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1.
Critical temperature for unilamellar vesicle formation in dimyristoylphosphatidylcholine dispersions from specific heat measurements. 总被引:1,自引:1,他引:0
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Using a heat conduction calorimeter with very high resolution (+/- 0.00005 J/degrees C.cm3), we have measured the specific heat CpL between 25 and 35 degrees C of dimyristoylphosphatidylcholine (DMPC) in aqueous dispersions. Previous studies of the temperature dependence of the chemical potential of DMPC in the L alpha phase (lamellar, liquid crystalline) indicated that a dispersion consisting only of unilamellar vesicles forms spontaneously at a critical temperature T* of 29.0 degrees C. Our present measurements show an anomaly in CpL between 28.70 and 29.50 degrees C: the curve for CpL versus T first decreases and then exhibits an inflection point at 28.96 degrees C before it flattens. This anomaly is attributed to the transformation from multilamellar dispersion to unilamellar vesicles at T* = 28.96 degrees C. Two independent properties of the CpL data also indicate T* is a critical point for the formation of unilamellar vesicles: (a) the time to reach equilibrium upon changing temperature increased dramatically between 28.7 and 28.96 degrees C, increasing as (T* - T)-1; at T > T* the dramatic "slowing-down" phenomenon was not observed. This slowing-down near T* is a general characteristic of critical phenomena. (b) The free energy change for the multilamellar-unilamellar transformation was obtained from the CpL-T data over this temperature interval and found to be 3.2 J/mol or 0.016 ergs/cm2 of bilayer, in agreement with other estimates of the interaction energy between neutral bilayers. We conclude with a discussion of the implications for membrane bilayer stability of these newly identified dynamic properties of the transformation. 相似文献
2.
G. Menestrina C. Pederzolli M. Dalla Serra M. Bregante F. Gambale 《The Journal of membrane biology》1996,149(2):113-121
Escherichia coli hemolysin is known to cause hemolysis of red blood cells by forming hydrophilic pores in their cell membrane. Hemolysin-induced
pores have been directly visualized in model systems such as planar lipid membranes and unilamellar vesicles. However this
hemolysin, like all the members of a related family of toxins called Repeat Toxins, is a potent leukotoxin. To investigate
whether the formation of channels is involved also in its leukotoxic activity, we used patch-clamped human macrophages as
targets. Indeed, when exposed to the hemolysin, these cells developed additional pores into their membrane. Such exogenous
pores had properties very different from the endogenous channels already present in the cell membrane (primarily K+ channels), but very similar to the pores formed by the toxin in purely lipidic model membranes. Observed properties were:
large single channel conductance, cation over anion selectivity but weak discrimination among different cations, quasilinear
current-voltage characteristic and the existence of a flickering pre-open state of small conductance. The selectivity properties
of the toxin channels appearing in phospholipid vesicles were also investigated, using a specially adapted polarization/depolarization
assay, and were found to be completely consistent with that of the current fluctuations observed in excised macrophage patches.
Received: 14 August 1995/Revised: 2 October 1995 相似文献
3.
The relative weight of electrostatic interactions and hydrophobic forces in the process of membrane disruption caused by
E. coliα-haemolysin (HlyA) has been studied with a purified protein preparation and a model system consisting of large unilamellar
vesicles loaded with water-soluble fluorescent probes. Vesicles were prepared in buffers of different ionic strengths, or
pHs, and the net surface charge of the bilayers was also modified by addition of negatively (e.g., phosphatidylinositol) or
positively (e.g., stearylamine) charged lipids. The results can be interpreted in terms of a multiple equilibrium in which
α-haemolysin may exist: aggregated HlyA ⇄ monomeric HlyA ⇄ membrane-bound HlyA. In these equilibria both electrostatic and
hydrophobic forces are significant. Electrostatic forces become substantial under certain circumstances, e.g., membrane binding
when bilayer and protein have opposite electric charges. Protein adsorption to the bilayer is more sensitive to electrostatic
forces than membrane disruption itself. In the latter case, the irreversible nature of protein insertion may overcome electrostatic
repulsions. Also of interest is the complex effect of pH on the degree of aggregation of an amphipathic toxin like α-haemolysin,
since pH changes are not only influencing the net protein charge but may also be inducing protein conformational transitions
shown by changes in the protein intrinsic fluorescence and in its susceptibility to protease digestion, that appear to regulate
the presence of hydrophobic patches at the surface of the molecule, thus modifying the ability of the toxin to either aggregate
or become inserted in membranes.
Received: 29 October 1996/Revised: 4 February 1997 相似文献
4.
A novel method was developed for the direct examination of pairwise encounters between positively and negatively charged
phospholipid bilayer vesicles. Giant bilayer vesicles (unilamellar, 4–20 μm in diameter) prepared from 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine, a new cationic phospholipid derivative, were electrophoretically maneuvered into contact with
individual anionic phospholipid vesicles. Fluorescence video microscopy revealed that such vesicles commonly underwent fusion
within milliseconds (1 video field) after contact, without leakage. Fusion occurred at constant volume and, since flaccid
vesicles were rare, the excess membrane was not available after fusion. Hemifusion (the outer monolayers of each vesicle fused
while the inner monolayers remained intact) was inferred from membrane-bound dye transfer and a change in the contact area.
Hemifusion was observed as a final stable state and as an intermediate to fusion of vesicles composed of charged phospholipids
plus zwitterionic phospholipids. Hemifusion occurred in one of three ways following adhesion: either delayed with an abrupt
increase in area of contact, immediately with a gradual increase in area of contact, or with retraction during which adherent
vesicles dissociated from a flat contact to a point contact. Phosphatidylethanolamine strongly promoted immediate hemifusion;
the resultant hemifused state was stable and seldom underwent complete fusion. Although sometimes single contacts between
vesicles led to rupture of both, in other cases, a single vesicle underwent multiple fusion events. Direct observation has
unequivocally demonstrated the fusion of two, isolated bilayer-bounded bodies to yield a stable, non-leaky product, as occurs
in cells, in the absence of proteins.
Received: 25 November 1998/Revised: 23 March 1999 相似文献
5.
M. Behzadipour R. Ratajczak K. Faist P. Pawlitschek A. Trémolières M. Kluge 《The Journal of membrane biology》1998,166(1):61-70
The present study deals with the phenotypic adaptation of tonoplast fluidity in the CAM plant Kalancho? daigremontiana to changes in growth temperature. Tonoplast fluidity was characterized by measuring fluorescence depolarization in membranes
labeled with fluorescent fatty acid analogues and by following formation of eximeres in membranes labeled by eximere-forming
fluorophores. With both techniques it was found that exposure of the plants to higher growth temperature compared with the
control decreased the fluidity of the tonoplast while exposure to lower growth temperature caused the opposite. Three hours
of high temperature treatment (raised from 25°C to 35°C; ``heat shock') were sufficient to decrease the tonoplast fluidity
to roughly the same extent as growth under high temperature for 30 days. The phenotypic response of tonoplast fluidity to
changes in growth temperature was found only in the complete membrane, not however in the lipid matrix deprived of the membrane
proteins. Heat treatments of the plants decreased the lipid/protein ratio while exposure to low temperature (for 30 days)
increased it. Heat treatments led to a decrease in the percentage of linolenic acid (C18:3) and linoleic acid (C18:2), heat
shock and low temperature treatments induced an increase in the percentage of linoleic acid (C18:3), with concomitant decrease
in the percentage of linoleic acid (C18:2). However, in the case of heat shock, increase in linolenic acid concerned mainly
monogalactosyldiacylglycerol, while with low temperature treatment linoleic acid increased in phosphatidylcholine. Both treatment
of the plants with high and low temperature led to a slight decrease in the contribution of phosphatidylcholine and phosphoethanolamine
to the total phospholipid content of the tonoplast. High-temperature treatment of the plants not only decreased the phospholipid/protein
ratio in the tonoplast, but also led to the occurrence of a 35 kDa polypeptide in the tonoplast which cross-reacted with an
antiserum against the tonoplast H+-ATPase holoenzyme. The important role of membrane proteins in bringing about the phenotypic rigidization of the tonoplast
was mimicked by reconstitution experiments showing that incorporation of the proteins isolated from the tonoplast into phosphatidylcholine
vesicles decreased the fluidity of this membrane system. As to be expected from the analyses in the natural membrane, the
degree of this effect depended on the phospholipid/protein ratio.
Received: 4 March 1998/Revised: 28 July 1998 相似文献
6.
S.E. Gasanov M.A. Alsarraj N.E. Gasanov E.D. Rael 《The Journal of membrane biology》1997,155(2):133-142
Membrane-active toxins from snake venom have been used previously to study protein-lipid interactions and to probe the physical
and biochemical states of biomembranes. To extend these studies, we have isolated from Naja naja kaowthia (cobra) venom a cytotoxin free of detectable phospholipase A2 (PLA2). The amino acid composition, pI (10.2), and net charge of the cytotoxin compares well with membrane-active toxins isolated
from venoms of other cobras. The cytotoxin, shown by a spin label method, associates with PLA2 in buffers at pH values between 7.0 and 5.0, but not at pH 4.0. It is suggested that cytotoxin and PLA2 (pI close to 4.8) associate electrostatically in the native venom. The effect of the cytotoxin on model phospholipid membranes
was studied by EPR of spin probes in oriented lipid multilayers and 1H-NMR of sonicated liposomes. The cytotoxin did not significantly affect the packing of lipids in pure phosphatidylcholine
(PC) membranes and in PC membranes containing 10 mol% phosphatidic acid (PA) or cardiolipin (CL). However, the cytotoxin induced
an increase in membrane permeability and formation of nonbilayer structures in PC membranes containing 40 mol% of PA or CL.
The purified cytotoxin was cytocidal to Jurkat cells, but had little effect on normal human lymphocytes. However, both Jurkat
cells and normal lymphocytes were killed equivalently when treated with 10−9
m PLA2 and 10−5
m cytotoxin in combination. From its effect on model membranes and Jurkat cells, it is suggested that purified cytotoxin preferentially
targets and disrupts membranes that are rich in acidic phospholipids on the extracellular side of the plasma membrane.
Received: 20 March 1996/Revised: 25 September 1996 相似文献
7.
Yu. A. Ivashchuk-Kienbaum 《The Journal of membrane biology》1996,151(3):247-259
A method has been developed to monitor changes of the membrane potential across vesicle membranes in real time. Using the
potential-sensitive fluorescent dye indocyanine and on the basis of a water/lipid redistribution model, a calculation procedure
has been introduced to estimate the membrane potential in vesicles with incorporated cytochrome-c oxidase. Physical parameters,
such as vesicle size distribution and density of the lipid bilayer were estimated and used as calculation parameters. By extrapolation
of the transient potential change to zero time, the initial rate of the potential change (dU/dt) could be calculated. It is also shown, that the initial potential change (dU/dt) may be used to study the proton/electron stoichiometry of cytochrome-c oxidase incorporated in the vesicles.
Received: 28 September 1995/Revised: 6 February 1996 相似文献
8.
Vestergaard MC Yoda T Hamada T Akazawa Ogawa Y Yoshida Y Takagi M 《Biochimica et biophysica acta》2011,1808(9):2245-2251
The effect of temperature change(s) on the dynamics of giant unilamellar vesicles containing oxidized and non-oxidized cholesterol was investigated and characterized. We have demonstrated that (i) major cholesterol auto-oxidation products, 7β-hydroxycholesterol (7β) and 7-ketocholesterol (7keto), rendered vesicles more responsive to temperature changes; (ii) 7keto imparted greater thermo-induced membrane dynamics than 7β; (iii) 7β and 7keto vesicles synergistically were more thermo-responsive than the individual oxysterols; (iv) the thermo-responsiveness of 7keto-containing vesicles was equivalent to that of 25 hydroxycholesterol (25OH)-containing vesicles; and (v) we have characterized the observed membrane dynamics. The results provide a new plausible mechanism: oxidative-stressed membranes in conjunction with temperature change induce membrane dynamics. These findings improve the mechanisms reported previously that attributed the induced dynamics solely to membrane oxidation. 相似文献
9.
L-lactate transport mechanism across rat jejunal enterocyte was investigated using isolated membrane vesicles. In basolateral
membrane vesicles l-lactate uptake is stimulated by an inwardly directed H+ gradient; the effect of the pH difference is drastically reduced by FCCP, pCMBS and phloretin, while furosemide is ineffective.
The pH gradient effect is strongly temperature dependent. The initial rate of the proton gradient-induced lactate uptake is
saturable with respect to external lactate with a K
m
of 39.2 ± 4.8 mm and a J
max of 8.9 ± 0.7 nmoles mg protein−1 sec−1. A very small conductive pathway for l-lactate is present in basolateral membranes. In brush border membrane vesicles both Na+ and H+ gradients exert a small stimulatory effect on lactate uptake. We conclude that rat jejunal basolateral membrane contains
a H+-lactate cotransporter, whereas in the apical membrane both H+-lactate and Na+-lactate cotransporters are present, even if they exhibit a low transport rate.
Received: 22 October 1996/Revised: 11 March 1997 相似文献
10.
M.G. Leonardi P. Parenti M. Casartelli B. Giordana 《The Journal of membrane biology》1997,159(3):209-217
The mechanical properties of brush border membrane vesicles, BBMV, from rabbit kidney proximal tubule cells, were studied
by measuring the initial and final equilibrium volumes of vesicles subjected to different osmotic shocks, using cellobiose
as the impermeant solute in the preparation buffer.
An elevated intracellular hydrostatic pressure was inferred from osmotic balance requirements in dilute solutions. For vesicles
prepared in 18 and 85 mosm solutions, these pressures are close to 17 mosm (290 mm Hg). The corresponding membrane surface tension is 6.0 × 10−5 N cm−1 while the membrane surface area is expanded by at least 2.2%. When these vesicles are exposed to very dilute solutions the
internal hydrostatic pressure rises to an estimated 84 mosm (1444 mm Hg) just prior to lysis. The corresponding maximal surface tension (pre-lysis) is 18.7 × 10−5 N cm−1, and the maximal expansion of membrane area is 6.8%. The calculated area compressibility elastic modulus was 2.8 × 10−3 N cm−1.
Received: 8 August 1996/Revised: 4 March 1997 相似文献
11.
The mitochondrial outer membrane channel, VDAC, is thought to serve as the major permeability pathway for metabolite flux
between the cytoplasm and mitochondria. The permeability of VDAC to citrate, succinate, and phosphate was studied in channels
reconstituted into planar phospholipid membranes. All ions showed large changes in permeability depending on whether the channel
was in the open or in the low conductance, ``closed' state, with the closed state always more cation selective. This was
especially true for the divalent and trivalent anions. Additionally, the anion flux when the voltage was zero was shown to
decrease to 5–11% of the open state flux depending on the anion studied. These results give the first rigorous examination
of the ability of metabolites to permeate through VDAC channels and indicate that these channels can control the flux of these
ions through the outer membrane. This lends more evidence to the growing body of experiments that suggest that the outer mitochondrial
membrane has a much more important role in controlling mitochondrial activity than has been thought historically.
Received: 4 November 1996/Revised: 8 January 1997 相似文献
12.
In our study we investigated hemispherical phospholipid bilayer membranes and phospholipid vesicles made from hexadecaprenyl
monophosphate (C80-P), dioleoylphosphatidylocholine (DOPC) and their mixtures by voltammetric and transmission electron microscopy (TEM) techniques.
The current-voltage characteristics, the membrane conductance-temperature relationships and the membrane breakdown voltage
have been measured for different mixtures of C80-P/DOPC. The membrane hydrophobic thickness and the activation energy of ion migration across the membrane have been determined.
Hexadecaprenyl monophosphate decreased in comparison with DOPC bilayers, the membrane conductance, increased the activation
energy and the membrane breakdown voltage for the various value of C80-P/DOPC mole ratio, respectively. The TEM micrographs of C80-P, DOPC and C80-P/DOPC lipid vesicles showed several characteristic structures, which have been described. The data indicate that hexadecaprenyl
monophosphate modulates the surface curvature of the membranes by the formation of aggregates in liquid-crystalline phospholipid
membranes. We suggest that the dynamics and conformation of hexadecaprenyl monophosphate in membranes depend on the transmembrane
electrical potential. The electron micrographs indicate that polyprenyl monophosphates with single isoprenyl chains form lipid
vesicular bilayers. The thickness of the bilayer, evaluated from the micrographs, was 11 ± 1 nm. This property creates possibility
of forming primitive bilayer lipid membranes by long single-chain polyprenyl phosphates in abiotic conditions. It can be the
next step in understanding the origin of protocells.
Received: 10 January 2000/Revised: 7 June 2000 相似文献
13.
E.I. Pécheur J. Sainte-Marie A. Bienvenüe D. Hoekstra 《The Journal of membrane biology》1999,167(1):1-17
Processes such as endo- or exocytosis, membrane recycling, fertilization and enveloped viruses infection require one or more
critical membrane fusion reactions. A key feature in viral and cellular fusion phenomena is the involvement of specific fusion
proteins. Among the few well-characterized fusion proteins are viral spike glycoproteins responsible for penetration of enveloped
viruses into their host cells, and sperm proteins involved in sperm-egg fusion. In their sequences, these proteins possess
a ``fusion peptide,' a short segment (up to 20 amino acids) of relatively hydrophobic residues, commonly found in a membrane-anchored
polypeptide chain. To simulate protein-mediated fusion, many studies on peptide-induced membrane fusion have been conducted
on model membranes such as liposomes and have employed synthetic peptides corresponding to the putative fusion sequences of
viral proteins, or de novo synthesized peptides. Here, the application of peptides as a model system to understand the molecular details of membrane
fusion will be discussed in detail. Data obtained from these studies will be correlated to biological studies, in particular
those that involve viral and sperm-egg systems. Structure-function relationships will be revealed, particularly in the context
of protein-induced membrane perturbations and bilayer-to-nonbilayer transition underlying the mechanism of fusion. We will
also focus on the involvement of lipid composition of membranes as a potential regulating factor of the topological fusion
site in biological systems.
Received: 3 August 1998/Revised: 15 October 1998 相似文献
14.
Unilamellar vesicle populations having a narrow size distribution and mean radius below 100 nm are preferred for drug delivery applications. In the present work, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was used to prepare giant unilamellar vesicles (GUVs) by electroformation and multilamellar vesicles (MLVs) by thin film hydration. Our experiments show that in contrast to MLVs, a single-pass extrusion of GUVs through track-etched polycarbonate membranes at moderate pressure differences is sufficient to produce small liposomes having low polydispersity index. Moreover, we observe that the drug encapsulating potential of extruded liposomes obtained from GUVs is significantly higher compared to liposomes prepared by extrusion of MLVs. Furthermore, our experiments carried out for varying membrane pore diameters and extrusion pressures suggest that the size of extruded liposomes is a function of the velocity of GUV suspensions in the membrane pore. 相似文献
15.
Prenner E Sommer A Maurer N Glatter O Gorges R Paltauf F Hermetter A 《The Journal of membrane biology》2000,174(3):237-243
Choline phospholipids are the major constituents of the outer layer of the erythrocyte membrane. To investigate their lateral
membrane organization we determined the fluorescence lifetime properties of diphenylhexatriene analogues of phosphatidylcholine,
choline plasmalogen, (the respective enolether derivative), and sphingomyelin inserted into the outer layer of hemoglobin-free
ghosts. Fluorescence lifetimes were recorded by time-resolved phase and modulation fluorometry and analyzed in terms of Continuous
Lorentzian distributions. To assess the influence of membrane proteins on the fluorescence lifetime of the labeled lipids
in the biomembrane, lipid vesicles were used as controls. In general, the lifetime distributions in the ghost membranes are
broad compared to vesicles. Phosphatidylcholine and sphingomyelin exhibit very similar lifetime distributions in contrast
to an increased plasmalogen lifetime heterogeneity in both systems. Orientational effects of side chain mobilities on the
observed lifetimes can be excluded. Fluorescence anisotropies revealed identical values for all three labeled phospholipids
in the biomembrane.
Received: 22 July 1999/Revised: 6 January 2000 相似文献
16.
In vivo studies with leaf cells of aquatic plant species such as Elodea nuttallii revealed the proton permeability and conductance of the plasma membrane to be strongly pH dependent. The question was posed
if similar pH dependent permeability changes also occur in isolated plasma membrane vesicles. Here we report the use of acridine
orange to quantify passive proton fluxes. Right-side out vesicles were exposed to pH jumps. From the decay of the applied
ΔpH the proton fluxes and proton permeability coefficients (PH+) were calculated. As in the intact Elodea plasma membrane, the proton permeability of the vesicle membrane is pH sensitive, an effect of internal pH as well as external
pH on PH+ was observed. Under near symmetric conditions, i.e., zero electrical potential and zero ΔpH, PH+ increased from 65 × 10−8 at pH 8.5 to 10−1 m/sec at pH 11 and the conductance from 13 × 10−6 to 30 × 10−4 S/m2. At a constant pH
i
of 8 and a pH
o
going from 8.5 to 11, PH+ increased more than tenfold from 2 to 26 × 10−6 m/sec. The calculated values of PH+ were several orders of magnitude lower than those obtained from studies on intact leaves. Apparently, in plasma membrane
purified vesicles the transport system responsible for the observed high proton permeability in vivo is either (partly) inactive
or lost during the procedure of vesicle preparation. The residue proton permeability is in agreement with values found for
liposome or planar lipid bilayer membranes, suggesting that it reflects an intrinsic permeability of the phospholipid bilayer
to protons. Possible implications of these findings for transport studies on similar vesicle systems are discussed.
Received: 5 April 1995/Revised: 28 March 1996 相似文献
17.
The presence of proteins in lipid bilayers always decreases the excimer formation rate of pyrene and pyrene lipid analogues
in a way that is related to the protein-to-lipid ratio. Energy transfer measurements from intrinsic tryptophans to pyrene
have shown (Engelke et al., 1994), that in microsomal membranes, the excimer formation rate of pyrene and pyrene fatty acids
is heterogeneous within the membrane plane, because a lipid layer of reduced fluidity surrounds the microsomal proteins. This
study investigates whether of not liposomes prepared from egg yolk phosphatidylcholine with incorporated gramicidin A give
results comparable to those from microsomal membranes. The results indicate that the influence of proteins on the lipid bilayer
cannot be described by one unique mechanism: Small proteins such as gramicidin A obviously reduce the excimer formation rate
by occupying neighboring positions of the fluorescent probe and thus decrease the pyrene collision frequency homogeneously
in the whole membrane plane, while larger proteins are surrounded by a lipid boundary layer of lower fluidity than the bulk
lipid.
The analysis of the time-resolved tryptophan fluorescence of gramicidin A incorporated liposomes reveals, that the tryptophan
quenching by pyrene is stronger for tryptophans located closely below the phospholipid headgroup region because of the pyrene
enrichment in this area of the lipid bilayer.
Received: 29 December 1996/Revised: 15 May 1996 相似文献
18.
Calcium-induced fusion of liposomes was studied with a view to understand the role of membrane tension in this process. Lipid
mixing due to fusion was monitored by following fluorescence of rhodamine-phosphatidyl-ethanolamine incorporated into liposomal
membrane at a self-quenching concentration. The extent of lipid mixing was found to depend on the rate of calcium addition:
at slow rates it was significantly lower than when calcium was injected instantly. The vesicle inner volume was then made
accessible to external calcium by adding calcium ionophore A23187. No effect on fusion was observed at high rates of calcium
addition while at slow rates lipid mixing was eliminated. Fusion of labeled vesicles with a planar phospholipid membrane (BLM)
was studied using fluorescence microscopy. Above a threshold concentration specific for each ion, Ca2+, Mg2+, Cd2+ and La3+ induce fusion of both charged and neutral membranes. The threshold calcium concentration required for fusion was found to
be dependent on the vesicle charge, but not on the BLM charge. Pretreatment of vesicles with ionophore and calcium inhibited
vesicle fusion with BLM. This effect was reversible: chelation of calcium prior to the application of vesicle to BLM completely
restored their ability to fuse. These results support the hypothesis that tension in the outer monolayer of lipid vesicle
is a primary reason for membrane destabilization promoting membrane fusion. How this may be a common mechanism for both purely
lipidic and protein-mediated membrane fusion is discussed.
Received: 27 September 1999/Revised: 22 March 2000 相似文献
19.
Brush border membrane vesicles, BBMV, from eel intestinal cells or kidney proximal tubule cells were prepared in a low osmolarity
cellobiose buffer. The osmotic water permeability coefficient P
f
for eel vesicles was not affected by pCMBS and was measured at 1.6 × 10−3 cm sec−1 at 23°C, a value lower than 3.6 × 10−3 cm sec−1 exhibited by the kidney vesicles and similar to published values for lipid bilayers. An activation energy E
a
of 14.7 Kcal mol−1 for water transport was obtained for eel intestine, contrasting with 4.8 Kcal mol−1 determined for rabbit kidney proximal tubule vesicles using the same method of analysis. The high value of E
a
, as well as the low P
f
for the eel intestine is compatible with the absence of water channels in these membrane vesicles and is consistent with
the view that water permeates by dissolution and diffusion in the membrane. Further, the initial transient observed in the
osmotic response of kidney vesicles, which is presumed to reflect the inhibition of water channels by membrane stress, could
not be observed in the eel intestinal vesicles. The P
f
dependence on the tonicity of the osmotic shock, described for kidney vesicles and related to the dissipation of pressure
and stress at low tonicity shocks, was not seen with eel vesicles. These results indicate that the membranes from two volume
transporter epithelia have different mechanisms of water permeation. Presumably the functional water channels observed in
kidney vesicles are not present in eel intestine vesicles. The elastic modulus of the membrane was estimated by analysis of
swelling kinetics of eel vesicles following hypotonic shock. The value obtained, 0.79 × 10−3 N cm−1, compares favorably with the corresponding value, 0.87 × 10−3 N cm−1, estimated from measurements at osmotic equilibrium.
Received: 28 January 1999/Revised: 15 June 1999 相似文献
20.
《生物化学与生物物理学报:生物膜》2023,1865(3):184116
Unilamellar liposomes often are employed in investigations of lipid-protein interactions and the delivery of drugs in therapies for disease. Also, related lipid-containing nanoparticles have been developed as elements of a new class of mRNA vaccines. We show that only unilamellar films form in equilibrium lipid dispersions, at temperature values {T*} that depend on the identities of the lipids (e.g., T* ≈ 29 °C for DMPC). Thermodynamic analysis confirms that films at air-water surfaces can be used to monitor the properties of the lipid vesicles that form in the dispersion. When T > T*, critical exponents describing film properties as T approaches T* are μ ≈ 1.4 and ν ≈ 0.7, which are close to values for the interfacial tension and the correlation length of density fluctuations at fluid interfaces. These results, and observations that within the bilayer the lateral diffusion of fluorescent lipid probes demonstrates increases at T*, suggest that unilamellar vesicles at T* are a transition state between two different multilamellar structures. We generalize the thermodynamic arguments to explain the linkage between lipid structures in the surface and bulk dispersion within more complex samples, showing that dispersions containing total lipid extracts of cell membranes have properties similar to those in dispersions containing single lipids. Information from various independent studies indicates that T* noted for bilayer membranes of a population of cells is identical to the temperature at which the growth or gestation of the cells occurs in vivo. Examples include whole-cell lipid extracts obtained from bacteria, and poikilothermic and homeothermic animals. 相似文献