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Biomedical subjects

S W Hui

Publications and source records attributed to S W Hui.

At least 127 records · Page 7Linked to original sources
PubMed1983

Effects of physical states of phospholipids on the incorporation and cytochalasin B binding activity of human erythrocyte membrane proteins in reconstituted vesicles.

Proteoliposomes were reconstituted from a Triton extract of human erythrocyte membrane proteins and a mixture of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) of varying ratios. With mixtures of egg PC and soybean PE, the protein/lipid ratio of the reconstituted vesicles was maximal at 25% PC and 75% PE, the composition which is known to have a maximum bilayer disruption (highest occurrence of lipidic particles seen by freeze-fracture electron microscopy). With mixtures of 1-palmitoyl-2-oleoyl-PC and dilinoleoyl-PE, which gave vesicles with few isolated lipidic particles at room temperature, the effect was less pronounced. The specific activity of the cytochalasin B (CB) binding protein in the reconstituted vesicles, on the other hand, was increased monotonically up to severalfold as the PC content was increased in the egg PC/soybean PE mixture. A similar increase was observed when soybean PE was partially substituted by dimyristoyl-PC, cholesterol, or transphosphatidylated PE from egg PC. These findings indicate that preexisting defects in the lipid bilayer promote protein incorporation into the bilayer during reconstitution whereas reduction of the bilayer fluidity facilitates the CB binding activity in the reconstituted vesicles.

Binding Sites↗
PubMed1983

Polymorphic phase behaviour of dilinoleoylphosphatidylethanolamine and palmitoyloleoylphosphatidylcholine mixtures. Structural changes between hexagonal, cubic and bilayer phases.

The polymorphic phase behaviour of dilinoleoylphosphatidyethanolamine (DLPE) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) is investigated by freeze-fracture electron microscopy, X-ray diffraction and 31P-NMR. The structures at 5% or less POPC are predominantly inverted hexagonal (HII), whereas at 15% or more POPC, the structure is mostly bilayer (L), interrupted by defects (lipidic particles). A cubic phase structure is observed in the transition range between H and L phases; the cubic arrangement deteriorates at higher temperatures into an amorphous aggregate of spherical units. Both cubic and amorphous structures contribute to the isotropic 31P resonance, with no preference for PC or PE partitioning in the isotropic motion as observed by high resolution NMR. The existence of the cubic phase seems to depend cirtically on the homogeneity and the degree unsaturation of the phospholipids.

Freeze Fracturing↗
PubMed1983

The nature of lipidic particles and their roles in polymorphic transitions.

The structural transition between bilayer (L alpha), inverted hexagonal (HII) and inverted cubic (CII) phases in mixtures of unsaturated phosphatidylethanolamines (PE) and phosphatidylcholines (PC) were investigated. Freeze fracture electron micrographs of intermediate stages of phase transitions showed that CII was a stable intermediate form between the L alpha--HII transition. The electron microscopic observation was supported by X-ray diffraction and 31P-NMR results. Detailed morphology revealed that during the L alpha--CII transition, interlamellae attachment points (conical lipidic particles) connect adjacent bilayers to form arrays of entrapped water pockets (inverted micelles). These water-containing spherical units were packed in a cubic lattice. In the CII to HII transition, these spherical units were linearly connected to form tubes. During the L alpha--HII transition, a ripple pattern was observed across the otherwise smooth lamellar. The troughs of the ripples were transformed into linear connections between adjacent bilayers, thereby converting multilayer structures into parallel tubes. No lipidic particles were involved in this type of transition. We show that there are different mechanisms involved in the L alpha, HII, CII polymorphic transitions, and that different types of 'lipidic particles' representing different molecular organizations may be involved in each case. Models of these transitions are proposed.

Lipid Bilayers↗
PubMed1982

Phase behavior of the major lipids of tetrahymena ciliary membranes.

The major lipids of Tetrahymena membranes have been purified by thin-layer and high pressure liquid chromatography and the phosphatidylethanolamine and aminoethylphosphonate lipids were examined in detail. 31P-NMR, X-ray diffraction and freeze-fracture electron microscopy were employed to describe the phase behavior of these lipids. The phosphatidylethanolamine was found to form a hexagonal phase above 10 degrees C. The aminoethylphosphonate formed a lamellar phase up to 20 degrees C, but converted to a hexagonal phase structure at 40 degrees C. Small amounts of phosphatidylcholine stabilized the lamellar phase for the aminoethylphosphonate. 31P-NMR spectra of the intact ciliary membranes were consistent with a phospholipid bilayer at 30 degrees C, suggesting that phosphatidylcholine in the membrane stabilized the lamellar form, even though most of the lipid of that membrane prefers a hexagonal phase in pure form at 30 degrees C. 31P-NMR spectra also showed a distinctive difference in the chemical shift tensor of the aminoethylphosphonolipid, when compared to that of phosphatidylethanolamine, due to the difference in chemical structure of the polar headgroups of the two lipids.

Animals↗
PubMed1981

The condensing effect of glucagon on phospholipid bilayers.

Glucagon forms discoidal particles with dimyristoylphosphatidylcholine at temperatures below the phase transition. Under these conditions and at a lipid to protein molar ratio of 20 : 1, glucagon is observed to induce a closer packing of the phospholipid bilayer. Similar effects are observed upon the interaction of glucagon with dilauroylphosphatidylcholine. In the region of the phase transition the discoidal particles are observed by freeze-fracture electron microscopy to undergo end-to-end association leading to the formation of multilamellar structures containing only a few layers and having a large internal volume. Above the phase transition temperature the properties of the lipid appear to be unperturbed by glucagon according to either freeze-fracture or densitometer studies. These results support the importance of phospholipid phase transitions in peptide-lipid interactions.

Densitometry↗
PubMed1981

Interactions of La2+ with phosphatidylserine vesicles: binding, phase transition, leakage, 31P-NMR and fusion.

The interaction of La2+ with phosphatidylserine vesicles is studied by differential scanning calorimetry, 140La binding, 31P-NMR chemical shifts and relaxation rates, carboxyfluorescein and [14C]sucrose release, X-ray diffraction and freeze-fracture electron microscopy. In the presence of La3+ concentrations above 1 mM and an incubation temperature of 38 degrees C, i.e., at the phase transition temperature of the complex La/phosphatidylserine, the binding ratio of La/lipid exceeds a 1/3 ratio, reaching saturation at a 1/2 ratio. Analysis, employing a modified Gouy-Chapman equation, indicates a significant increase in the intrinsic binding constant of La/phosphatidylserine when the La3+ concentrations exceeds the threshold concentration for leakage. The analysis illustrates that at the molecular level the binding of La3+ can be comparable to or even weaker than that of Ca2+, but that even when present at smaller concentrations La3+ competes with and partially displaces Ca2+ from membranes or other negatively charged surfaces. The results suggest that the sequence La3+ greater than Ca2+ greater than Mg2+ reflects both the binding strength of these cations to phosphatidylserine as well as their ability to induce leakage, enhancement of 31P spin-lattice relaxation rates, fusion and other structural changes. The leakage, fusion, and other structural changes are more pronounced at the phase transition temperature of the La/lipid complex.

Electron Spin Resonance Spectroscopy↗
PubMed1981

Membrane fusion through point defects in bilayers.

Fusion between bilayers of mixed egg phosphatidylcholine and soybean phosphatidylethanolamine was induced by freezing and thawing. Contact points between bilayers were observed by freeze fracture electron microscopy, and isotropic molecular motional averaging was detected by phosphorus-31 nuclear magnetic resonance under fusion conditions. A molecular model of point defect structure is proposed as an intermediate stage of fusion.

Freeze Fracturing↗
PubMed1981

Lipid-polyethylene glycol interactions: I. Induction of fusion between liposomes.

Fusion between unilamellar vesicles of both egg phosphatidylcholine and bovine phosphatidylserine was induced by polyethylene glycol. Aggregation and fusion events were monitored by electron microscopy and turbidity measurements. The threshold concentration of polyethylene glycol for aggregation and fusion is found to be independent of lipid concentration. Typically, aggregation of phosphatidylcholine vesicles starts at 2.5% (wt/wt) polyethylene glycol, but fusion is not significant until the polyethylene glycol concentration reaches 35%. Multilamellar vesicles were formed as a result of fusion.

Animals↗
PubMed1981

Lipid-polyethylene glycol interactions: II. Formation of defects in bilayers.

The kinetic properties of L-leucine transport across the human red blood cell membrane was analyzed according to the simple pore and carrier theory of Lieb and Stein (Biochim. Biophys. Acta, 1974, 373: 165-177 and 178-196) at 25 degrees C, pH 7.4. Several methods were used in order to obtain a thorough kinetic description of L-leucine transport. A rejection of the simple pore model was suggested from the result of zero-trans influx and zero-trans and equilibrium-exchange efflux experiments. Several predictions from the simple carrier model, based on the requirement of consistency among different kinetic parameters, were tested in infinite experiments, i.e. experiments performed at a high concentration of substrate at one of the faces of the membrane. The simple pore model was rejected, but no crucial evidence against a simple carrier model, which displays symmetric properties at 25 degrees C, was found in the concentration range considered (0.002-68 mM). The relative magnitudes of the rate constants of the translocation process are discussed, and it is concluded (a) that both the dissociation and translocation of carrier-complex is faster than the translocation of the empty carrier, (b) that no translocation step is rate determining, and (c) that the carrier-complex is equally distributed across the membrane at equilibrium. The present work provides a unique example of a carrier-mediated transport mechanism which displays symmetric properties. L-leucine transport in red blood cells may be a convenient system for studying molecular mechanisms of facilitated transport.

Cell Fusion↗
PubMed1981

Geometry of phase-separated domains in phospholipid bilayers by diffraction-contrast electron microscopy.

The sizes and shapes of solidus (gel) phase domains in the hydrated molecular bilayers of dilauroylphosphatidylcholine/dipalmitoylphasphatidylcholine (DLPC/DPPC) (1:1) and phosphatidylserine (PS)/DPPC (1:2) are visualized directly by low dose diffraction-contrast electron microscopy. The temperature and humidity of the bilayers are controlled by an environmental chamber set in an electron microscope. The contrast between crystalline domains is enhanced by electron optical filtering of the diffraction patterns of the bilayers. The domains are seen as a patchwork in the plane of the bilayer, with an average width of 0.2-0.5 micrometer. The percentage of solidus area measured from diffraction-contrast micrographs at various temperatures agrees in general with those depicted by known phase diagrams. The shape and size of the domains resemble those seen by freeze-fracture in multilamellar vesicles. Temperature-related changes in domain size and in phase boundary per unit area are more pronounced in the less miscible DLPC/DPPC mixture. No significant change in these geometric parameters with temperature is found in the PS/DPPC mixture. Mapping domains by their molecular diffraction signals not only verifies the existance of areas of different molecular packing during phase separation but also provides a quantitative measurement of structural boundaries and defects in lipid bilayers.

Biophysical Phenomena↗
PubMed1981

The mouse pale ear pigment mutant as a possible animal model for human platelet storage pool deficiency.

The mouse pigment mutant pale ear, ep/ep, which has a defect in kidney lysosomal enzyme secretion, had prolonged bleeding on experimental injury. Platelet counts and platelet protein did not differ from normal. There was, however, a deficiency in the platelet dense granule contents, serotonin, ATP, and ADP. Furthermore, a marked reduction of platelet dense granules was observed by electron microscopy. The results suggest that pale ear is a useful animal model in the study of platelet storage pool disease. Studies on this mutant and other pigment mutants have established that one gene can regulate at least three subcellular organelles, including the melanosome, the lysosome, and the platelet dense granule.

Adenosine Diphosphate↗
PubMed1980

Temperature-dependent morphological and phase behavior of sphingomyelin.

Aqueous dispersions of bovine brain sphingomyelin were studied as a function of temperature. 31P-NMR, X-ray diffraction, and negative-stain and freeze-fracture electron microscopy were used to determine the morphology and phase structure at several temperatures. 31P-NMR indicated a change in phase structure with an increase in temperature. Evidence was found only for the lamellar phase at all temperatures studied with X-ray diffraction. Electron microscopy unexpectedly revealed the spontaneous development of small unilamellar vesicles at elevated temperatures, consistent with the 31P-NMR data, in the absence of any outside disturbances.

Freeze Fracturing↗
PubMed1980

Radiation damage of phosphatidylcholine bilayers effects of temperature and hydration.

Structural damage by 100 kV electron irradiation in hydrated and dehydrated lecithin bilayers of different acyl chain lengths were measured by the alteration of electron diffraction patterns. At temperatures below the transition temperatures (Tc), the total diffraction intensities of both hydrated and dehydrated bilayers decreased at similar exponential rates, starting at a cumulative dose of 2 x 10(-4) C/cm2. The intensity anisotropy along diffraction rings was not noticeably altered by radiation. At temperatures less than 6 degrees from Tc, the total diffraction intensity of hydrated bilayers increased initially with radiation, while the intensity anisotropy along diffraction rings was also changed by radiation. Unsupported and hydrated bilayers at temperatures well below Tc ruptured at a cumulative dose of 10(-3) C/cm2, whereas those near or above Tc were more resistant to radiation-induced rupture. The results are discussed in terms of molecular damage, bilayer movement and annealing.

Chemical Phenomena↗