DIFFERENT EFFECTS OF ENZYME-GENERATED CERAMIDES AND DIACYLGLYCEROLS IN PHOSPHOLIPID MEMBRANE-FUSION AND LEAKAGE

When large unilamellar vesicles consisting of sphingomyelin:phosphatid ylethanolamine:cholesterol (2:1:1 molar ratio) are treated with sphing omyelinase, production of ceramides in the bilayer is accompanied by l eakage of vesicle aqueous contents and by vesicle aggregation in the a bsence of lipid mixing or vesicle fusion, This is in contrast to the s ituation of atidylcholine:phosphatidylethanolamine:cholesterol (2:1:1 molar ratio) liposomes when treated with phospholipase C. In that case , in situ generation of diacylglycerol leads to vesicle aggregation fo llowed by vesicle fusion in the absence of leakage (Nieva, J. L., Goni , F. M., and Alonso, A, (1989) Biochemistry 28, 7364-7367). Moreover, when ceramides (5-10 mol %) are included in the formulation of the pho sphatidylcholine-containing vesicles, they reduce the lag time of phos pholipase C-induced fusion, although they are less active than diacylg lycerols in this respect. P-31 NMR studies of aqueous lipid dispersion s show that diacylglycerols as well as ceramides induce a thermotropic lamellar to non-lamellar phase transition in both phospholipid:choles terol mixtures under study although sphingomyelin-containing bilayers are more stable than those containing phosphatidylcholine, and ceramid e is less active than diacylglycerol in promoting non-lamellar phase f ormation. These observations are relevant to both the physiological ro le of ceramides and the current views on the mechanism of membrane fus ion.