Abstract
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.
Highlights
Both phosphatidylcholine (PC) and sphingomyelin (SM) are the major phospholipids in the outer leaflet of the hepatocyte canalicular membrane
The decrease of micellar phospholipid occurred both in egg yolk sphingomyelin (EYSM)- and buttermilk sphingomyelin (BMSM)-containing systems
Similar results have been reported after incubation with bile salts of isolated vesicular hepatocyte canalicular membrane subfractions [34, 35] or erythrocytes [36]
Summary
Both phosphatidylcholine (PC) and sphingomyelin (SM) are the major phospholipids in the outer leaflet of the hepatocyte canalicular membrane. 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.—Eckhardt, E. With the aid of ultrarapid cryofixation and electron microscopic imaging, Crawford et al [4] could visualize significant amounts of unilamellar vesicles within the canalicular lumen, consistent with a vesicular mode of cholesterol and PC secretion These findings do not exclude the possibility that detergent bile salts, after their secretion into the canalicular lumen, could micellize considerable amounts of cholesterol and PC from the outer leaflet of the canalicular membrane. Another matter of debate is why PC is the predominant phospholipid in bile: both sphingomyelin (SM) and PC are the major structural phospholipids in the outer leaflet of the hepatocyte canalicular membrane
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