Lipid microdomains in dimyristoylphosphatidylcholine-ceramide liposomes

Binary membranes composed of dimyristoylphosphatidylcholine (DMPC) and natural ceramide (up to a mole fraction X-cer = 0.25) were investigated by measuring the excimer:monomer fluorescence emission intensity ratio I-E:I-M for the pyrene labelled phospholipid probe 1-palmitoyl-2[(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC), by monitoring fluorescence polarization of diphenylhexatriene (DPH), as well as using differential scanning calorimetry (DSC). Increasing X-cer>0.10 both below and above the main transition temperature T-m increased I-E:I-M for PPDPC maximally approximately 1.6-fold at X-cer = 0.25. Above T-m, and when X-cer approaches 0.10, fluorescence polarization P for DPH increases steeply, reflecting an overall decrease in acyl chain motions. At X-cer = 0.10 there is a discontinuity in P and upon further increase in the content of ceramide a smaller, yet significant increase in P is evident, DSC revealed ceramide to increase the pretransition temperature until at X-cer exceeding 0.07 this transition was no longer evident. Simultaneously, increasing X-cer up to 0.05 increased T-m from 23.9 to 24.6 degrees C. Total enthalpy Delta H-m of the main transition diminished progressively upon increase in X-cer up to approximate to 0.10. Above this concentration of ceramide a new endotherm became evident at 22.5 degrees C, and exceeding X-cer = 0.14 this endotherm became dominant. Our results indicate an enrichment of the pyrene labelled phospholipid analog into microdomains concomitant with the formation of a distinct ceramide-enriched phase at X-cer > 0.10. (C) 1997 Elsevier Science Ireland Ltd.