Model studies of lignified fiber fermentation by human fecal microbiota and its impact on heterocyclic aromatic amine adsorption.

This study examined how shifts in pH and fiber fermentation may alter the adsorption of mutagenic heterocyclic aromatic amines (HAAs) 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-9H-pyrido[2,3-b]indole (AalphaC) to dietary fiber in the human small intestine and colon. Nonlignified and artificially lignified maize cell walls were fermented in vitro with human fecal microbiota for 0, 8, or 24h. We then assessed the adsorption of HAAs to unfermented fiber at pH 6.5 and to unfermented and fermented fibers at pH 7.4 to mimic conditions in the small intestine and colon, respectively. HAAs were effectively adsorbed to lignified fiber by up to 74% at pH 6.5 and by up to 68% at pH 7.4. Increasing the lignin content of unfermented fiber from 0.4% to about 14% increased HAA adsorption by two- to three-fold. This increase in lignification reduced microbial fiber degradation from 51% to minimum 8% after 24h of fermentation, whereas variations in the guaiacyl and syringyl makeup of lignin had smaller but significant impacts on fiber degradation. A 24h fermentation decreased the AalphaC adsorption to lignified fiber at pH 7.4 by up to one-third, while PhIP adsorption was not affected. Our results indicate that lignification increases the adsorption of hydrophobic HAAs to fiber but shifts in pH and fermentation may somewhat diminish adsorption of some HAAs as fiber passes from the small intestine into and through the colon.

Moderate ferulate and diferulate levels do not impede maize cell wall degradation by human intestinal microbiota.

The degradation of plant fiber by human gut microbiota could be restricted by xylan substitution and cross-linking by ferulate and diferulates, for example, by hindering the association of enzymes such as xylanases with their substrates. To test the influence of feruloylation on cell wall degradability by human intestinal microbiota, nonlignified primary cell walls from maize cell suspensions, containing various degrees of ferulate substitution and diferulate cross-linking, were incubated in nylon bags in vitro with human fecal microbiota. Degradation rates were determined gravimetrically, and the cell walls were analyzed for carbohydrates, ferulate monomers, dehydrodiferulates, dehydrotriferulates, and other minor phenolic constituents. Shifting cell wall concentrations of total ferulates from 1.5 to 15.8 mg/g and those of diferulates from 0.8 to 2.6 mg/g did not alter the release of carbohydrates or the overall degradation of cell walls. After 24 h of fermentation, the degradation of xylans and pectins exceeded 90%, whereas cellulose remained undegraded. The results indicate that low to moderate levels of ferulates and diferulates do not interfere with hydrolysis of nonlignified cell walls by human gut microbiota.

Influence of lignification and feruloylation of maize cell walls on the adsorption of heterocyclic aromatic amines.

Both epidemiological and experimental data indicate that a diet rich in fiber may reduce cancer risk. One possible mechanism is by adsorbing carcinogens and transporting them out of the body without metabolic activation. We investigated the role of fiber lignification and feruloylation on the adsorption of four of the most relevant heterocyclic aromatic amines in food: 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), and 2-amino-9H-pyrido[2,3-b]indole (AalphaC). Adsorption experiments, under conditions mimicking the small intestine, were carried out using nonlignified and artificially lignified primary maize walls with defined lignin and ferulate/diferulate concentrations and defined lignin compositions. Lignin concentration and composition both influenced the adsorption of heterocyclic aromatic amines, especially the more hydrophobic types. Heterocyclic aromatic amine adsorption increased with lignin concentration. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-9H-pyrido[2,3-b]indole were better adsorbed by guaiacyl-rich lignins, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline by syringyl-rich lignins, whereas the adsorption of 2-amino-3-methylimidazo[4,5-f]quinoline was not clearly influenced by lignin composition. Nonlignified cell walls adsorbed lesser amounts of heterocyclic aromatic amines. Variations in cell wall feruloylation had no effect on heterocyclic aromatic amine adsorption.

Isolation and structural characterisation of 8-O-4/8-O-4- and 8-8/8-O-4-coupled dehydrotriferulic acids from maize bran.

Two new dehydrotriferulic acids were isolated from saponified maize bran insoluble fiber using Sephadex LH-20 chromatography followed by semi-preparative RP-HPLC. Based on UV-spectroscopy, mass spectroscopy and one- and two-dimensional NMR experiments, the structures were identified as 8-O-4,8-O-4-dehydrotriferulic acid and 8-8(cyclic),8-O-4-dehydrotriferulic acid. Which of the possible phenols in the initially formed 8-8-dehydrodiferulate was etherified by 4-O-8-coupling with ferulate has been unambiguously elucidated. The ferulate dehydrotrimers which give rise to these dehydrotriferulic acids following saponification are presumed, like the dehydrodiferulates, to cross-link polysaccharides. Neither dehydrotriferulic acid described here involves a 5-5-dehydrodiferulic acid unit; only the 5-5-dehydrodimer may be formed intramolecularly. However, whether dehydrotriferulates are capable of cross-linking more than two polysaccharide chains remains open. Although the levels of the isolated ferulate dehydrotrimers are lower than those of the ferulate dehydrodimers, the isolation now of three different dehydrotriferulates indicates that trimers contribute to a strong network cross-linking plant cell wall polysaccharides.

Semipreparative isolation of dehydrodiferulic and dehydrotriferulic acids as standard substances from maize bran.

A method for the isolation of diferulic and triferulic acids in quantities and purity that comply with the requirements for their use as standard substances was developed. The procedure includes alkaline hydrolysis of destarched maize bran and ether extraction of liberated phenolic compounds. Following a first purification by liquid-liquid extraction Sephadex LH-20 chromatography is performed. This step is the core of the method and allows the separation of monomeric and dimeric/trimeric substances. A good pre-separation of di- and triferulic acids (purity in most cases >75%) is also achieved. Further separation and purification is carried out by semipreparative RP18-HPLC. Using this rapid, easy to handle, and moderately priced separation procedure it is possible to obtain approx. 41 mg 8-O-4'-diferulic acid, 27 mg 5-5'-diferulic acid, 12 mg 8-5'-diferulic acid (benzofuran form), 16 mg 8-5'-diferulic acid (open form), 11 mg 8-5'-diferulic acid (decarboxylated form), 7 mg 8-8'-diferulic acid (cyclic form), 5 mg 8-8'-diferulic acid (open form), and 10 mg 5-5',8'-O-4"-triferulic acid out of 20 g destarched maize bran. The incorporation of minor modifications allows a further upscaling of this procedure.