Saponins are naturally occurring chemical compounds, with the properties of soaps and detergents. They occur in different tissues of a large number of plant species including many common Indian food and medicinal plants. Although predominent in angiosperms, saponins also occur in some ferns (species of Polypodium and Cyclamen) and possibly algae. They even occur in some marine animals and snake venom as well.

During the course of our work we observed that saponins have varied and important therapeutic benefits, and also that a large number of food and medicinal plants contain saponins. The saponin containing species of food and medicinal plants discovered during our work through erythrocyte lysis (Sathyananda, 1989; Sharu Raj, 1990; Sathyanarayana Bhat, 1993; Sharon, 1994; Shubha Rani, 1995), are indicated in Appendices 32, 33 and 34.

Despite their importance in various contexts, saponins are relatively unknown. This account is an effort to draw attetion to them, in view of their varied therapeutic effects.

The general account of saponins given here was summarised mainly from Gibbs (1974), Birk (1969), Oakenfull and Sidhu (1989), Hostettmann et al., (1991), Sangeetaa and Kameswara Rao (1993), Kameswara Rao and Sangeetaa (1993), Sangeetaa (1994) and Kameswara Rao and Sangeetaa (1997).

CHARACTERISTICS OF SAPONINS

Saponins are generally identified by their bitter taste, ability to foam in aqueous solutions and to lyse erythrocytes. However, there are exceptions. While some saponins do not foam, a few others do not lyse erythrocytes. Glycyrrhizin, the saponin from the roots of Glycyrrhiza glabra and leaves of Abrus precatorius, is 60 times sweeter than cane sugar and is a non-calory alternate sweetener.

CHEMICAL NATURE OF SAPONINS

Saponins are triterpenoid or steroid compounds. In nature, they are complexed with simple sugars, which are not essential for their biological activity. More than one kind of saponins may occur in the same species. Tea plant has five different saponins; soybean has at least four. Both triterpenoidal and steroidal saponins may occur in the same species.

The molecular weights of saponins range from 1000 to 1500 daltons (Dorsaz and Hostettmann, 1986).

Saponins are synthesised by a very common metabolic pathway starting from acetyl coenzyme A. Mevalonic acid and then squalene are the intermediary products for both the triterpenoidal and steroidal saponins. Upto the point of formation of squalene, the synthesis of cholesterol (and other steroids) and saponins proceeds on the same lines.

DETECTION OF SAPONINS

The conventional methods of detecting saponins in plant tissues are not very sensitive and resulted in missing their detection, particularly when they occur in minute parts of plants and/or in minute quantities. During our lectin assay we found that erythrocytes, particularly the erythrocytes of the snake head fish, are very useful in detecting saponins (Kameswara Rao and Sangeetaa, 1993). Using this method we discovered saponins in a large number of species of food and medicinal plants, that were largely missed by earlier workers, particularly in very small parts like stigmas and microscopic parts like pollen (Sangeetaa, 1991; Kameswara Rao and Sangeetaa, 1993; Sangeetaa and Kameswara Rao, 1993; Sangeetaa, 1994). The sensitivity of erythrocytes to lysing by saponins is in the increasing order of human, animal and fish.

TOXICITY OF SAPONINS

Saponins are toxic to a variety of organisms, from bacteria to higher plants and animals. Populations of aquatic animals such as fish and molluscs are controlled by using saponin containing plants. Saponins are also toxic to different degrees to mammals particularly, when introduced into the circulatory system. Saponins mainly act through solubilising and destabilising the membrane systems of cells, by complexing with the steroidal componets of the membranes.

Toxicity of saponins to humans has been over emphasised. Saponins in plant (parts) used as food and herbal medicine are being consumed by us regularly for ages, without any discernible adverse effects.

USES OF SAPONINS

One of the most traditional uses of saponins is as a cleansing agent. The common soapnut (Shikakai; Acacia concinna) and the south Indian soapnut (Reetha; Antawala; Kunkudu; Sapindus emarginatus) are minor forest products abundant in saponins, that have traditionally been used to wash hair and delicate cloth. Although saponins were not realised as the chief agents, several saponin containing plants have been a part of many Ayurvedic formulations. They have several applications in research. Over years the industrial and therapeutic applications of saponins have grown to be very many.