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Biliary lipid secretion probably involves both 'micellization' and 'vesiculization' of bile-canalicular membrane lipids. Several hydrophilic organic anions inhibit the secretion of lipids into the bile without altering bile salt secretion [Verkade, Vonk and Kuipers (1995) Hepatology 21, 1174-1189]. Hydrophobic organic anions do not interfere with biliary lipid secretion. We investigated whether the organic-anion-induced inhibition of biliary lipid secretion in vivo could be attributed to inhibition of micellization, by the application of in vitro models of micellization. Carboxyfluorescein was entrapped in a self-quenching concentration in small unilamellar vesicles (SUV) composed of cholesterol/egg phosphatidylcholine (molar ratios 0, 0.2 and 0.5). Certain organic anions clearly affected the bile-salt-induced release of fluorescence from these SUV, reflecting interference with micellization. However, the effects of hydrophilic and hydrophobic organic anions did not correspond with their effects on biliary lipid secretion in vivo, irrespective of the bile salt species used (taurocholate, taurodeoxycholate or tauroursodeoxycholate) and of the lipid composition of the SUV. Ultracentrifugation and dynamic light-scattering studies indicated that organic anions do interact with bile salt/ phosphatidylcholine/cholesterol mixed micelles, but that they do not inhibit micellization, for example by competing with phosphatidylcholine and/or cholesterol for incorporation into mixed micelles. In conclusion, the present in vitro data indicate that the in vivo mechanism of organic-anion-induced inhibition of biliary lipid secretion is not mediated by inhibition of micellization.

In this study, the interaction of mucin and concanavalin A-binding proteins isolated from human bile with cholesterol/phospholipid vesicles was investigated. Using resonance energy transfer assays originally developed by Struck, Hoekstra and Pagano [(1981) Biochemistry 20, 4093-4099], no significant protein-induced fusion or aggregation of vesicles was demonstrated. Instead of fusion, these proteins induced destabilization of cholesterol/phospholipid vesicles, as monitored by release of entrapped carboxyfluorescein. A good correlation (rho = 0.81) was obtained between the extent of leakage and the nucleation-promoting activity of the concanavalin A-binding proteins. We conclude that aggregation or fusion of cholesterol/phospholipid vesicles is not an obligatory step in cholesterol crystallization. Biliary protein-induced crystallization seems to be preceded by vesicle disruption.