Asymmetric distribution of phosphatidylcholine and sphingomyelin between micellar and vesicular phases. Potential implications for canalicular bile formation.

Both phosphatidylcholine (PC) and sphingomyelin (SM) are the major phospholipids in the outer leaflet of the hepatocyte canalicular membrane. Yet, the phospholipids secreted into bile consist principally (>95%) of PC. In order to understand the physical;-chemical basis for preferential biliary PC secretion, we compared interactions with bile salts (taurocholate) and cholesterol of egg yolk (EY)SM (mainly 16:0 acyl chains, similar to trace SM in bile), buttermilk (BM)SM (mainly saturated long (>20 C-atoms) acyl chains, similar to canalicular membrane SM) and egg yolk (EY)PC (mainly unsaturated acyl chains at sn-2 position, similar to bile PC). Main gel to liquid-crystalline transition temperatures were 33. 6 degrees C for BMSM and 36.6 degrees C for EYSM. There were no significant effects of varying phospholipid species on micellar sizes or intermixed-micellar/vesicular bile salt concentrations in taurocholate-phospholipid mixtures (3 g/dL, 37 degrees C, PL/BS + PL = 0.2 or 0.4). Various phases were separated from model systems containing both EYPC and (EY or BM)SM, taurocholate, and variable amounts of cholesterol, by ultracentrifugation with ultrafiltration and dialysis of the supernatant. At increasing cholesterol content, there was preferential distribution of lipids and enrichment with SM containing long saturated acyl chains in the detergent-insoluble pelletable fraction consisting of aggregated vesicles. In contrast, both micelles and small unilamellar vesicles in the supernatant were progressively enriched in PC. Although SM containing vesicles without cholesterol were very sensitive to micellar solubilization upon taurocholate addition, incorporation of the sterol rendered SM-containing vesicles highly resistant against the detergent effects of the bile salt. These findings may have important implications for canalicular bile formation.