DIGESTION OF CERAMIDE BY HUMAN-MILK BILE SALT-STIMULATED LIPASE

Background: There is a renewed interest in metabolism of sphingolipids because of their role in signal transduction. Sphingomyelin is the do minating phospholipid in human milk but its metabolism and possible fu nction in the gastrointestinal tract of breast fed infants is unknown. We explored whether bile salt-stimulated milk lipase has a role in sp hingolipid metabolism. Methods: In vitro assays of sphingomyelinase an d ceramidase activities, using radiolabeled substrates, human milk sam ples and purified native and recombinant variants of bile salt-stimula ted milk lipase with or without known activators or inhibitors. Result s: Human whey and purified Lipase catalysed hydrolysis of palmitoyl-la beled ceramide with the highest rate around pH 8.5-9.0. 1 mg of lipase hydrolysed 0.7 mu mol ceramide in one hour at pH 8.5 in presence of 4 mM bile salt. The activity of whey was inhibited by antibodies toward s human bile salt-stimulated milk lipase, indicating that this lipase accounted for virtually all ceramidase activity in the milk. In contra st, bile salt-stimulated milk lipase showed no activity against sphing omyelin. However we give evidence of a separate, hitherto unknown, aci d sphingomyelinase in human milk. Under the used in vitro conditions t his sphingomyelinase could account for hydrolysis of half of milk sphi ngomyelin in one hour. Conclusions: Human milk bile salt-stimulated mi lk lipase hydrolyses ceramide and may thus have a role in sphingomyeli n digestion, but only after initial hydrolysis to ceramide and phospho rylcholine. Part of the latter could be carried out in the stomach by the acid milk sphingomyelinase now described. We speculate that these two milk enzymes may be of importance for optimal use of human milk sp hingolipids.