Phytoestrogens are defined as plants-derived compounds with estrogen-like activities. Isoflavones and lignans have been categorized as phytoestrogens, based on their chemical structures and activities.1,2) Phytoestrogens have been explored in the field of metabolic degradation by intestinal bacteria. [3][4][5][6][7][8][9] In particular, the biotransformation of plant lignans, such as secoisolariciresinol diglucoside (SDG), pinoresinol diglucoside (PDG), arctiin, and matairesinol to mammalian lignans, enterodiol (END, 1) and enterolactone (ENL, 2) have extensively been studied. The metabolic processes by intestinal bacteria include deglucosylation, ring cleavage, demethylation, dehydroxylation and oxidation. Mammalian lignans lacking of phenolic hydroxyl groups at the para position of aromatic rings are different from plant lignans, 6) and dehydroxylation is an essential process on the metabolic conversion from plant lignans to mammalian lignans by intestinal bacteria.Eubacterium (E.) sp. strain SDG-2 was isolated by Wang et al. as a bacterium capable of reductive dehydroxylation in the processes of SDG to END (1) and ENL (2).10) E. sp. strain SDG-2 was similar in characteristics to E. lentum, which was reclassified to Eggerthella (Eg.) lenta by phylogenic identification.11) In this study, we determined phylogenic affiliation of E. sp. SDG-2 with 16S rRNA gene-based identification using polymerase chain reaction with proper primers.END (1) and ENL (2) have two enantiomeric, mirror image forms (1a and 1b; 2a and 2b) (Fig. 1). Secoisolariciresinol (SECO), prepared by incubation of PDG with intestinal bacteria, was a (Ϫ)-form. 9) On the other hand, SECO, aglycone of SDG isolated from flaxseed, was a (ϩ)-form. 10)Although there were a few reports on the biotransformation of these lignans by intestinal bacteria, 12-14) enantioselective conversion was not well confirmed. Therefore, metabolic specificity between enantimers raised our interest in investigation. We prepared two sets of enantiomers, such as (ϩ)-and (Ϫ)-dihydroxyenterodiol (DHEND, 3a and 3b, respectively), November 2007 2113 Enantioselective Dehydroxylation of Enterodiol and Enterolactone Precursors by Human Intestinal Bacteria

Plant lignans, such as pinoresinol diglucoside, secoisolariciresinol diglucoside and arctiin, are metabolized to mammalian lignans, enterolactone or enterodiol, by human intestinal bacteria. Their metabolic processes include deglucosylation, ring cleavage, demethylation, dehydroxylation and oxidation. Here we isolated an intestinal bacterium capable of demethylating arctigenin, an aglycone of arctiin, to 2,3-bis(3,4-dihydroxybenzyl)butyrolactone (1) from human feces, and identified as an Eubacterium species (E. sp. ARC-2), which is similar to Eubacterium limosum on the basis of morphological and biochemical properties and 16S rRNA gene sequencing. By incubating with E. sp. ARC-2, arctigenin was converted to 1 through stepwise demethylation. Demethylation of arctigenin by E. sp. ARC-2 was tetrahydrofolate-and ATP-dependent, indicating that the reaction was catalyzed by methyltransferase. Moreover, E. sp. ARC-2 transformed secoisolariciresinol to 2,3-bis(3,4-dihydroxybenzyl)-1,4-butanediol by demethylation.