Biology Reference
In-Depth Information
1997a/b, 1999b). One of the roles of tannins in plant tissues, particularly
in those around the vascular bundle where their concentration is
generally high, might have to do with the prevention or at least the
retardation of oxidation in the plant body.
The antioxidant activity of tannins was initially demonstrated by
their suppression of the autoxidation of ascorbic acid (Yoshida,
et al.
,
1981). The inhibitory effect of tannins on Cu(II)-catalyzed autoxidation
of ascorbic acid was examined by kinetic studies and ESR measurements
showing that the inhibitory effects by several ellagitannins (
e.g.
,
geraniin, mallotusinic acid and corilagin), and ellagic acid, which is
produced by hydrolysis of ellagitannins, are markedly higher than that by
polyphenols of low molecular masses, such as gallic acid, and also
significantly higher than that by pentagalloylglucose (PGG). These
effects are attributable to the potent radical scavenging activity of
ellagitannins as substantiated by signals of stable free radicals in their
ESR spectra. Unlike ellagitannins, polyphenols of low molecular masses
usually gave unstable or no ESR signals. However, the antioxidant effect
of ellagic acid, in spite of its rather small size, is quite high in accordance
with the high stability of its free radical (Fujita
et al
., 1987).
The radical scavenging capacity of ellagitannins of various chemical
structures has also been evaluated on the basis of their effects on the 1,1-
diphenyl-2-picrylhydrazyl (DPPH) radical (Fig. 1.12). Ellagitannins
generally showed more potent effects than α-tocopherol and ascorbic
acid, as measured by the discoloration of the purple DPPH radical
solution. The generation of stable free radical from an alkyl gallate upon
scavenging the DPPH radical was demonstrated by ESR measurements
and by high-yielding isolation of the dialkyl ester of
hexahydroxydiphenoic acid (HHDP) produced by mutual coupling of
transient C-centered galloyl radicals (Yoshida
et al
., 1989b). The
antioxidant activity of ellagitannins, thus attributable to their radical
scavenging effect, was also evidenced by their significant inhibitory
action on the carbon tetrachloride- and galactosamine-induced
cytotoxicities in primary cultured rat hepatocytes (Hikino
et al
., 1985).
The antioxidant
effect of tannins on lipids in biological systems was
shown by inhibition of lipid peroxidation induced by adenine 5'-
diphosphate (ADP) and ascorbic acid in rat liver mitochondria, and by
