Biology Reference
In-Depth Information
4.1 Introduction
Plant polyphenols, including mainly hydrolyzable and condensed
tannins, are widely distributed in the plant kingdom and thought to be
biosynthesized and accumulated as defensive compounds against
microorganisms, herbivores, and oxidative stress (Barbehenn
et al.
,
2005, Haslam, 1996, Scalbert, 1991, Robbins
et al.
, 1987). Among plant
polyphenols, tannins have the characteristic property of precipitating
proteins, which leads for example to the unpleasant astringent taste and
inhibition of digestive enzymes in the digestive tract of plant predators.
Although the toxicity of tannins is lower than that of qualitative defense
compounds, such as toxic alkaloids and terpenoids, tannins are
accumulated in much higher concentration in many cases and are
designated as quantitative defense compounds (Zucker, 1983). The
inhibition of food digestion by tannins is sometimes severe for animals
(Mehansho
et al.
, 1983, 1987). However, paradoxically, it is regarded as
being beneficial for human health in developed countries, because the
inhibition of digestive enzymes decreases the excess absorption of sugars
and triglycerides from the intestine, hence lowering plasma sugar and
lipid levels (Hara and Honda, 1990, Nakai
et al.
, 2005). The
physicochemical nature of the interaction of tannins with proteins,
especially with salivary proline-rich proteins, has been extensively
studied and the results unveiled the importance of the hydrophobic
association in aqueous media (Charlton,
et al.
, 1996, 2002a, 2002b,
Baxter
et al.
, 1997, Luck
et al.
, 1994, Cai
et al.
, 1990). The
hydrophobicity of tannin molecules strongly depends on their structure,
and it was shown that the molecular flexibility rather than the number of
phenolic hydroxyl groups is the most determinant factor.
Compared to gallo- and condensed tannins, ellagitannins exhibit a
great structural diversity. Although the overwhelming majority of
ellagitannins feature β-
D
-glucose as their core unit, ellagitannins based
on other cyclic polyols (Nishimura
et al.
, 1984a, 1986),
O
- or
C
-
glucosides (Nishimura
et al.
, 1984b, Tanaka
et al.
, 1993a, 2005], 1-
O
-
caffeoyl glucose (Jiang
et al.
, 2001) and even hydroxytriterpenes (Ageta
et al.
, 1988; Chen
et al.
, 1993) have also been identified. As for the acyl
groups of ellagitannins, their characteristic hexahydroxydiphenoyl
