Saponins and Cardenolides

Introduction Steroidal saponins Triterpenoid Saponins Biological Activity of Saponins Saponins in Food and Forage Plants Saponins and Insects Medicinal Uses of Saponins Alteration of Taste Properties Sweeteners Cardenolides Biosynthesis 

Distribution of Cardiac Glycosides Biological Activity of Cardiac Glycosides Interactions with Insects and Other Animals Medicinal Properties Arrow Poisons References 

Saponins are classified into two major groups based on the structure of the aglycones that are known collectively as sapogenins. Steroidal sapogenins usually contain 27 carbon atoms those of triterpenoid sapogenins contain 30. Approximately 360 sapogenins and 750 triterpene glycoside structures are known, but it is only recently that full structure 

The complexity of saponins results from the variety of combinations of the different sapogenins and numerous sugars. For example, the saponins of alfalfa, Medicago sativa, (Fabaceae) vary in composition as well as in absolute amount. Some alfalfa varieties contain as many as 30 different saponins (Applebaum and Birk, 1979). 

Many of the biological properties for which saponins are known are due to this hydrophobic/hydrophilic asymmetry and consequent ability to lower surface tension (Gershenzon and Croteau, 1991 Hostettmann et al., 1991). In general, both the sugar and the aglycone are necessary for activity. Saponins have soap-like properties in aqueous solutions. This is a major reason for acute toxicity when they are injected intravenously. These compounds have been used to capture fish (as piscicides) since ancient times (Howes, 1930). Saponins in the diet are more toxic to cold-blooded than to warm-blooded animals. Most saponins hemolyze red blood cells (Marston and Hostettmann, 1991). 

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Glvcosidases and Thioglvcosidases. Of the glycosides produced by plants (e.g., iridoids, saponins, phenolics, cardenolides, and alkaloids), perhaps the best studied in terms of influence upon insects and other animals are the cyanogenic glycosides and the glu-cosinolates (2.17-19). 

Xysmalobium undulatum (L.) Ait. F. [lishogwe, milk bush](Asclepiadaceae) tuber infusion is used for diarrhoea and headaches. The tuber contains flavonoids, glycosides, polyphenols, saponins and steroids 15). The steroids contained in the tuber are cardenolides 40). 

Asclepias fruticosa L. [lubetjane, milkweed] (Asclepiadaceae) stem bark concoction is used in the treatment of asthma. The stem bark contains alkaloids, flavonoids, polyphenols, saponins and steroids (15). The aerial parts contain cardenolides (55). The powdered leaf is used as a snuff for the treatment of pulmonary tuberculosis, producing violent and prolonged sneezing in the process (28). 

Annona senegalensis stem bark is used for the treatment of hysteria and a constituent of its root bark has been found efiective for treating cancer using sarcoma 180 ascites cells (75). The bark and root are used in the treatment of sexually transmitted diseases (18). The root bark contains cardenolides, glycosides and saponins. The stem bark contains anthranoids, cardenolides, glycosides saponins and terpenoids (50,18). Its fraits are edible and are taken for diarrhoea, dysentery and vomiting (23,33). 

In mammals, the first and limiting step in the bios)mthesis of all C21 and C20 steroids is the conversion of cholesterol into pregnenolone. Cholesterol is also supposed to be a precursor of pregnanes, cardenolides and steroid saponins in plants. Analogous to the formation of steroids in animals, this reaction is thought to be catalysed by side-chain cleavage cytochrome P450scc (SCCE). 

Anthospermum rigidum Eckl. Zeyh. [sambulela] (Rubiaceae) leaf infusion is also used for pubic lice. The leaf contains flavonoids, polyphenols and tannins (75). Studies have also shown that apart from the antibacterial properties of these plants, they also have additional medicinal values. The root bark of Andrachne ovalis shows insecticidal activity and is a remedy for snake bites and epilepsy (16,28). The alcohohc extract of the leaves is strongly molluscicidal and can be used in the vector control of hilharzia (67). The leaf decoction is used for persistent dizziness (11). The stem bark contains alkaloids, anthranoids, cardenolides and polyphenols. The root bark contains alkaloids, cardenohdes, polyphenols and saponins (50). 

The present volume reflects these developments, and there is a growing emphasis on bioactive natural products. Articles in this volume include those on structure-activity relationships of highly sweet natural products, chemical constituents of cchinodenns, diterpenoids from Rabdosia and Eremophila sp., structural studies on saponins, marine sesquiterpene quinoncs and antimicrobial activity of amphibian venoms. The reviews on bioactive metabolites of Phomopsis, cardenolide detection by ELISA, xenocoumacins and bioactive dihydroisocoumarins, CD studies of carbohydrate-molybdate complexes, oncogene function inhibitors from microbial secondary metabolites and Gelsemium and Lupin alkaloids present frontier developments in several areas of natural product chemistry. It is hoped that the present volume, which contains articles by eminent authorities in each field, will be received with the same enthusiasm as the previous volumes of this series. 

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