2.8 Saponin Glycosides
In general a group of plant glycosides commonly referred to as saponin glycosides, usually share in different extents, the following two specific characteristics namely:
(a) They produce foam in aqueous solution, and
(b) They cause haemolysis of Red Blood Corpuscles (RBC).
The saponin glycosides are broadly regarded as haemotoxic in nature by virtue of the fact that
they afford the haemolysis of erythrocytes, which render most of them as ‘fish poisons’. Invaribaly, they possess a bitter and acrid taste, besides causing irritation to mucous membranes. They are mostly amorphous in nature, soluble in alcohol and water, but insoluble in non-polar organic solvents like benzene, n-hexane etc.
Interestingly, the naturally occurring plant materials consisting of saponin glycosides have been extensively employed in various parts of the globe for their exclusive detergent characteristics, for instance: In South Africa the bark of Quillaia saponaria belonging to family Rosaceae and in Europe the root of Saponaria officinalis belonging to family Caryophyllaceae.
Sapogenins—The aglycone of the saponin glycosides are collectively known as sapogenins. Sapotoxins—the harmful and poisonous sapogenine/ saponins are aften referred to as sapotoxins. Based on the nature of the ‘aglycone’ residue present in the saponin glycosides, they are broadly classified into the following two categories, namely:
(i) Tetracyclic triterpenoid saponins (or Steroidal saponins), and
(ii) Pentacyclic triterpenoid saponins.
These two categories of saponin glycosides will be discussed with suitable examples from plant sources in the sections that follows: