Interactions between biliary lipid micelles and intestinal brush border membranes investigated by 1H and 31P nuclear magnetic resonance.

The effect of taurocholate and lecithin-cholesterol-taurocholate mixed micelles on the structure of isolated intestinal brush border membranes was investigated by nuclear magnetic resonance (NMR). Rabbit brush border membranes isolated by a Mg2+ precipitation step were chosen for this study because of their stability and integrity as revealed by 31P NMR. Incubation of taurocholate with the brush border membranes does not induce significant solubilization of these membranes even when the taurocholate/phospholipid ratio reaches 3.0. 1H NMR studies indicate that taurocholate is included in the membrane bilayer at low concentration (3 mM). However this biliary salt produces a size diminution of the vesicles when its concentration increases. Incorporation of lecithin or lecithin-cholesterol in micelles of taurocholate and subsequent incubation with brush border membranes lead simultaneously to a decrease in the 31P NMR isotropic/bilayer line ratio, and to an increase in delta sigma. These results indicate a protective effect of these compounds against lytic damage of taurocholate. Furthermore the equilibrium distribution of lecithin between mixed micelles and the membrane bilayer is strongly in favour of complete integration of micellar components in the bilayer. These data suggest that uptake of lipids from the micellar phase by isolated brush border membranes involves an interaction of the micelles with membranes followed by a fusion process.