TERPENOIDS AND STEROIDS 



141 



The saponins were originally named because of their soap-like characteristics. 

 They are powerful surface active agents which cause foaming when shaken with water 

 and in low concentration often produce hemolysis of red blood cells. In very dilute solu- 

 tion they are quite toxic to fish, and plants containing them have been used as fish poisons 

 for hundreds of years. They have also been implicated as a contributing cause of bloat 

 in cattle on some forage crops. Certain saponins have become important in recent years 

 because they may be obtained in good yields from some plants and are used as starting 

 material for the synthesis of steroid hormones to be used in medicine. The saponins have 

 no known function in plants but have been shown to stimulate the growth of pea embryos 

 (17). 



Two types of saponins are recognized- -glycosides of triterpenoid alcohols, and 

 glycosides of a particular steroid structure described as having a spiroketal side chain. 

 Both types are soluble in water and ethanol but insoluble in ether. Their aglycones, 

 called sapogenins, are prepared by acid or enzymatic hydrolysis and without the sugar 

 residues have the solubility characteristics of other sterols. A few of the steroidal sa- 

 pogenins are distinguished by having a cis A/Bring juncture. Steroidal saponins are most 

 common in the families Liliaceae, Amaryllidaceae and Dioscoraceae. 



The spiroketal steroid nucleus has the following structure: 



Rings E and F contain the same basic carbon skeleton as common animal steroids but 

 lack the extra carbon atoms found in most plant sterols. 



The triterpenoid saponins may have as their aglycones such compounds as oleanolic 

 acid which also occur uncombined with sugars. In some cases though, the aglycones are 

 known only as sapogenins. Oleanane-type sapogenins are much more common than either 

 ursane or lupane types. 



Some sapogenin structures are given in Table 11. Glycosylation is generally at 

 C-3. Several different monosaccharides are usually present as an oligosaccharide. 

 Uronic acids may also be present. For example, digitonin is derived from digitogenin 

 plus an oligosaccharide composed of 1 xylose, 2 glucose, and 2 galactose units. Several 

 different saponins may all have the same sapogenin but different sugars. 



CARDIAC GLYCOSIDES 



The cardiac glycosides, cardenolides, or heart poisons bear a structural resem- 

 blance to the steroid saponins and have the same solubility and foaming characteristics. 

 They are distinguished from other steroid glycosides by an unsaturated lactone ring at- 

 tached at C-17, a cis-juncture of rings C and D, a 14/3 hydroxy group, and by the peculiar 

 sugars composing them. 



