Evaluation of hydralazine and procainamide effects on fibroblast membrane fluidity

In this study the membrane fluidity of fibroblasts under different pharmacological treatment was investigated. Two drugs, hydralazine and procainamide, were used to treat the immortalized mouse NIH 3T3 and hamster B14 fibroblasts. Membrane lipid dynamics was measured by fluorescence spectroscopy and electron spin resonance techniques. Two kinds of fluorescent probes (TMA-DPH and 12-(9-anthroyloxy)-stearic acid (12-AS)) and two spin labels (5-doxylstearic acid (5-DS) and 12-doxylstearic acid (12-DS)) were used to monitor fluidity in the upper polar and in the hydrophobic core regions of the lipid bilayer. The drugs influenced the membrane hydrophobic core, of which hydralazine induced fluidization and procainamide increased the rigidity. The membrane fluidity at the surface of the lipid bilayer was not modified by the drugs which indicates that both drugs intercalated mainly into the inner core of the cell membrane.     

Ionizing radiation-induced structural modification of human red blood cells

Summary The effect of gamma radiation on red blood cells have been examined using a spin labeling method. For this purpose two spin labels were used to monitor membrane fluidity: methyl 5-doxylpalmitate (Met 5-DP) and methyl 12-doxylstearate (Met 12-DS). The irradiation of red cells with the doses of 200 and 500 Gy caused decrease of microviscosity in certain regions of lipid bilayer (as indicated by Met 5-DP and Met 12-DS spectra) but did not affect lipid order parameter. The behavior of two other spin labels, maleimide(4-malei-mido-2,2,6,6-tetramethylpiperidine-1-oxyl) and TEMPONE (4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl) indicated:1) conformational changes of membrane proteins,2) modification of cell internal peptides and proteins,3) decreased internal viscosity of red blood cells.     

Exhaustive exercise affects fluidity and alpha-tocopherol levels in brain synaptosomal membranes of normal and vitamin E supplemented rats

Alpha-tocopherol level and fluidity were studied in the neuronal membrane of rat brain after exhaustive exercise. The order parameter, 5-doxyl-stearic acid (5-DS), which is utilized for assessing the fluidity of the lipid bilayer closer to the hydrophilic face of the membrane, decreased in the pons-medulla oblongata, and the motion parameter, 16-doxyl-stearic acid (16-DS) for the core of the lipid bilayer, decreased in the cortex, hippocampus, hypothalamus and striatum, whereas it increased in the cerebellum after exercise. The w/s ratio of n-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)-maleimido (maleimido-TEMPO) for the conformation of SH-protein also decreased in the hippocampus and midbrain after exercise. These changes were not observed in alpha-tocopheryl acetate supplemented rats after exercise. Although the levels of 5-DS, 16-DS and maleimido-TEMPO were affected by alpha-tocopheryl acetate in rat neuronal membranes, fluidity changes were reversible with exercise.     

Alterations in human red blood cell membrane properties induced by the lipopolysaccharide from Proteus mirabilis S1959

The effect of lipopolysaccharide (LPS, endotoxin), isolated from Proteus mirabilis S1959 strain, on red blood cell (RBC) membranes in whole cells as well as on isolated membranes was studied. Lipid membrane fluidity, conformational state of membrane proteins and the osmotic fragility of RBCs were examined using electron paramagnetic resonance spectroscopy and spectrophotometric method. Lipid membrane fluidity was determined using three spin-labeled fatty acids: 5-, 12- and 16-doxylstearic acid (5-, 12- and 16-DS). The addition of LPS S1959 to RBC suspension resulted in an increase in membrane fluidity, as indicated by 12-DS. At the concentrations of 0.5 and 1 mg/ml, LPS treatment led to a significant (P<0.05) increase in lipid membrane fluidity in the deeper region of lipid bilayer (determined by 12-DS). The conformational changes in membrane proteins were determined using two covalently bound spin labels, 4-maleimido-2,2,6,6-tetramethylpiperidine-1-oxyl and 4-iodoacetamido-2,2,6,6-tetramethylpiperidine-1-oxyl (ISL). The highest concentration of endotoxin significantly (P<0.05) decreased the relative rotational correlation time of ISL and significantly (P<0.05) increased the osmotic fragility of RBCs. The effect of endotoxin was much more profound in isolated membranes than in intact cells treated with LPS. At the concentrations 0.5 and 1 mg/ml, LPS led to a significant increase in h(w)/h(s) ratio. These results indicated increased membrane protein mobility, mainly in the spectrin-actin complex in membrane cytoskeleton. These data suggest that LPS-induced alterations in membrane lipids and cytoskeleton proteins of RBCs lead to loss of membrane integrity.     

Interaction of doxorubicin and idarubicin with red blood cells from acute myeloid leukaemia patients

Doxorubicin (DOX) and idarubicin (IDA) are anthracycline antibiotics, widely used in human cancer treatment. The present study addressed the effects of these two drugs on lipid bilayer fluidity, protein conformation and microviscosity in erythrocytes from acute myeloid leukaemia patients, using electron spin resonance (ESR) spectroscopy and fluorescence measurements. Only DOX caused statistically significant changes in the parameters examined. Within 30 min of drug injection, changes were observed in the fluidity of the hydrophobic parts of the lipid bilayer and erythrocyte membrane protein conformation. These changes persisted for up to 24h. Analysis of the EPR Tempamine spectrum also showed that the microviscosity of the erythrocyte interior increased during the early stages of the drug effect. Idarubicin, in contrast, caused no identifiable change in any of the parameters studied and therefore seems to be safe for erythrocytes. We conclude that IDA is markedly less toxic than DOX to erythrocytes from acute myeloid leukaemia patients.     

The Negative Effect of Soy Extract on Erythrocyte Membrane Fluidity: An Electron Paramagnetic Resonance Study

A decrease of erythrocyte membrane fluidity can contribute to the pathophysiology of hypertension. Soy products, which are used as alternative therapeutics in some cardiovascular conditions, contain various isoflavones (genistein, daidzein, and their glucosides, genistin and daidzin), which can incorporate cellular membrane and change its fluidity. The aim of this study was to examine the effects of soy extract (which generally corresponds to the soy products of isoflavone composition) on erythrocyte membrane fluidity at graded depths. We used electron paramagnetic resonance spectroscopy and fatty acid spin probes (5-DS and 12-DS), the spectra of which are dependent on membrane fluidity. After being treated with soy extract, erythrocytes showed a significant (P = 0.016) decrease of membrane fluidity near the hydrophilic surface, while there were no significant changes of fluidity in deeper hydrophobic membrane regions. These results suggest that soy products containing high levels of genistein and isoflavone glucosides may not be suitable for use in hypertension because they decrease erythrocyte membrane fluidity.     

Response of human blood platelet membrane to sodium selenite.

It was demonstrated that incubation of blood platelets with sodium selenite (1-100 microM) resulted in a dose- and time-dependent loss of platelet thiols (both glutathione and protein -SH groups). The effects of sodium selenite on platelet membrane lipid fluidity by the EPR spin-labelling method was also investigated. We showed there were no alterations in membrane fluidity at the deeper regions (12-DOXYL-Ste) in lipid bilayer, a slight increase (approx. 7%, p < 0.03) of h +1/h0 for spin probe 5-DOXYL-Ste was monitored. The amount of Triton-insoluble protein fraction isolated from platelets after incubation (60 min) with selenite was significantly elevated (p < 0.006). It has been suggested that limited increase in lipid fluidity at the surface regions in the lipid bilayer of the platelet membrane in selenite-treated platelets may be the result of alteration in lipid-protein interactions caused by protein conformational changes.     

The nitroxides pirolin and pirolid protect the plasma membranes of rat cardiomyocytes against damage induced by anthracyclines

This study was performed to evaluate the protective effects of pyrroline and pyrrolidine nitroxides Pirolin, PL, and Pirolid, PD, on the plasma membranes of rat cardiomyocytes treated in vitro with anthracycline drugs aclarubicin (ACL) and doxorubicin (DOX). The influence of two concentrations of drugs (10 and 20 microM) and nitroxides (0.1 and 1 mM) as well as their combinations (a drug and a nitroxide) on membrane fluidity was investigated. The plasma membranes of cardiomyocytes were labelled with a hydrophobic fluorescence probe 12-AS and membrane fluidity was estimated on the basis of the fluorescence anisotropy of the probe. We found that aclarubicin and doxorubicin induced a significant dose-dependent decrease in membrane fluidity, whereas the nitroxides (PL and PD) caused its increase. Preincubation of cardiomyocytes with Pirolin entirely protected plasma membranes of these cells against damage caused by DOX. In the same conditions no protective effect of Pirolid was observed. What is more, Pirolid in combination with DOX caused fluidisation of the plasma membranes of cardiomyocytes. Both nitroxides at low concentration (0.1 mM) protected plasma membranes against rigidification induced by aclarubicin, while high concentration (1 mM) was ineffective and caused fluidisation of the plasma membranes of cardiomyocytes.     

Partitioning and membrane disordering effects of dopamine antagonists: influence of lipid peroxidation, temperature, and drug concentration.

The effect of four dopamine antagonists (spiperone, haloperidol, pimozide, and domperidone) on the lipid order of caudate nucleus microsomal membranes and on liposomes from membrane lipid extracts was evaluated and related to the partition coefficients (Kp) of the drugs. Lipid membrane order was determined by fluorescence polarization using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe of the membrane core and 1-[4-(trimethylammonium)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) as a probe of the membrane surface. Dopamine antagonists decrease the fluorescence polarization of both probes, indicating that they disorder the membrane lipids at different depths. Pimozide and domperidone, the drugs with higher Kp values, are more effective at decreasing the polarization of DPH, a probe of the membrane core, than that of TMA-DPH. In contrast, spiperone and haloperidol, which have lower values for Kp, induce more significant decreases in TMA-DPH depolarization, a probe of the membrane surface. These findings indicate that higher partition coefficients of the drugs are directly correlated with an increase of fluidity in the hydrophobic core of brain membranes. Ascorbate/Fe(2+)-induced membrane lipid peroxidation increases membrane order. Membrane lipid peroxidation decreases the partition coefficients of the dopamine antagonists tested. Increasing temperature (4-37 degrees C) decreases membrane order, but temperature effect is less evident after lipid peroxidation. The disordering effect of dopamine antagonists increases with increasing drug concentrations (1-15 microM), a maximum being observed at 10 microM. However, this effect is also less evident after membrane lipid peroxidation. We can conclude that dopamine antagonists and membrane lipid peroxidation affect membrane lipid order and that the action of these drugs is dependent on initial bilayer fluidity. Membrane lipid peroxidation increases membrane order while dopamine antagonists show a disordering effect of membrane phospholipids. This disordering effect can indirectly influence the activity of membrane proteins and it is one of the mechanisms through which membrane function can be altered by these drugs.     

Carbamylation of proteins leads to alterations in the membrane structure of erythrocytes

The effect of the sodium cyanate-induced carbamylation (carbamoylation) of proteins in erythrocytes was studied using spin labelling and spectrophotometric methods. The experiments were conducted in whole blood and in erythrocytes in phosphate buffer using 25 mmol/L of sodium cyanate. Lipid membrane fluidity was determined using three spin-labelled fatty acids: 5-, 12- and 16-doxylstearic acids (5-DS, 12-DS, 16-DS). Internal viscosity was measured with Tempamine, using also EPR spectroscopy. Osmotic fragility was determined spectrophotometrically. Incubation of whole blood with sodium cyanate led to an increase in lipid membrane fluidity in the deeper region of the lipid layer, indicated by 12- and 16-doxylstearic acid, and a decrease near the surface (5-DS). Statistically significant results were obtained for the internal viscosity and osmotic fragility of erythrocytes. An increase in internal viscosity and increase in osmotic fragility were found in erythrocytes after incubation of whole blood, as well as in erythrocytes incubated with sodium cyanate in buffer. Alterations in internal viscosity were stronger in erythrocytes incubated with sodium cyanate in blood than in erythrocytes in the buffer. On the other hand, higher osmotic fragility was observed for erythrocytes in the buffer.     

Mechanism of the chilling-induced decrease in proton pumping across the tonoplast of rice cells

The ATP-generated proton pumping across tonoplast vesicles from chilling-sensitive Boro rice (Oryza sativa L. var. Boro) cultured cells was markedly decreased by chilling at 5 degrees C for 3 d. The membrane fluidity of core hydrophobic and surface hydrophilic regions of the lipid bilayer was measured by steady-state fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene and trimethylammonium 1,6-diphenyl-1,3,5-hexatriene and by electron spin resonance spectroscopy of 16- and 5-doxyl stearic acid, respectively. The fluidity of the surface region of the lipid bilayer of the tonoplast vesicles decreased by chilling. The fluidity of the surface region of the liposomes and the proton pumping across the reconstituted proteoliposomes with tonoplast H+-ATPase decreased with increasing content of the glycolipids. The proton pumping across chimera proteoliposomes was reduced by chilling only when it was reconstituted in the presence of tonoplast glycolipids from chilled Boro cells. These data suggest that the reduction in ATP-generated proton pumping across the tonoplast by chilling is due to the decrease in the fluidity of the surface region of the lipid bilayer of the tonoplast, which is caused by the changes in glycolipids.     

Kinetics and localisation of haemin-induced lipoprotein oxidation

Abstract

Haemin (iron (III)-protoporphyrin IX) is a degradation product of haemoglobin in circulating erythrocytes. Haemin may play a key oxidising agent for lipoprotein oxidation in patients with haemolytic anaemia. In this study, kinetic changes in chemical composition and target sites of haemin-induced LDL and HDL oxidation were investigated. Haemin initially induced the loss of α-tocopherol, followed by accumulation of lipid hydroperoxide (LP) and alteration of core lipid fluidity. The absence of LP in HDL was explained by the antioxidant activity of PON in addition to α-tocopherol. The target site of haemin was evaluated by ESR spin labelling with 5- and 16-doxyl steric acids. In the presence of t-BuOOH and haemin, ESR signal decay of the doxyl moiety demonstrated the initiation phase and the propagation phase of lipid peroxidation. The results of the lag time and the rate of signal decay indicated that haemin is located near the 16th carbon atom of the fatty acid chain in the phospholipid layer. The analyses of motion parameters, order parameter (S) of 5-DS and rotational correlation time (τ) of 16-DS, supported the observation that the lipid properties changed near the hydrophobic region rather than at the surface region of lipoproteins. Moreover, ESR spin labelling demonstrated that haemin molecules but not iron ions caused lipoprotein oxidation. In conclusion, haemin is a potent inducer of lipoprotein oxidation, and the target site for this oxidation is near the hydrophobic core of the lipoprotein leading to the loss of antioxidant activities and changes in lipid composition and physical properties.     

S-100 protein-induced changes in the physical state of synaptosomal particulate fractions as monitored by spin labels

This report documents changes in the physical state of synaptosomal particulate fractions (SYN) upon binding of S-100 protein, as monitored by spin labels. Studies were conducted on SYN labeled with either 5-doxylstearic acid or 16-doxylstearic acid, which probe the polar region and the hydrophobic core of the lipid bilayer, respectively. S-100 perturbs to some extent both the polar surface and the hydrophobic core of SYN in a time- and temperature-dependent manner. Ca2+ is essential for S-100 to perturb the membranes. K+ almost completely inhibits the S-100 perturbing effect if present in the incubation medium, but fails to reverse the S-100-induced changes if added after S-100 has interacted with SYN. At room temperature and below, the overall S-100 effect registered after about 30 min of association of the protein with SYN is an increase in the fluidity of both the surface and the interior of the membranes. Spectra registered at intervals at room temperature indicate that the S-100 perturbing effect on the membrane surface is practically monophasic, consisting of an increase in fluidity, while that on the membrane interior is multiphasic, consisting of a decrease in fluidity during the first 10 min of association, followed by an increase in fluidity during the subsequent 20 min and a return to starting values during the second 30 min of association. Around 37 degrees C, on the contrary, a decrease in fluidity is registered in both regions. The data suggest that S-100 induces a spatial rearrangement of membrane components (proteins) involved in the specific binding and/or partially penetrates into the lipid bilayer.     

Electron spin resonance studies on the dynamics of phosphatidylcholine — sulfatide model membranes

The effects of sulfatide on the fluidity and surface dynamics of bilayered and micellar model membranes of egg phosphatidylcholine containing sulfatide were studied by electron spin resonance (ESR). 5-Nitroxystearic acid and 15-nitroxystearic acid were employed as spin-label probes for the region close to the surface and that close to the hydrophobic core of lipid structures. In the vesicular structures, the signals generated by 5-nitroxystearic acid showed that the presence of sulfatide reduced the mobility of the hydrocarbon chains around the probe. The effect increased with increasing glycolipid concentration. The decrease in membrane fluidity was also monitored with the 15-nitroxystearic acid probe, although to a lesser extent. We think that sulfatide causes strong side-to-side head-group interactions on the bilayer surface, causing the lipid chains to assemble in a more rigid fashion, though this effect may be balanced in part by the disordered mechanical coupling of glycolipid acyl chains in theapposite faces of the hydrophobic core of the bilayer. Reduction of this mechanical coupling between apposite lipids when there was transition from a bilayered to a micellar structure resulted in a further increase in the order of the system.     

Effect of the functional groups of tocopherol molecules on lipid viscosity of mitochondria

The influence of tocopherol and its analogue (oxychroman) on the microviscosity of mitochondrial lipids was studied, using spin labels. The viscosity of the lipid bilayer was shown to enhance with the increase in the antioxidant content in the membrane. Small concentrations of alpha-tocopherol (10(-5)-10(-6) mol/l) were shown to increase, while large concentrations (10(-3)-10(-4) mol/l) decreased the fluidity of the lipid bilayer. The influence of alpha-tocopherol on fluidity of the lipid bilayer depending on its concentration could be realized in two ways: by direct influence on the lipid bilayer and via reception. It was shown that alterations in the viscosity of the lipid bilayer depend on chroman cycle of tocopherols, while the temperature of structural transfer and effective energy of activation depend on the lateral phytyl chain.     

An EPR investigation of surfactant action on bacterial membranes

The effects of the surfactants, alcohol ethoxylate, amine ethoxylate, amine oxide and SDS on cell membranes were investigated using the lipid soluble spin label 5-doxyl stearic acid (5-DS). Electron paramagnetic resonance (EPR) spectroscopy revealed that the action of the surfactants was to significantly increase membrane fluidity of Proteus mirabilis, Staphylococcus aureus and Saccharomyces cerevisiae. The action of these surfactants as biocides was investigated and found to be dependent on the type of organism tested. There was, however, no direct correlation between enhanced membrane fluidity observed due to the action of the surfactants and biocidal activity. Data presented suggest that perturbing the fluidity of the cytoplasmic membrane is not immediately responsible for cell death.     

Relevance of lipid polar headgroups on boron-mediated changes in membrane physical properties

Using liposomes composed of either brain phosphatidylcholine (PC), or binary mixtures of PC and phosphatidylserine (PS), galactolipids (GL), phosphatidylinositol (PI), cardiolipin (CL), phosphatidic acid (PA), or phosphatidylethanolamine (PE), we investigated the effects of graded amounts of boric acid (B, 0.5-1000 microM) on the following membrane physical properties: (a) surface potential, (b) lipid rearrangement through lateral phase separation, (c) fluidity, and (d) hydration. Incubation of the different populations of vesicles with B was associated with a small, but statistically significant, increase in membrane surface potential in PC, PC:PS, PC:GL, PC:PI, PC:PA, and PC:PE liposomes. B-induced lipid lateral rearrangement through lateral phase separation in PC, PC:PA, and PC:PE liposomes; but had no effects on PC:PS, PC:GL, and PC:PI liposomes. In PC liposomes B affected membrane fluidity at the water-lipid interface without affecting the hydrophobic core of the bilayer. In all the other binary liposomes studied, B increased membrane fluidity in both, the hydrophobic portion of the membrane and in the anionic domains. The above was associated with a decrease in the fluidity of the cationic domains. B (10-1000 microM) decreased membrane hydration regardless the composition of the liposomes. The obtained results demonstrate the ability of B to interact with membranes, and induce changes in membrane physical properties. Importantly, the extent of B-membrane interactions and the consequent effects were dependent on the nature of the lipid molecule; as such, B had greater affinity with lipids containing polyhydroxylated moieties such as GL and PI. These differential interactions may result in different B-induced modulations of membrane-associated processes in cells.     

Identification of membrane-contacting loops of the catalytic domain of cytochrome P450 2C2 by tryptophan fluorescence scanning

The catalytic domain of cytochrome P450 is thought to contact the lipid core of the endoplasmic reticulum membrane based on antibody epitope accessibility, protease susceptibility, and hydrophobic surfaces present on P450 structures of solubilized forms of the proteins. Quenching by nitroxide spin label-modified phospholipids of the fluorescence of tryptophan residues substituted into cytochrome P450 2C2, modified to contain tryptophan only at position 120, was used to identify regions of P450 inserted into the lipid core and to estimate the depth of penetration. Consistent with the proposed models of cytochrome P450-membrane interaction, the fluorescence of tryptophans inserted at residues 36 and 69 in the two segments of P450 2C2 flanking the A-helix and at residue 380 in the beta2-2 strand was quenched by nitroxide spin labels on carbon 5 of the fatty acid tails of the phospholipids within the lipid bilayer. The fluorescence of tryptophan at 380 was also strongly quenched by a spin label on carbon 12 of the fatty acids suggesting it was deepest in the membrane. However, fluorescence of tryptophan substituted at residue 225 in the F-G loop, which was predicted to be in the lipid bilayer, was not quenched by the spin labels at carbons 5 and 12 of the fatty acids. The pattern of quenching of fluorescence for tryptophans at the other positions tested, 80, 189, 239, and 347, was similar to the parent protein indicating they were not inserted into the lipid bilayer as expected. The results are consistent with an orientation of cytochrome P450 2C2 in the membrane in which positions 36, 69, and 380 are inserted into the lipid bilayer and residues 80 and 225 are near or within the phospholipid headgroup region. In this orientation, the F-G loop, which contains residue 225, could form a dimerization interface as was observed in the P450 2C8 crystal structure (Schoch, G. A., et al. (2004) J. Biol. Chem. 279, 9497).     

Change in membrane fluidity induced by lectin-mediated phase separation of the membrane and agglutination of phospholipid vesicles containing glycopeptides

Changes in membrane fluidity induced by lectin addition to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles containing synthetic glycopeptides were measured by depolarization of the fluorescent probes 8-anilino-1-naphthalenesulfonate (ANS) and 1,6-diphenyl-1,3,5-hexatriene (DPH). In the present synthesized glycopeptides, N-acetylglucosamine (GlcNAc) and a tripeptide were connected by aliphatic chains of different lengths. A pyrenyl group, which is introduced to the peptide moiety, acted as a probe to characterize the distribution of glycopeptides in the membrane on the basis of its excimer formation. The glycopeptide was shown to be distributed to DPPC vesicles with the peptide moiety buried in the hydrophobic core of the lipid bilayer and the glyco moiety exposed to the outside of the membrane. By the addition of wheat germ agglutinin (WGA) to the vesicles containing the glycopeptides, intravesicular cross-linking of glycopeptides in the membrane and aggregation of vesicles were observed. The intravesicular cross-linking was antagonized by GlcNAc above the phase transition temperature. However, the dissociation of aggregation required the addition of a stronger antagonist, N,N'-diacetylchitobiose. The addition of the glycopeptide to DPPC vesicles above the phase transition temperature decreased the membrane fluidity. However, a succeeding addition of WGA caused a large increase of membrane fluidity at either the surface or the hydrophobic core of the lipid bilayer membrane. This increase of membrane fluidity was attributed to two factors by use of two kinds of antagonists having different potencies: one is a WGA-mediated cross-linking of glycopeptides in the membrane, and the other is a close contact of vesicles on aggregation.     

Effect of lipid composition on rat liver nuclear membrane fluidity

Nuclear membrane fluidity is measured in rat liver by use of the fluorescence anisotropy of two probes: diphenylhexatriene and its cationic derivative trimethylammonium-diphenylhexatriene. It has been shown that, in 2-month-old rat liver cells, the bilayer surface is less fluid than the hydrophobic core. The fluidity was higher in 6-day-old rat liver nuclei, in which both the amount of cholesterol and the cholesterol/phospholipid ratio decreased. The influence of the single phospholipids, and in particular of phosphatidylcholine, has been studied by increasing the phosphatidylcholine with a choline base exchange reaction in isolated nuclear membranes. After this reaction, the fluorescence anisotropy of the bilayer surface increased, whereas at the hydrophobic core it decreased. Analysis of fatty acid composition shows an increase of phosphatidylcholine unsaturated fatty acids. The results show that the fluidity of nuclear membranes changes in relation to the lipid content and to the fatty acid composition. The role of nuclear membrane fluidity in cell function is discussed. © 1997 John Wiley & Sons, Ltd.