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Addition of 1 led to a digestibility reduction of 23% (Figure
5). The same inhibition was observed when 3 or (-)-4 [(R)-4]
was added (25 and 23%, respectively). Previous results have
shown that (S)-2, the metabolite formed by hydrolysis of the
glucoside 1, possesses the same effect on the digestibility as 1
(10). A much higher activity was found for (+)-4 [(S)-4], which
reduced the digestibility 46%. The activity of the racemate
(37%) corresponds to an average between (S)- and (R)-4.
5, the compound found in feces but not in rumen fluid,
possessed the highest activity of the compounds tested, exhibit-
ing 57% inhibition.
Active Metabolite in the Rumen Fluid. Our conclusion
about the active metabolite formed in rumen fluid is based on
the results from the present and earlier studies (7, 10). In
summary, in rumen fluid in vitro, three or four metabolites [(S)-
2, 3, and one or both enantiomers of 4] are formed from 1 (7).
Thus, at least one of these five compounds must cause the
observed reduction of digestibility. From studies of the activity
and metabolite concentrations versus incubation time, the
activity was found to be linearly correlated to the concentration
of 4 (in this study the stereochemistry was not determined) (7).
This indicates that either (S)- or (R)-4 or both enantiomers is/
are active. In another study (10), it was shown that incubation
with (()-2 gave a lower digestibility inhibitory activity than
(S)-2. It was also found that (S)-2 was totally metabolized to 3
and 4, whereas one-fourth of the racemic 2 remained unreacted
within the incubation time. Even in this study the activity was
correlated to the concentration of 4. Addition of (R)-4 gave the
same digestibility reduction as addition of 1 or 3, whereas (S)-4
gave a much higher activity (Figure 5). Racemic 4 gave the
activity corresponding to an average of (R)- and (S)-4. This
shows that (R)-4 and not the (S)-enantiomer is formed in the
rumen fluid. Addition of 1, (S)-2, 3 or (R)-4 gives the same
activity in vitro (Figure 5 and ref 10). This would be the case
if 1 and the two first formed metabolites are inactive but
converted to the last and active one, (R)-4, within the incubation
time. Our conclusions based on these results are that (R)-
centrolobol is formed in the rumen fluid in vitro and is the active
metabolite causing the digestibility-inhibiting effect observed
from platyphylloside.
ACKNOWLEDGMENT
Bo¨rje Ericson is acknowledged for performing the digestibility
tests.
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H.; Ueno, T.; Tsurushima, T. Synthesis of a germination self-
inhibitor, (-)-gloeosporone, and related compounds and evalu-
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Chem. 1974, 39, 3904-3906.
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Received for review March 15, 2004. Revised manuscript received June
18, 2004. Accepted June 22, 2004. Pharmacia & UpJohn, the Swedish
Council for Forestry and Agricultural Research, and the Swedish
Hunter’s Association are acknowledged for financial support.
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