Biology Reference
In-Depth Information
1.6.2 Indexes of tannin binding activity and reversal of tannin
biological activities
The leather making activity of tannins is attributed to their aptitude to
form multiple hydrogen bonds to collagen in hide. The binding of tannins
with alkaloids has been exploited for preparing some medicines such as
complexes of tannic acid with
inter alia
berberine and diphenhydramine
in order to suppress the offensive taste of these compounds. Gallotannin
mixtures have been mainly used for this purpose because of their ready
availability. The efficacy with which a given tannin molecule binds to
hemoglobin or methylene blue relatively to that of tannic acid JP (
i.e.
,
Japanese Pharmacopoeia) (RA or RMB) or to that of geraniin (RAG or
RMBG) offers convenient indexes that serve to rapidly evaluate the
binding activities of various tannins (Okuda
et al
., 1985). The latter
indexes RAG and RMBG are more reliable than the former ones,
because of the structural uniformity of the standard compound geraniin.
The effects of tannins on enzymes can drastically vary and even be
reversed depending upon the concentration at which the tannin molecule
is used, as well as upon the structural class to which it belongs. For
example, the inhibitory effects of the ellagitannins chebulinic acid and
granatin B on
Streptococcus mutans
, a carcinogenic bacterium, at 10
-5
M
are less potent than those observed at 10
-6
M, but they are reinforced by
further increasing the concentration of these ellagitannins (Kakiuchi
et
al
., 1986). Geraniin, mallotusinic acid, chebulinic acid and chebulagic
acid enhanced adrenocorticotropic hormone (ACTH)-induced lipolysis in
fat cells at a concentration of 20 μg/ml or 5 μg/ml, but all of these
ellagitannins have no effect on the insulin-induced lipogenesis from
glucose. These activities of ellagitannins are contrary to those observed
with condensed tannins, which weakly inhibited ACTH-induced
lipolysis, whereas they enhanced insulin-stimulated lipogenesis from
glucose (Kimura
et al
., 1983).
1.6.3 Antioxidant activities
One of the most notable activities of tannins and related polyphenols is
their potent antioxidant activity (Okuda
et al.
, 1992b, 1993b, Okuda,
