The metabolism of the plant lignans matairesinol, secoisolariciresinol, pinoresinol, syringaresinol, arctigenin, 7-hydroxymatairesinol, isolariciresinol, and lariciresinol by human fecal microflora was investigated to study their properties as mammalian lignan precursors. The quantitative analyses of lignan precursors and the mammalian lignans enterolactone and enterodiol were performed by HPLC with coulometric electrode array detector. The metabolic products, including mammalian lignans, were characterized as trimethylsilyl derivatives by gas chromatography-mass spectrometry. Matairesinol, secoisolariciresinol, lariciresinol, and pinoresinol were converted to mammalian lignans only. Several metabolites were isolated and tentatively identified as for syringaresinol and arctigenin in addition to the mammalian lignans. Metabolites of 7-hydroxymatairesinol were characterized as enterolactone and 7-hydroxyenterolactone by comparison with authentic reference compounds. A metabolic scheme describing the conversion of the most abundant new mammalian lignan precursors, pinoresinol and lariciresinol, is presented.

Twenty-four plant lignans were analyzed by high-performance liquid chromatography-tandem mass spectrometry in bran extracts of 16 cereal species, in four nut species, and in two oilseed species (sesame seeds and linseeds). Eighteen of these were lignans previously unidentified in these species, and of these, 16 were identified in the analyzed samples. Four different extraction methods were applied as follows: alkaline extraction, mild acid extraction, a combination of alkaline and mild acid extraction, or accelerated solvent extraction. The extraction method was of great importance for the lignan yield. 7-Hydroxymatairesinol, which has not previously been detected in cereals because of destructive extraction methods, was the dominant lignan in wheat, triticale, oat, barley, millet, corn bran, and amaranth whole grain. Syringaresinol was the other dominant cereal lignan. Wheat and rye bran had the highest lignan content of all cereals; however, linseeds and sesame seeds were by far the most lignan-rich of the studied species.

Asian people who are vegetarian or semi-vegetarian show a low incidence of and mortality from hormone-dependent diseases, such as breast and prostate cancers, compared with Western people, whose diets are rich in animal protein and fats.1) Consumption of fiber-rich whole grain food elevates a serum mammalian lignan, enterolactone (ENL).2,3) Furthermore, both very low and very high plasma concentrations of ENL are associated with an increased breast cancer risk, 4) and urinary excretion of mammalian lignans [enterodiol (END) and ENL] is low in patients with breast cancer.5,6) All these findings suggest a close relationship between vegetarian diet, mammalian lignans and hormone-dependent diseases. The origins of END and ENL found in human biological fluids and in feces are plant lignans contained in fibrerich whole grain food, which are transformed by intestinal microflora in the proximal colon. 7,8) Until recently, secoisolariciresinol and matairesinol are the only known precursors of END and ENL. 7-9) Pinoresinol, lariciresinol, syringaresinol, 7-hydoxymatairesinol and arctigenin are newly identified enterolactone precursors, 10,11) and a possible metabolic pathway for pinoresinol (formation of mammalian lignans via lariciresinol and secoisolariciresinol) was proposed based on the biosynthetic pathway of secoisolariciresinol and matairesinol in plants. 10) Pinoresinol diglucoside is a major antihypertensive principle of Tu-Chung (the bark of Eucommia ulmoides), used in traditional Chinese medicine, 12) and pinoresinol is contained in cereals, particularly in whole-grain rye products, 1) olive oil, 13) and in various Picea, Pinus and Abies etc.14) The present study was designed for a better understanding of the metabolism of pinoresinol diglucoside by human intestinal microflora, as well as isolation of the bacterial strains responsible for the respective reactions in the transformation. ResultsTransformation of Pinoresinol Diglucoside (PDG, 1) by a Human Intestinal Bacterial Mixture After anaerobic incubation of pinoresinol diglucoside (1) with a bacterial mixture of human feces, the culture was extracted with nBuOH and the extract was subjected to Diaion HP-20, Sephadex LH-20, preparative thin-layer chromatography (TLC) and RP-18 column chromatography. Fifteen metabolites (2-16) were isolated and identified by electron impact mass (EI-MS), one dimensional (1D) and 2D-NMR, and circular dichroic (CD) spectroscopy.Compound 2 was detected as a major metabolite after 6 h of incubation of 1 with a human fecal suspension. The [a] D 25 value measured in MeOH was ϩ69°. The 1 H-and 13 C-NMR spectra (Table 1) were in good agreement with those reported for (ϩ)-pinoresinol, bearing a (7S,7ЈS,8S,8ЈS)-configuration (Fig. 1). 15,16) Compound 3 was a second metabolite when the reaction mixture was monitored by TLC. Its molecular ion peak (m/z 360 [M] ϩ ) in the EI-MS spectrum was 2 mass units (2H) higher than that of 2, suggesting that 3 is a reduced product of 2. The [a] D 25 value in MeOH was ϩ30°. 17) Compound 3 was deter...

No abstract