Saponins from Triterpenes

Saponins are plant compounds, close relatives of the cardiac glycosides. They consist of a sterol or triterpenoid joined to a sugar. They make a strong foam when shaken with water. They are bitter-tasting and generally toxic to lower forms of life. They have been much used by primitive people for killing fish.

The defensive glands of adult beetles of Platyphora, Leptinotarsa and Desmogramma have been found to contain triterpene saponins. Since insects cannot make triterpenes, and the plants they were feeding on do not contain saponins, bio synthetic studies were made to find how the insects acquired them. Feeding Platyphora kollari with [2,2,3-2H3]|3-amyrin (a- and (3-amyrin are widely distributed plant triterpenoids) produced up to 40% of the labelled saponin shown in Figure 7.13. The enzymes necessary for the oxidation of a methyl group of (3-amyrin and coupling of the product with sugars are generally present in all living organisms.

deuterated ß-amyrin

HOOCV

HOHgC^O ^VOT

HOHgC^O ^VOT

a triterpenoid saponin

Figure 7.13 By feeding deuterated p-amyrin to adult Platyphora kollari beetles, they have been shown to be able to convert it into a labelled saponin

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