ANTI-NUTRITIONAL CONSTITUENT OF COLOCASIA ESCULENTA ...

12.1.2.3 Steroidal saponin
The triterpenes and the plant steroids are broadly descnbed as saponins (Kelly, 2005).
Sterols fonn an important group among the steroidal saponins. Like triterpenes,
phytosterols are synthesized in the isoprenoid pathway. Triteq>enes and plant sterols are
differentiated by the strncture of the carbon skeleton: triterpene saponins have a 30­
carbon skeleton and sterols have a 27/29-earbon skeleton (Ukpabi and Ukpabi, 2003).
Sterols were initially descnbed as compounds having a structure similar to that of
cholesterol, but this definition did not take their stereochemistry into account. Nes (1977)
proposed a new definition, descnbing sterols as compounds developing from squalene or
its oxide by a cyclization process. Therefore, the biosynthetic route to plant sterols also
follows an isoprenoid biosynthetic pathway with isopentenyl pyrophosphate, derived
from mevalonate, as the key building block (Piironen, 2000). However, in photosynthetic
organisms (as opposed to yeast and fungi), it differs in that the important intermediate in
the route from squalene is cycloartenol rather than lanosterol (GIbbons et at., 1971). The
basic sterol from which other sterol structures are defined is 5a-cholestan-31J-01 (Sterols,
2008). The phytosterols include campesterol, beta-sitosterol, and stigmasterol etc.
Steroidal saponins are comprised of two major classes: furostanol glycosides with an
open side chain at C-22 and spirostanol glycosides with a closed spiroketal ring at C-22
(Figure B2-1.4). Furostanol glycosides are the foundation from which spirostanol
glycosides are fonned through hydrolysis ofthe C-26 sugar and spontaneous cyclisation
ofthe side chain from the spiroketal (Inoue and Ebizuka, 1996).
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