The binding of bile salts by vegetable fibre

Abstract

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.

Zusammenfassung

Die Binding von Salzen des Gallensaftes durch Ballaststoffe spielt eine wichtige Rolle im Cholesterol-Metabolismus des Menschen. In den meisten der Adsorbtions-Studien der Salze des Gallensaftes in vitro wurde weder die chemische Zusammensetzung der Fiber-Präparate beschrieben-eventuell ganz allgemein-noch betrachtete man die Wirkung gleichzeitig ausgefällter Substanzen, wie z.B. der Protein. Daher erschien es erforderlich, die Adsorbtion von Natriumcholat (NaC) und Natriumtaurocholat (NaCT) durch wohldefiniertes Zellwand-Material (CWM) von parenchymatischem, lignifiziertem und kutinisiertem Gewebe reifer Samen der Feuerbohne (Phaseolus coccineus) sowie von Porree (Allium ampeloprasum varporrum) unter variierten experimentellen Bedingungen zu untersuchen. Dies bezog sich auf die Identifizierung der polymeren Gruppen, die für die Adsorbtion verantwortlich sind. Die Ergebnisse weisen entscheidende Unterschiede in den Adsorbtions-Charakteristika der Zellwand-Präparate bei verschiedenen pH-Werten auf. Über einige der Ergebnisse soll im folgenden berichtet werden.

Das CWM der verschiedenen Gewebe wurde gewonnen durch mehrmals durchgeführte Extraktion, indem das Material mit 1%igem wäßrigen Natrium-Deoxycholat und mit Phenol/Essigsäure/Wasser (2:1:1, G/V/V) feucht in der Kugelmühle behandelt worden war. Versuche mit markiertem Deoxycholast zeigten, daß die Endpräparate zu vernachlässigende Mengen von adsorbiertem Deoxycholat enthielten.

Adsorbtions-Studien mit den Porree-Präparaten zeigten, daß die Adsorbtins-Kapazität der parenchymatischen Gewebe beträchtlich höher war als das der kutinisierten. Hieraus folgt, daß Pektine in der H-Form-und nich Lignin oder Kutin-wahrscheinlich die hauptsächlichen bindenden Agentien von Cholaten sind. Die Adsorbtion von Natrium-Taurocholat war bei den verschiedenen Präparaten vom pH-Wert unabhängig. Es werden die Folgerungen aus diesen Ergebnissen besprochen.