Chem. Pharm. Bull. 51(4) 378—384 (2003)

tone, were reported from human and animal species. They were given the name mammalian lignans, because unlike plant lignans they carry phenolic hydroxy groups only in the meta position of the aromatic ring and they were assumed to play an important role in the prevention of hormone-dependent diseases, such as breast cancer and prostate cancer. The origins of enterodiol and enterolactone were later found to be plant lignans, such as secoisolariciresinol and matairesinol, in vegetarian food, and to be transformed by gut microflora in the proximal colon. As enterodiol and enterolactone are similar in partial structure to estradiol, they were expected to have estrogenic or antiestrogenic activity and so far to influence hormone-dependent diseases. Recently, arctiin, which is a glycoside of arctigenin and abundant in the seeds of Arctium lappa, was reported to show a protective effect against 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced mammary carcinogensis in female rats with oral administration. Arctiin was shown to be transformed to two metabolites (arctigenin and AM2) by rat intestinal flora. The transformation of secoisolariciresinol diglucoside (SDG) to enterodiol and enterolactone by human intestinal bacteria was studied in our department, and two bacterial strains, Peptostreptococcus sp. SDG-1 and Eubacterium sp. SDG-2, responsible for the conversion of SDG were isolated. Knowing the differences in bacterial species and their numbers between human and rat intestinal flora, we assumed that the respective transformations of arctiin (1) would be different. In the present paper, we report the transformation of 1 by human intestinal bacteria and the influence of its metabolites on the growth of human breast cancer MCF-7 cells. Results Transformation of Arctiin (1) by Human Intestinal Flora After anaerobic incubation of arctiin (1) with a bacterial mixture of human feces, the culture was extracted with acidified n-BuOH and the extract was subjected to Diaion HP-20, Sephadex LH-20, silica gel, and RP-18 column chromatography. Six metabolites (2—7) (Fig. 1) were isolated and identified by electron impact mass (EI-MS), one dimensional (1D) and two dimensional (2D)-NMR, and circular dichroic (CD) spectroscopy. Compound 2 was detected as the first metabolite from 1 by thin-layer chromatography (TLC). The EI-MS showed a molecular ion peak at m/z 372, 162 mass units (C6H10O5) less than that of 1. The H(see Experimental) and C-NMR 378 Chem. Pharm. Bull. 51(4) 378—384 (2003) Vol. 51, No. 4