Identification of an alkaline sphingomyelinase activity in human bile

The hydrolysis of sphingomyelin has been found to generate important signals regulating cell proliferation, differentiation and apoptosis. However, the enzymes responsible for digestion of dietary sphingomyelin have not been well documented. This study demonstrates the occurrence of a sphingomyelinase (SMase) in both human hepatic bile and gallbladder bile. The enzyme was equally found in both bacteria negative and positive bile samples and in samples obtained from patients with or without gallbladder diseases. A bacteria-free gallbladder bile was used for characterization. It was found that bile SMase hydrolyzed sphingomyelin to phosphorylcholine and ceramide with negligible activity against either phosphatidylcholine or p-nitrophenyl phosphate. The enzyme preferred an alkaline condition and the optimal pH was 9. The activity of this alkaline SMase was bile salt dependent and was fully activated by 4-6 mM bile salts. Triton X-100, the non-ionic detergent did not activate bile SMase. Ca2+ and Mg2+ ions had no significant effect at optimal bile salt concentration. The molecular mass of this enzyme was about 85 kDa as measured by Sephadex G200 gel chromatography. In conclusion, we demonstrated a SMase in bile which differs markedly from the known acid and neutral SMase. Its potential important roles in sphingomyelin digestion and gallbladder diseases require further investigation.