Résumé
In order to study the basic physical phenomena underlying complex lipid transbilayer movement in biological membranes, we have measured the transmembrane diffusion of spin-labelled analogues of sphingolipids in phosphatidylcholine (PC) large unilamellar vesicles in the absence or presence of cholesterol, going from a fluid ( liquid disordered) ld, phase to a more viscous, liquid ordered (lo), phase. We have found cholesterol to reduce the transverse diffusion of glucosylceramide (GlcCer) and galactosylceramide (GalCer) in a concentration-dependent manner. However, surprisingly, we could neither detect any influence of cholesterol on the rapid flip-flop of ceramide nor on the flip-flop of dihydroceramide, for which the 1/2 of flip-flop remains in the order of 1 minute at 20°C in the presence of cholesterol. As a consequence of rapid flip-flop of ceramide in both the lo and the ld phase, ceramide is likely to distribute between the two monolayers of a membrane, and could in principle partition into segregated domains in each side of the plasma membrane of eukaryotic cells.