Sphingomyelin exhibits greatly enhanced protection compared with egg yolk phosphatidylcholine against detergent bile salts

Inclusion of phosphatidylcholine within bile salt micelles protects against bile salt-induced cytotoxicity. In addition to phosphatidylcholine, bile may contain significant amounts of sphingomyelin, particularly under cholestatic conditions. We compared protective effects of egg yolk phosphatidylcholine (similar to phosphatidylcholine in bile), egg yolk sphingomyelin (mainly 16:0 acyl chains) and dipalmitoyl phosphatidylcholine against taurocholate in complementary in vitro studies. Upon addition of taurocholate-containing micelles to sonicated egg yolk phosphatidylcholine vesicles, subsequent micellization of the vesicular bilayer proved to be retarded when phospholipids had also been included in these micelles in the rank order: egg yolk phosphatidylcholine < dipalmitoyl phosphatidylcholine < sphingomyelin. Hemolysis of erythrocytes and LDH release by CaCo-2 cells after addition of taurocholate micelles were strongly reduced by including small amounts of sphingomyelin or dipalmitoyl phosphatidylcholine in these micelles (PL/(PL + BS) ≥ 0.1), whereas egg yolk phosphatidylcholine provided less protection. Amounts of non-phospholipid-associated bile salts (thought to be responsible for cytotoxicity) in egg yolk phosphatidylcholine- containing micelles were significantly higher than in corresponding sphingomyelinor dipalmitoyl phosphatidylcholine-containing micelles (tested at PL/(PL + BS) ratios 0.1, 0.15, and 0.2). LDH release upon incubation of CaCo-2 cells with taurocholate simple micelles at these so-called 'intermixed micellar-vesicular' concentrations was identical to LDH release upon incubation with corresponding taurocholate-phospholipid mixed micelles. In conclusion, we found greatly enhanced protective effects of sphingomyelin and dipalmitoyl phosphatidylcholine compared to egg yolk phosphatidylcholine against bile salt-induced cytotoxicity, related to different amounts of non- phospholipid-associated bile salts. These findings may be relevant for protection against bile salt-induced cytotoxicity in vivo.