Conversion of (3S,4R)-Tetrahydrodaidzein to (3S)-Equol by THD Reductase: Proposed Mechanism Involving a Radical Intermediate

To elucidate the mechanism of (3S)-equol biosynthesis, (2,3,4-d(3))-trans-THD was synthesized and converted to (3S)-equol by THD reductase in Eggerthella strain Julong 732. The position of the deuterium atoms in (3S)-equol was determined by H-1 NMR and H-2 NMR spectroscopy, and the product was identified as (2,3,4(alpha)-d(3))-(3S)-equol. All the deuterium atoms were retained, while the OH group at C-4 was replaced by a hydrogen atom with retention of configuration. To explain the deuterium retention in this stereospecific reduction, we propose a mechanism involving radical intermediates.