A newly isolated rod-shaped, gram-negative anaerobic bacterium from human feces, named Julong 732, was found to be capable of metabolizing the isoflavone dihydrodaidzein to S-equol under anaerobic conditions. The metabolite, equol, was identified by using electron impact ionization mass spectrometry, 1 H and 13 C nuclear magnetic resonance spectroscopy, and UV spectral analyses. However, strain Julong 732 was not able to produce equol from daidzein, and tetrahydrodaidzein and dehydroequol, which are most likely intermediates in the anaerobic metabolism of dihydrodaidzein, were not detected in bacterial culture medium containing dihydrodaidzein. Chiral stationary-phase high-performance liquid chromatography eluted only one metabolite, S-equol, which was produced from a bacterial culture containing a racemic mixture of dihydrodaidzein. Strain Julong 732 did not show racemase activity to transform R-equol to S-equol and vice versa. Its full 16S rRNA gene sequence (1,429 bp) had 92.8% similarity to that of Eggerthella hongkongenis HKU10. This is the first report of a single bacterium capable of converting a racemic mixture of dihydrodaidzein to enantiomeric pure S-equol.There has been growing interest in the effects of dietary phytoestrogenic isoflavones on human health because isoflavones possess diverse physiological activities, including anticarcinogenic (1, 13), antimutagenic (12), and antioxidant (17) activities, as well as antiproliferative effects against tumor cells (14). The main components of dietary phytoestrogenic isoflavones are daidzein, genistein, and their glycosides daidzein and genistein, which are distributed predominantly in leguminous plants, such as soybeans.After ingestion, isoflavones are subjected to diverse metabolic processes in the intestinal tract prior to absorption. The metabolites of daidzein include dihydrodaidzein, tetrahydrodaidzein, equol, and O-desmethylangolensin, and these metabolites have been detected in human urine (9,18,19). The crucial role of gut microflora in the metabolism of isoflavones in human beings has also been previously explored (9). In vivo studies proved that bacteria in the gastrointestinal tract play an important role in determining the magnitude and pattern of isoflavone bioavailability (30). Setchell et al. (28) demonstrated that incubation of textured vegetable protein with human fecal bacteria resulted in the formation of equol, which among isoflavone metabolites was known to be the most effective in stimulating an estrogenic response (6, 25). However, only 30 to 40% of the population can produce equol from daidzein (25,27). In vitro studies proved interindividual differences in the bacteria responsible for equol production. Equol is a chiral molecule that can exist in two enantiomeric forms with potentially different biological activities (22). A recent study demonstrated that the binding affinity of a natural-form S-equol to estrogen receptor ␤ is 13 times higher than that of R-equol and 2 times higher than that of (Ϯ)equol, whereas R-equol showed a pr...

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