The binding of bile salts by vegetable fibre

The binding of bile salts by dietary fibre plays an important role in cholesterol metabolism in man. In most of the adsorption studies of bile salts in vitro, the chemical composition of the fibre preparations has not been described, even broadly, nor has the effect of co-precipitated compounds (e.g. proteins) been considered. Hence, it seemed useful to investigate the adsorption of Na cholate (NaC) and Na taurocholate (NaTC) by well-defined cell wall material (CWM) from parenchymatous, lignified and cutinised tissues of mature runner bean pods as well as leeks under a variety of experimental conditions. This study was to identify the groups of polymers which are responsible for adsorption. The results showed dramatic differences in the adsorption characteristics of the wall preparations at different pH values. Some of the findings are reported below. The CWM from the various tissues was prepared by sequentially extracting the wet ball-milled tissues with 1% aq. Na deoxycholate and phenol/acetic acid/water (2:1:1, w/v/v). Experiments with labelled deoxycholate showed that the final preparations contained negligible amounts of adsorbed deoxycholate. Since the amount of residual starch in the preparations was small, no attempt was made to remove it; however, if required, this could be completely removed by extraction with 90% aq. dimethyl sulphoxide. The particle size of the preparations varied from 25–50 μm. For adsorption studies the following preparations were used: (1) whole-, depeetinated-and delignified-CWM from parenchymatous and lignified tissues of runner beans, together with the H and Na forms; (2) CWM of runner beans at different stages of maturity; (3) CWM of whole-and decutinised-leaves and cutinised tissues of leeks and (4) carboxymethyl cellulose and amberlite resin. The binding of bile salts was measured by a modified isotope-dilution procedure. The neutral sugars from the polysaccharides were determined as alditol acetates by GLC and an estimate of the uronic acid content of the preparations was obtained by a modified carbazole method. Experiments on the effect of pH on the binding of cholate by the various preparations showed that the adsorption capacity was very much dependent on the pH. The binding increased as the pH decreased. These experiments were complicated by the precipitation cholic acid at a pH value of <4. Nevertheless, an interaction between cholate and CWM persisted in acid solutions. Removal of pectic substances and lignin from the runner bean preparations resulted in a decrease of the adsorption capacity. The results suggested that the adsorption is greatest under conditions in which the ionization of cholic acid and the acidic groups of the cell wall polymers is at its lowest. Following these studies, determinations were made of the adsorption of cholate from an aqueous solution (without buffer) by wall preparations in the H and Na forms. With all the preparations the adsorption was greatest when these were in the H form. Adsorption studies with the preparations from leeks showed that the adsorption capacity of the parenchymatous tissues was considerably more than that of cutinised tissues. Hence, pectic substances in the H form (an not lignin or cutin) are probably the principal binding agents of cholate. The adsorption of Na taurocholate by the various preparations was independent of pH value. The implications of these results are discussed.