Saponin

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Saponins are glycosides of steroids, steroid alkaloids (steroids with a nitrogen function) or triterpenes found in plants, especially in the plant skins where they form a waxy protective coating. Some authors ditinguish a third class of saponin the alkaloid saponins. They dissolve in water to form a stable soapy froth, this is thought to be due to their amphiphillic nature. Saponins are believed to be useful in the human diet for controlling cholesterol, but some (including those produced by the soapberry) are poisonous if swallowed and can cause urticaria (skin rash) in many people. Any markedly toxic saponin is known as a sapotoxin.

Saponins have been identified in:

Soapberry and many other members of the family Sapindaceae, including buckeyes
Soapwort
Conkers/horse chestnuts
Digitalis (as digitonin)
Grape skin[1]
Gotu Kola
Chickweed
Olives
Panax (as ginsenoside)
Jiaogulan
Quillaia saponaria (member of the Rosaceae family)
Rambutan
Soybeans
Yucca
Liquorice
Aloe
Quinoa
Bacopa monnieri
Chlorophytum species
Chlorogalum species, soap plants
Tuberous cucurbit species
Echinodermata
Tribulus terrestris (as protodioscin)
Asparagus (as protodioscin)

and many other plants used in medicine or as food items.

In particular the saponins from Quillaia saponaria are used in veterinary vaccines as adjuvant(eg. Foot-and-mouth disease vaccines, helping to enhance the immune response). Initially the crude fraction was used. Later on a purified mixture, called Quil A was developped by Dalsgaard which was more effective and caused less local side reactions. Still Quil A is a mixture of more than 25 different saponin molecules. One of them, the saponin QS21, is being investigated for possible beneficial adjuvant effects on the human immune system.

Saponins are also mild detergents and are used commercially aswell as for research. They are used in the British museum as a mild detergent to gently clean ancient manuscripts. In laboratory studies saponins can be used at 0.04%-0.2% to permeabilize ("make holes in") the plasma membrane as well as the membranes of internal organelles such as ER and Golgi but does not penetrate the nuclear membrane. Therefore it is used in intracellular histochemistry staining to allow antibody access to intracellular proteins.

Because of its reversible nature on cells and its ability to permeabilize cells without destroying cell morphology,it is used in laboratory applications to treat live cells in order to facilitate peptide or reagents such as antibodies to enter cells instead of the harsher detergent triton X-100. It is also done on whole cell preparations such as cell smears and cytospins where the cell membrane is intact. It can also be done on frozen sections but is not used on fixed tissue sections. To preserve the permeabilizing effect, saponin has to be used in all processes involved in the staining steps or otherwise removed after reagent of interest has reached the cell.