Coexistence of immiscible mixtures of palmitoylsphingomyelin and palmitoylceramide in monolayers and bilayers.

A combination of lipid monolayer- and bilayer-based model systems has been applied to explore in detail the interactions between and organization of palmitoylsphingomyelin (pSM) and the related lipid palmitoylceramide (pCer). Langmuir balance measurements of the binary mixture reveal favorable interactions between the lipid molecules. A thermodynamically stable point is observed in the range approximately 30-40 mol % pCer. The pSM monolayer undergoes hyperpolarization and condensation with small concentrations of pCer, narrowing the liquid-expanded (LE) to liquid-condensed (LC) pSM main phase transition by inducing intermolecular interactions and chain ordering. Beyond this point, the phase diagram no longer reveals the presence of the pSM-enriched phase. Differential scanning calorimetry (DSC) of multilamellar vesicles reveals a widening of the pSM main gel-fluid phase transition (41 degrees C) upon pCer incorporation, with formation of a further endotherm at higher temperatures that can be deconvoluted into two components. DSC data reflect the presence of pCer-enriched domains coexisting, in different proportions, with a pSM-enriched phase. The pSM-enriched phase is no longer detected in DSC thermograms containing >30 mol % pCer. Direct domain visualization has been carried out by fluorescence techniques on both lipid model systems. Epifluorescence microscopy of mixed monolayers at low pCer content shows concentration-dependent, morphologically different pCer-enriched LC domain formation over a pSM-enriched LE phase, in which pCer content close to 5 and 30 mol % can be determined for the LE and LC phases, respectively. In addition, fluorescence confocal microscopy of giant vesicles further confirms the formation of segregated pCer-enriched lipid domains. Vesicles cannot form at >40 mol % pCer content. Altogether, the presence of at least two immiscible phase-segregated pSM-pCer mixtures of different compositions is proposed at high pSM content. A condensed phase (with domains segregated from the liquid-expanded phase) showing enhanced thermodynamic stability occurs near a compositional ratio of 2:1 (pSM/pCer). These observations become significant on the basis of the ceramide-induced microdomain aggregation and platform formation upon sphingomyelinase enzymatic activity on cellular membranes.