Saponins and Cardiac Glycosides


Saponins


Saponins are glycosides of triterpenoids or steroids. They are common in plants and are found in almost all groups of plants.

Many saponins are piscicides, that is, they have the ability to stun fish.

Saponins contribute to the toxicity of alfalfa and limit the usefulness of this forage crop in animal diets.

Saponins are also diverse in ginseng. Many of the effects of this common drug plant are due to these compounds. The anti-inflammatory activity of English ivy, Hedera helix, also is due to compounds of this group.

The sweet tasting compounds of licorice, Glycyrrhiza spp., are saponins. In large amount these compounds cause irregularities in heart action. Gymnemic acids from Gymnema sylvestre (Asclepiadaceae) suppress the perceived sweetness of sugar and to a lesser extent amino acids. Chewing the root of this plant blocks the sweet taste of sugar, saccharin and cyclamates for hours.

A recent popular herbal medicine, Tribulus terrestris, also contains saponins that are thought to be similar in action to certain steroidal compounds used by athletes.

The C27 steroidal saponins of Dioscorea species serve as chemical precursors for the synthesis of anti-inflammatory compounds and contraceptive drugs.

Cholesterol appears to be a usual precursor for steroidal saponins.

Saponins have soap-like properties. They are toxic in large quantities to many animals, but are also found in small quantities in most of the plants eaten by herbivores. Saponins cause changes in the microstructure of cell membranes and they lyse red blood cells.

Many saponins have molluscicidal activity. Those of Phytolacca species may have efficacy for control of snails that are intermediate hosts in the disease schistosomiasis.

Pentacyclic triterpene glycosides have been shown to be phytoalexins in oats.


Cardiac glycosides or cardenolides


Cardiac glycosides are glycosides of mostly C23-steroidal compounds. They have a characteristic 5- or 6-membered lactone ring. They are called cardiac glycosides because they modify heart action. Cardenolides inhibit the Na+-K+-ATPase pump in mammals.

This group of compounds is found in a large number of families, many of which are unrelated. There are about 400 known cardenolides.

A number of toads and frogs make cardiac active compounds that are steroidal, but not glycosidic, in nature.

Cardenolides are derived from steroidal precursors, probably cholesterol, via the intermediacy of pregnenolone or progesterone intermediates. The exact pathway does not seem to have been established with certainty.

Many questions about the probable acetate/malonate origin of the five or six membered lactone ring.

Most members of the family Asclepiadaceae contain cardiac glycosides. Many of the insects that feed on this plants sequester the glycosides. The story of monarch butterflies and milkweeds is well known. In this case the larvae eat the plant, and sequester a different complement of cardenolides from that which occurs in the plants. The glycosides are passed on to the adult insects. The adults are marked with warning coloration (aposematic labelling) and are not eaten by most predators. However, in the area in Mexico in which the butterflies overwinter, there are bird predators that eat the adults without apparent ill effects. Naive blue jays will eat one adult, but will reject others in subsequent feeding trials.

Many other insects that feed on plants of the Asclepiadaceae and Apocynaceae (recent data suggest that these should be considered confamilial as the Apocynaceae).

Several plants with cardiac glycosides or cardenolides are used medicinally. Among these are Digitalis species (Scrophulariaceae) and ouabain (Strophanthus species) (Apocynaceae). These plants are often used to treat heart problems.

A number of plants and insects that contain cardiac glycosides have been used to prepare arrow poisons.


Related Images

Assorted saponins

Sapogenins from soy beans and other plants

Arrow poisons from Lophopetalum toxicum (Celastraceae)

α-Hederin and hederosaponin

Sapogenins

Sweet tasting saponins

Cardiac glycosides

More cardiac glycosides

Additional cardiac glycosides



Lecture Slides

Plants with Saponins and Cardiac Glycosides



© David S. Seigler, Integrative Biology 425, Plant Secondary Metabolism, Department of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA. 217-333-7577. seigler@life.uiuc.edu.