoside B, are partially converted into digitoxin and gitoxin respectively ,which normally predominate in the dried leaf. Use:- Glycosides of the gitoxigenin series are less active than the corresponding members of the digitoxigenin-derived series. Digitoxin is the only compound routinely used as a drug, and it is employed in congestive heart failure and treatment of cardiac arrhythmias, particularly atrial fibrillation. Digitalis lanata Source:- Digitalis lanata (Scrophulariaceae), the Grecian foxglove, is a perennial or biennial herb from Southern and Central Europe, and differs in appearance from the red foxglove by its long narrow

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