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or in its galloyl moieties would enormously enhance the sensitivity and
specificity of such a test system.
3.7.2 Biosynthesis of a monomeric ellagitannin, tellimagrandin II
Extended screening programs based on the above strategy, employing
[
U
-
14
C]pentagalloylglucopyranose (
3
, Rausch and Gross, 1996) as
enzyme substrate, finally led to the discovery of a novel enzyme in
leaves of
Tellima grandiflora
,
a weed that is known as a rich source of
ellagitannins. As shown in Fig. 3.9 (panel A), several radioactive
products were formed from this substrate. Degradation of prominent
products among these afforded [
14
C]labeled glucose, [
14
C]gallic acid (
1
)
and [
14
C]ellagic acid (
6
, see Fig. 3.9, panel B). The major product was
identified as the monomeric ellagitannin, tellimagrandin II (
4
, Niemetz
et al.
, 2001).
Detailed investigations following this principal finding (Niemetz and
Gross, 2003a) led to the characterization of a soluble enzyme that
catalyzed the stereoselective oxidative coupling of the galloyl residues at
the 4,6-position of pentagalloylglucopyranose (
3
) to the (
S
)-HHDP unit
(
5
) of tellimagrandin II (
3
, see Fig. 3.10). The reaction was oxygen
dependent and did not require any other cofactor. Hydrogen peroxide
was a strong inhibitor, suggesting that the enzyme was not a peroxidase.
In contrast, carbon monoxide had no effect, for neither requirement for
NADPH nor association with the microsome fraction was observed. It
was thus concluded that the enzyme did not belong to cytochrome P450
monooxygenases, but was a member of the vast class of O
2
-dependent
phenol oxidases. Inhibitor studies led to the assumption that it had the
characteristics of a laccase, a class of enzymes that is common to fungi,
but is less distributed in plants (Mayer and Staples, 2002).
3.7.3 Biosynthesis of a dimeric ellagitannin, cornusiin E
Analysis of some unexpected side-products in these experiments,
obtained with crude enzyme preparations from
T. grandiflora
, indicated
the rather specific formation of another oxidation product derived from
