Transformation of trachelogenin, an aglycone of tracheloside from safflower seeds, to phytoestrogenic (−)-enterolactone by human intestinal bacteria

For the purpose of surveying naturally occurring precursors of oestrogenic substances, and their metabolic processes, to mammalian lignans such as enterodiol (END) and enterolactone (ENL), many plant lignans have been studied. Trachelogenin, an aglycone of tracheloside, occurring in the seeds of Carthamus tinctorius L. (safflower), was demonstrated to transform to seven metabolites, including (−)-ENL, by anaerobic incubation with a human faecal bacterial mixture, when the reaction was monitored by LC/MS. The structures of the metabolites were determined by spectroscopic means after a large-scale incubation and purification of the respective metabolites. Moreover, the ligand-binding affinity of these metabolites to oestrogen receptors (ERs) α and β was measured in comparison with that of (+)-ENL. (−)- and (+)-ENL were found to significantly bind to both ERα and β, in which an appreciable difference in affinity was observed between (+)- and (−)-ENL for ERβ, but not for ERα.