Artificial Lignification of Maize Cell Walls Does Not Affect In Vitro Bile Acid Adsorption

Cereal Chem. 85(l):14-18 Bile acid adsorption by lignified dietary fiber in the human intestine is proposed as a mechanism for lowering blood cholesterol level and reducing colon cancer risk. In this study, we investigated how the concentration and composition of lignin in fiber influences the in vitro adsorption of primary bile acids (glycocholate, taurocholate, and glycochenodeoxycholate) and a secondary bile acid (deoxycholate). Adsorption studies were performed by incubating nonlignified and artificially lignified maize cell walls (dehydrogenation polymer-cell walls) with bile acids under condiDiets rich in dietary fibers provide a multitude of health benefits. For example, dietary fibers may adsorb carcinogens such as heterocyclic aromatic amines and transport them out of the body without being absorbed (Ferguson and Harris 1996). Adsorption of bile acids (Fig. I) to dietary fibers is also widely discussed as a mechanism to prevent cholesterol related diseases (Kritchevsky 1996). Primary bile acids, mainly cholic acid (I. R 3 = H) and chenodeoxycholic acid (3, R 3 = H), are biosynthesized from cholesterol in the liver, conjugated with either glycine or taurine (R 3 = NH-CH2-COOH or NH-CH2-CH-S0 3 H) (Fig. I), and secreted into the upper region of the small intestine, where they take part in the enterohepatic circulation (Hofmann 1999; Mukhopadhyay and Maitra 2004). Secondary bile acids, mainly deoxycholic acid (4), are produced by deconjugation and dehydroxylation of primary bile acids by bacteria located in the human ileum and colon. According to Rossi et al (1987) cholate, chenodeoxycholate, and deoxycholate conjugates comprise 37, 39, and 16%, respectively, of the bile acids found in the human bile. Their physiological functions include solubilization and transport of lipids, activation of bile acid-activated lipase and cholesterol homeostasis (Mukhopadhyay and Maitra 2004). Primary bile acid adsorption by dietary fibers may increase bile acid excretion, thus stimulating additional bile acid synthesis and lowering blood cholesterol levels. In addition, adsorption of secondary bile acids may reduce their availability in the gut. Some secondary bile acids such as deoxycholate are implicated in promoting colon cancer growth and progression (Pai et al 2004). In vivo studies have shown that diets rich in cereal fibers increase fecal excretion of bile acids (Cummings et al 1976; Nagengast 1992). These observations are in agreement with in vitro adsorption studies showing that bile acids are adsorbed by dietary fibers, though to varying extents (Eastwood and Hamilton 1968; Story and Kritchevsky 1976; Kern et al 1978; Huang and Dural 1995; Kahlon and Woodruff 2003; Górecka et al 2005). Various adsorption studies positively correlate the extent of adsorption with bile acid hydrophobicity (Eastwood and Hamilton 1968; Huang and Dural 1995; Górecka et al 2002). Statements about the adsorption abilities of individual 'Department of Food Chemistry. University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany. 2 U.S. Dairy Forage Research Center, USDA-Agricultural Research Service, 1925 Linden Drive West, Madison, WI 53706. Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Avenue, St. Paul, MN 55108. 4 Corresponding author. Phone: +1-612-624-1764. Fax: +1-612-625-5272. E-mail: [email protected] doi:l 0.1 094/CCHEM .85-1 -0014 © 2008 AACC International, Inc. tions imitating the small intestine and distal colon. Artificially lignified cell walls had varying but defined lignin concentrations (4.8-19.0%) and compositions (varying from pure guaiacyl to pure syringyl lignins) but a uniform polysaccharide-protein matrix. Adsorption of bile acids by cell walls was in a range of 6-31% (4-26 nmol of bile acids/mg of cell walls), with glycochenodeoxychol ate showing the highest adsorption rates. Neither lignin concentration nor lignin composition influenced bile acid adsorption, thus disproving a major role of lignin in bile acid adsorption. dietary fiber constituents are contradictory. The main hydrophobic component of fiber, lignin (a complex macromolecule formed by oxidative radical coupling of mainly coniferyl and sinapyl alcohols) (Fig. 2) has often been implicated as an effective adsorber of bile acids (Eastwood and Hamilton 1968; Story and Kritchevsky 1976; Gallaher and Schneeman 1986; Huang and Dural 1995; GOrecka et al 2002; Sayar et al 2006). Other studies, however, question a predominant role of lignin in bile acid adsorption (Nixon et a1 1986; Elhardallou 1992; GOrecka et al 2005) and instead promote the importance of soluble, viscous dietary fiber compounds (Glore et al 1994) such as f3-glucans (Braaten et al 1994; Wood 2001) or psyllium (Fulgoni 2001) for blood cholesterol reduction. Since the adsorption of bile acids is now regarded as a complex process, depending not only on the cell wall constituents but also on the cell wall structure (Drzikova et al 2005), in vitro studies should go beyond work with isolated cell wall components to include whole cell walls with precisely defined matrix characteristics. The aim of this study was to clearly delineate whether lignin in a cell wall matrix influences the adsorption of glycocholate, taurocholate, glycochenodeoxycholate, and deoxycholate under conditions mimicking the small intestine and distal colon. Our in vitro adsorption studies were performed with artificially lignified maize cell walls, so-called dehydrogenation polymer (DHP)-cell walls, with defined lignin concentration and composition characteristics and a uniform protein-polysaccharide matrix.