Interaction of ceramides with phosphatidylcholine, sphingomyelin and sphingomyelin/cholesterol bilayers

Ceramides (Cers) may exert their biological activity through changes in mem brane structure and organization. To understand this mechanism, the effect of Cer on the biophysical properties of phosphatidylcholine, sphingomyelin (SM) and SM/cholesterol bilayers was determined using fluorescence probe te chniques. The Cers were bovine brain Cer and synthetic Cers that contained a single acyl chain species. The phospholipids were 1-palmitoyl-3-oleoyl-sn -glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glyero-3-phosphocho line (DPPC) and bovine brain, egg yolk and bovine erythrocyte SM. The addit ion of Cer to POPC and DPPC bilayers that were in the liquid-crystalline ph ase resulted in a linear increase in acyl chain order and decrease in membr ane polarity. The addition of Cer to DPPC and SM bilayers also resulted in a linear increase in the gel to liquid-crystalline phase transition tempera ture (T-M). The magnitude of the change was dependent upon Cer lipid compos ition and was much higher in SM bilayers than DPPC bilayers. The addition o f 33 mol% cholesterol essentially eliminated the thermal transition of SM a nd SM/Cer bilayers. However, there is still a linear increase in acyl chain order induced by the addition of Cer. The results are interpreted as the f ormation of DPPC/Cer and SM/Cer lipid complexes. SM/Cer lipid complexes hav e higher T(M)s than the corresponding SM because the addition of Cer reduce s the repulsion between the bulky headgroup and allows closer packing of th e acyl chains. The biophysical properties of a SM/Cer-rich bilayer are depe ndent upon the amount of cholesterol present. In a cholesterol-poor membran e, a sphingomyelinase could catalyze the isothermal conversion of a liquid- crystalline SM bilayer to a gel phase SM/Cer complex at physiological tempe rature. (C) 2001 Elsevier Science B.V. All rights reserved.