Asymmetric addition of ceramides but not dihydroceramides promotes transbilayer (flip-flop) lipid motion in membranes. Biophys

ABSTRACT Transbilayer lipid motion in membranes may be important in certain physiological events, such as ceramide signaling. In this study, the transbilayer redistribution of lipids induced either by ceramide addition or by enzymatic ceramide generation at one side of the membrane has been monitored using pyrene-labeled phospholipid analogs. When added in organic solution to preformed liposomes, egg ceramide induced transbilayer lipid motion in a dose-dependent way. Short-chain (C6 and C2) ceramides were less active than egg ceramide, whereas dihydroceramides or dioleoylglycerol were virtually inactive in promoting flip-flop. The same results (either positive or negative) were obtained when ceramides, dihydroceramides, or diacylglycerols were generated in situ through the action of a sphingomyelinase or of a phospholipase C. The phenomenon was dependent on the bilayer lipid composition, being faster in the presence of lipids that promote inverted phase formation, e.g., phosphatidylethanolamine and cholesterol; and, conversely, slower in the presence of lysophosphatidylcholine, which inhibits inverted phase formation. Transbilayer motion was almost undetectable in bilayers composed of pure phosphatidylcholine or pure sphingomyelin. The use of pyrene-phosphatidylserine allowed detection of flip-flop movement induced by egg ceramide in human red blood cell membranes at a rate comparable to that observed in model membranes. The data suggest that when one membrane leaflet becomes enriched in ceramides, they diffuse toward the other leaflet. This is counterbalanced by lipid movement in the opposite direction, so that net mass transfer between monolayers is avoided. These observations may be relevant to the physiological mechanism of transmembrane signaling via ceramides

Asymmetric Addition of Ceramides but not Dihydroceramides Promotes Transbilayer (Flip-Flop) Lipid Motion in Membranes

Transbilayer lipid motion in membranes may be important in certain physiological events, such as ceramide signaling. In this study, the transbilayer redistribution of lipids induced either by ceramide addition or by enzymatic ceramide generation at one side of the membrane has been monitored using pyrene-labeled phospholipid analogs. When added in organic solution to preformed liposomes, egg ceramide induced transbilayer lipid motion in a dose-dependent way. Short-chain (C6 and C2) ceramides were less active than egg ceramide, whereas dihydroceramides or dioleoylglycerol were virtually inactive in promoting flip-flop. The same results (either positive or negative) were obtained when ceramides, dihydroceramides, or diacylglycerols were generated in situ through the action of a sphingomyelinase or of a phospholipase C. The phenomenon was dependent on the bilayer lipid composition, being faster in the presence of lipids that promote inverted phase formation, e.g., phosphatidylethanolamine and cholesterol; and, conversely, slower in the presence of lysophosphatidylcholine, which inhibits inverted phase formation. Transbilayer motion was almost undetectable in bilayers composed of pure phosphatidylcholine or pure sphingomyelin. The use of pyrene-phosphatidylserine allowed detection of flip-flop movement induced by egg ceramide in human red blood cell membranes at a rate comparable to that observed in model membranes. The data suggest that when one membrane leaflet becomes enriched in ceramides, they diffuse toward the other leaflet. This is counterbalanced by lipid movement in the opposite direction, so that net mass transfer between monolayers is avoided. These observations may be relevant to the physiological mechanism of transmembrane signaling via ceramides

Bulk PPKTP by crystal growth from high temperature solution

: A2. Growth from high temperature solutions A3. Liquid phase epitaxy B2. Ferroelectric materials B2. Nonlinear optic materials a b s t r a c t Periodically-poled ferroelectric crystals show unprecedented efficiency and properties otherwise impossible to obtain. Unfortunately, the sample thickness obtainable today limits their use to low and moderate power application. With the aim of increasing the size of periodically domain-structured crystals with a controlled and regular grating period, we proposed an epitaxial growth process using seeds made of thin plates domain engineered by electric field poling. We demonstrated this process with the ferroelectric crystal KTiOPO 4 (KTP) which is one of the most promising candidate materials for that purpose. The poling step requires a sample exhibitin