Cholesterol enhances membrane-damaging properties of model bile by increasing the intervesicular-intermixed micellar concentration of hydrophobic bile salts

Bile salts are potent detergents that, at concentrations attained in bile a nd intestine, can disrupt cell membranes. Hepatic secretion of vesicles con taining lecithin and cholesterol appears to be critical in preventing bile salt damage to hepatobiliary epithelia. We hypothesize that the protective effect of biliary lipids results from lowering of the bile salt intervesicu lar intermixed micellar bile salt concentration (IMMC) to which epithelial membranes are exposed. We further hypothesize that increases in biliary cho lesterol, by reducing association of bile salts with vesicles and mixed mic elles, may increase bile toxicity by raising the bile salt IMMC. Method Large unilamellar lecithin vesicles (100 nm) with varying cholestero l:lecithin molar ratios (C:L) df 0, 0.5, and 1 were added to taurochenodeox ycholate (TCDCA), taurocholate (TCA), or taurodeoxycholate (TDCA) in Tris-b uffered saline, pH 7.4. Human erythrocyte ghosts (model target membrane), p repared by osmotic hemolysis and resealed with [C-14]inulin trapped inside, were added and incubated at 37 degrees C for 30 min and 4 h. Plasma membra ne disruption was quantified by [C-14]inulin release and bile salt IMMC was determined by ultrafiltration. Results. Membrane disruption started at a concentration of 0.5 mM for TDCA, 1 mM for TCDCA, and 2 mM for TGA and was complete within 4 h at concentrat ions of 1, 2, and 4 mM, respectively. Addition of 2 mM lecithin to 2 mM TDC A, 4 mM TCDCA or 5 mM TCA reduced or eliminated membrane leakage and lowere d the IMMC. For TDCA and TCDCA,the protective effect of vesicles was entire ly attributable to reduction in IMMC; in contrast for TCA, the protective e ffect exceeded that which would have been expected based solely on reductio n of the IMMC. Inclusion of cholesterol attenuated the binding of bile salt s to vesicles and raised the IMMC, thereby reducing the protective effect o f lecithin over the time course of these studies. Although there was loss o f phospholipid and cholesterol from the erythrocyte membranes on addition o f bile acids even in the presence of vesicles, the ratio of cholesterol to phospholipid in the erythrocyte membrane did not change. Conclusion, Lecithin protects against membrane disruption by hydrophobic bi le salts by lowering the IMMC. Cholesterol added to lecithin raises the bil e salt IMMC and reduces or eliminates this protective effect. This mechanis m of potentiation of bile salt toxicity by cholesterol may be an important contributor to the pathogenesis of gallbladder disease in cholesterol chole lithiasis. (C) 1999 Academic Press.