Stereospecificity of Daidzein metabolism leading to S-Equol by human intestinal bacterium

Abstract

S-Equol, a metabolite of the isoflavonoid daidzein in the human intestine, shows the most active estrogen activity. Stereospecific production of S-equol by a human intestinal anaerobic bacterium strain Julong732 was studied. Julong732 was known to metabolize dihydrodaidzein (DHD) into S-equol tetrahydrodaidzein (THD) [1] but its metabolic stereospecificity has not yet been elucidated. DHD and THD were prepared, and their stereoisomers were separated by reverse-phase and chiral stationary-phase high-performance liquid chromatography. The absolute configurations of DHD enantiomers and four THD stereoisomers were assigned by 1H NMR, 13C NMR, and CD spectroscopies [2,3]. The reaction of the cell-free extract with DHD showed that only 3S-DHD was transformed into 3S-equol. When each of the four stereoisomers of THD was reacted with the cell-free extract of Julong732, only 3R,4S-THD was metabolized by the extract, producing S-equol. 7,4'-Dihydroxyisoflav-3-ene (dehydroequol, DE), a putative intermediate during the anaerobic metabolism of DHD, was unable to produce S-equol.