plant surface microbiology.pdf

6 Signalling in the Rhizobia–Legumes Symbioses 105
Flavanone metabolites from animals and humans are summarized in
Table 1 (Heilmann and Merfort 1998).A number of flavanes, flavanoles, trans-
3-hydroxyflavanes, 6-hydroxyflavanones, 6-hydroxyflavanes, 4-hydroxyflavanes,
3,6-dihydroxy-flavanes, 3,4-dihydroxy-flavanes and methoxyflavanes
have been identified. The best studied flavane is catechin. The major metabolites
of this compound in humans are 3-hydroxybenzoic acid, 3-hydroxyphenylpropionic
acid, 3-hydroxyhippuric acid, 3,4-dihydroxyphenylbenzoic
acid, 5-(3,4-dihydroxy)-valerianic acid and d-(3-hydroxy-4-methoxyphenyl)g-valerolactone.
Metabolites excreted from mammalians can, in certain locations
of the soil, therefore also significantly increase the concentrations of
flavonoid and isoflavonoid metabolites which may affect the plant-microbe
interactions. Sulfation of flavonoids and phenolic dietary compounds by
cytosolic sulfotransferases has been studied in detail by Pai et al. (2001). The
mechanisms for a chemoprotective action of these compounds is the inhibition
of the bioactivation of carcinogens by the human cytosolic sulfotransferases.
The ten known cytosolic sulfotransferases have a different substrate
specificity, e.g., the isoform PST has a high activity with flavonoids, but no
activity with isoflavonoids (Pai et al. 2001).
A fluoroimmunoassay has been developed to detect small concentrations
of daidzein and genistein as phytoestrogens in blood plasma. After synthesis
of 4¢-O-carboxymethyl-daidzein and 4¢-O-carboxymethyl-genistein, these
compounds were linked to bovine serum albumin and used to immunize rabbits.
The antisera were cross-reactive with some isoflavonoids, but not with
flavonoids (Wang et al. 2000). The assays could detect daidzein and genistein
in the range between 1 and 370 nMol/l. The actual concentrations in the blood
plasma were in the range between 4 and 7 nMol/l. The correlation coefficient
between this fluoroimmunoassay and a reference method, using an isotope
dilution gaschromatography mass spectrometry, was in the range of 0.95–
0.99. Another method to detect low concentrations of estrogenic flavonoids
was the development of a recombinant yeast strain in which the human estrogen
receptor was stably integrated into the genome. The most active
flavonoids in this assay were naringenin, apigenin, kaempferol, phloretin,
Table 1. Metabolites from flavanones (Heilmann and Merfort 1998)
Flavane-4-a-ol 4¢-hydroxyflavane
Flavane-4-b-ol 4¢-hydroxyflavane-4-a-ol
Trans-3-hydroxyflavanone 4¢- hydroxyflavane-4-b-ol
Trans-3-hydroxyflavane-4-a-ol 3,6-dihydroxy-flavane-4-a-ol
Trans-3-hydroxyflavane-4-b-ol 3,6-dihydroxy-flavane-4-b-ol
Flavone-3-ol 3,4-dihydroxy-flavane-4-a-ol
6-Hydroxyflavanone 3,4-dihydroxy-flavane-4-b-ol
6-Hydroxyflavane-4-a-ol 4¢-hydroxy-3¢-methoxyflavane-4-a-ol
6-Hydroxyflavane-4-b-ol 4¢-hydroxy-3¢-methoxyflavane-4-b-ol