2834 J. Agric. Food Chem., Vol. 57, No. 7, 2009
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Jurzysta, M. Antimicrobial activity of saponins from Medicago
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(10) Bialy, Z.; Jurzysta, M.; Mella, M.; Tava, A. Triterpene saponins
from aerial parts of Medicago arabica L. J. Agric. Food Chem.
2004, 52, 1095–1099.
A very interesting feature of saponins from M. arabica
described in this study is the identification of queretaroic acid
and its 2ꢀ-hydroxy derivative, 2ꢀ,3ꢀ,30-trihydroxyolean-12-en-
28-oic acid, as new aglycons for saponins of Medicago species.
The most common aglycons isolated so far within the Medicago
genus are medicagenic acid, zhanic acid, bayogenin, hederage-
nin, caulophyllogenin, oleanolic acid, 2ꢀ-hydroxy oleanolic acid
and soyasapogenols (3). Recently the 2ꢀ,3ꢀ-dihydroxy-23-oxo-
12-en-28-oic acid has been identified as a novel aglycon in M.
arborea (12) and M. hybrida (13).
Distribution of queretaroic acid is rather rare in plants. This
triterpenoid was first isolated from the Mexican cactus Le-
maireocereus queretaroensis (30) and subsequently found in
several other cacti (31-33), suggesting a systematic relevance
of its presence in such plants. Queretaroic acid has been found
sporadically as the aglycon component of saponins from plants
belonging to the Convolvulaceae (Pharbitis nil) (34), Verben-
aceae (Clerodendron serratum) (35) or Chenopodiaceae (Che-
nopodium quinoa) (36) families.
Queretaroic acid has the olean-12-ene skeleton and, together
with glycyrrhetic acid, is one of the few naturally occurring
triterpenes which is oxygenated at C-30. Isolation of this
compound in M. arabica should therefore be considered of
importance. As it described above and shown in Figure 2, the
olean-12-ene skeleton characterizes all the saponins extracted
from the aerial parts of M. arabica, and in general all the
saponins isolated from Medicago species (3). Queretaroic acid
is supposed to be synthesized in vivo by a CYP P450 dependent
hydroxylation of oleanolic acid (37). In this context, if a
biosynthetic pathway leads from oleanolic acid (VII) to queretaroic
acid (II), 2ꢀ,3ꢀ,30-trihydroxyolean-12-en-28-oic acid (I) should also
be considered as likely to originate from a CYP P450 dependent
hydroxylation of 2ꢀ-hydroxy oleanolic acid (V). To our knowledge,
this last new sapogenin has never been found in plants.
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