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content of 60-70% ginkgolides and bilobalide.
40
Having the crude mixture of gink-
golides (and bilobalide), it is a challenge to isolate the individual ginkgolides. A
simple improvement in separation has been achieved by van Beek and Lelyveld
with silica gel impregnated with sodium acetate, which disrupts the internal hydro-
gen bonds between the hydroxyl groups.
41
Because ginkgolides lack common chromophores, ultraviolet detection is not
suitable and other detection methods such as refractive index (RI), evaporation light
scattering detection (ELSD), and mass spectrometry (MS) have been used. Several
recent reports use high performance liquid chromatography (HPLC) MS to separate
and quantify the content of ginkgolides and bilobalide, using either electrospray
ionization (ESI)
42
or atmospheric pressure chemical ionization (APCI).
43,44
As
an alternative to MS, ELSD has also been successfully applied to quantify ginkgo-
lides and bilobalide in G. biloba extracts.
45,46
7.1.3 Biosynthesis of Ginkgolides
Early attempts to reveal the biosynthetic route of ginkgolides, using [2-
14
C]-acetate
and dl-[2-
14
C]-mevalonate, suggested the overall terpenoid origin of ginkgolides,
and it was believed that ginkgolides were biosynthesized through the conventional
mevalonate pathway.
47
However, only recently it was realized that the two conven-
tional precursors, dimethylallyl pyrophosphate (DMAPP) and isopentenyl pyropho-
sphate (IPP), can participate in what is now known as the nonmevalonate pathway.
Independently, Rohmer and Arigoni et al. showed the existence of the nonmeva-
lonate or deoxylulose phosphate pathway, in which pyruvate and glyceraldehyde
3-phosphate react to produce 2C-methyl-
D
-erythritol 2,4-cyclodiphosphate and ulti-
mately DMAPP and IPP.
48-51
Actually, Arigoni et al. were studying the ginkgolide
biosynthesis with a G. biloba embryo system and
13
C-labeled glucose, and they
showed that ginkgolides were biosynthesized via the nonmevalonate pathway.
49
O
O
P
O
O
O
O
O
O
O
P
H
O
P
O
O
OH
OH
OH
pyruvate
glyceraldehyde
3-phosphate
2
C
-methyl-D-erythritol
2,4-cyclodiphophate
O
P
P
DMAPP
ginkgolides
O
P
P
IPP
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