Importance of cholesterol-phospholipid interaction in determining dynamics of normal and abetalipoproteinemia red blood cell membrane

Acanthocytic red blood cells in patients with abetalipoproteinemia have a decrease membrane fluidity that is associated with increased sphingomyelin/phosphatidylcholine (SM/PC) ratios. Here we describe studies designed to gain better insight into (i) the interrelationship between the composition of lipoprotein and red blood cell membrane in abetalipoproteinemia patients and normal controls; and (ii) how the differences in lipid composition of the red blood cell membrane affect its fluidity. The increased SM/PC ratio found in abetalipoproteinemia plasma high density lipoproteins (HDL) (3 times greater than controls) was paralleled by an increase in this ratio in acanthocytic red cells, but to a lesser degree (almost twice greater than control red cells). Cholesterol/phospholipid mole ratios (C/P) were increased 3-fold in abetalipoproteinemia HDL, but only slightly increased in red cells compared to controls values. As in the controls, 80-85% of abetalipoproteinemia red cell sphingomyelin was found to be in the outer half of the erythrocyte membrane. Membrane fluidity was defined in terms of microviscosity (eta) between 5 and 42 degrees C by the fluorescent polarization of 1,6-diphenylhexatriene (DPH) present in erythrocyte ghost membranes. At all temperatures, membrane microviscosity was higher in abetalipoproteinemia ghosts than controls, but these differences decreased at higher temperatures (12.34 vs 9.79 poise, respectively at 10 degrees C; 4.63 vs 4.04 poise at 37 degrees C). These differences were eliminated after oxidation of all membrane cholesterol to cholest-4-en-3-one by incubation with cholesterol oxidase. Following cholesterol oxidation, the membrane microviscosity decreased in patient ghosts more than in normal red blood cells so that at all temperatures no significant differences were present relative to control ghosts, in which the apparent microviscosity was also diminished but to a lesser degree. Therefore, although increased SM/PC ratios in abetalipoproteinemia may be responsible for decreased erythrocyte membrane fluidity, these effects are dependent upon normal interactions of cholesterol with red cell phospholipid.     

Importance of cholesterol-phospholipid interaction in determining dynamics of normal and abetalipoproteinemia red blood cell membrane

Acanthocytic red blood cells in patients with abetalipoproteinemia have a decreased membrane fluidity that is associated with increased sphingomyelin/phosphatidylcholine (SM/PC)§ ratios. Here we describe studies designed to gain better insight into (i) the interrelationship between the composition of lipoprotein and red blood cell membrane in abetalipo-proteinemia patients and normal controls; and (ii) how the differences in lipid composition of the red blood cell membrane affect its fluidity. The increased SM/PC ratio found in abetalipoproteinemia plasma high density lipoproteins (HDL) (3 times greater than controls) was paralleled by an increase in this ratio in acanthocytic red cells, but to a lesser degree (almost twice greater than control red cells). Cholesterol/phospholipid mole ratios (C/P) were increased 3-fold in abetalipoproteinemia HDL, but only slightly increased in red cells compared to controls values. As in the controls, 80–85% of abetalipo-proteinemia red cell sphingomyelin was found to be in the outer half of the erythrocyte membrane. Membrane fluidity was defined in terms of microviscosity ({ie116-1}) between 5 and 42°C by the fluorescent polarization of 1,6-diphenylhexatriene (DPH) present in erythrocyte ghost membranes. At all temperatures, membrane microviscosity was higher in abetalipoproteinemia ghosts than controls, but these differences decreased at higher temperatures (12.34 vs 9.79 poise, respectively, at 10°C; 4.63 vs 4.04 poise at 37°C). These differences were eliminated after oxidation of all membrane cholesterol to cholest-4-en-3-one by incubation with cholesterol oxidase. Following cholesterol oxidation, the membrane microviscosity decreased in patient ghosts more than in normal red blood cells so that at all temperatures no significant differences were present relative to control ghosts, in which the apparent microviscosity was also diminished but to a lesser degree. Therefore, although increased SM/PC ratios in abetalipoproteinemia may be responsible for decreased erythrocyte membrane fluidity, these effects are dependent upon normal interactions of cholesterol with red cell phospholipid.     

Interaction of hopanoids with phosphatidylcholines containing oleic and ω-cyclohexyldodecanoic acid in lipid bilayer membranes

Lipid bilayer membranes were made from hopanoid phosphatidylcholine mixtures dissolved in decane. The specific capacity of the mixed membranes was found to increase with increasing hopanoid content. This indicates an interaction between hopanoids and lipids which leads to a reduction of the chemical potential of the solvent in the membranes.The structural properties of mixtures of hopanoids and phosphatidylcholines were investigated using charged probe molecules, the negatively charged lipophilic ions dipicrylamine (DPA) and tetraphenylborate (TØB) and the positively charged potassium complex PV-K⁺ (PV, cyclo (D-Val-L-Pro-L-Val-D-Pro)₃). The transport properties of the lipophilic ions in the mixed membranes indicate that the electrical properties like dipolar potential and surface potentials of phosphatidylcholine membranes are not changed by the insertion of the hopanoids. The translocation rate constant K of the PV-K⁺ complex is drastically reduced in the hopanoid phosphatidylcholine membranes with increasing hopanoid content. This effect is discussed on the basis of an alteration of the microviscosity in the mixed membranes. There exists a close analogy between the action of cholesterol and hopanoids in bilayer membranes from phosphatidylcholines.A bilayer membrane composed of di-ω-cyclohexyldodecanoyl-phosphatidylcholine (DCPC) was found to possess a higher specific capacity as compared to other phosphatidylcholines. Also a lower translocation rate constant for PV-K⁺ was observed which may be caused by the relative high microviscosity of this lipid even above the phase transition temperature.     

Possible approaches to creating phospholipid preparations--acceptors of membrane cholesterol

The capability of liposomes with plant phosphatidylcholine to extract cholesterol from erythrocyte membrane was investigated. It was shown, that plant phosphatidylcholine extracted 42% of membrane cholesterol, without causing haemolysis. This cholesterol extraction was responsible for the decrease of membrane microviscosity and for the increase of Na+, K+-ATPase activity. It is suggested that it is possible to create the extractive agent of cholesterol with plant phosphatidylcholine.     

Fatty acid composition and properties of the liver microsomal membrane of rats fed diets enriched with cholesterol.

Male rats were fed diets containing olive (OO) or evening primrose (EPO) oil (10% w/w), with or without added cholesterol (1% w/w). After 6-week feeding, the lipid and fatty acid compositions, fluidity, and fatty acid desaturating and cholesterol biosynthesis/esterification related enzymes of liver microsomes were determined. Both the OO and EPO diets, without added cholesterol, increased the contents of oleic and arachidonic acids, respectively, of rat liver microsomes. The results were consistent with the increases in delta 9 and delta 6 desaturation of n-6 essential fatty acids and the lower microviscosity in the EPO group. Dietary cholesterol led to an increase in the cholesterol content of liver microsomes as well as that of phosphatidylcholine (PC). The cholesterol/phospholipid and PC/PE (phosphatidylethanolamine) ratios were also elevated. Fatty acid composition changes were expressed as the accumulation of monounsaturated fatty acids, with accompanying milder depletion of saturated fatty acids in rat liver microsomes. In addition, the arachidonic acid content was lowered, with a concomitant increase in linoleic acid, which led to a significant decrease in the 20:4/18:2 ratio in comparison to in animals fed the cholesterol-free diets. Cholesterol feeding also increased delta 9 desaturase activity as well as membrane microviscosity, whereas it decreased delta 6 and delta 5 desaturase activities. There was a very strong correlation between fluidity and the unsaturation index reduction in the membrane. Furthermore, the activity of hydroxymethylglutaryl-CoA reductase increased and the activity of acyl-CoA:cholesterol acyltransferase decreased in liver microsomes from both cholesterol-fed groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lipid composition of the rat liver nuclear matrix as affected by hydrocortisone

It has been found that the rat liver nuclear matrix contains a small amount of phospholipids (2%) and neutral lipids (1.6%). The injection of hydrocortisone increased the nuclear matrix phospholipid content and reduced its neutral lipid levels. A marked increase in sphingomyelin content observed was accompanied by the reduction in phosphatidylcholine level. The decrease in neutral lipid content took place mainly at the expense of a sharp lowering of triglyceride and cholesterol ether levels. The latter may turn into free cholesterol, thus increasing its content. The data obtained testify to steroid hormone influence on lipid metabolism in intranuclear structures. The observed changes in lipid composition may be related to specific hydrocortisone-induced activation of genome.     

Effect of membrane cholesterol on dimyristoylphosphatidylcholine-induced vesiculation of human red blood cells

During incubation of intact human erythrocytes with sonicated dimyristoylphosphatidylcholine (DMPC) vesicles, the cells change their discoid morphology to form echinocytes and finally give rise to the release of membrane vesicles. In this process, the red cell membrane accumulates DMPC and loses up to 15% of its cholesterol. On the other hand, replacement of 25% of the endogenous phosphatidylcholine species by DMPC without affecting the cholesterol level of the erythrocytes can be achieved by incubation with DMPC/cholesterol (1:1, mol/mol) sonicated vesicles in the presence of the phosphatidylcholine-specific phospholipid-transfer protein from bovine liver. This replacement also gives rise to an echinocytic cell morphology, but no membrane vesiculation can be observed. However, the vesiculation process can as yet be initiated upon a subsequent decrease of the cholesterol level, by incubation of those modified cells in the presence of sonicated vesicles of pure egg phosphatidylcholine. Incubation of native erythrocytes with pure egg phosphatidylcholine vesicles, on the other hand, results in cholesterol depletion, but does neither induce the formation of echinocytes nor the release of membrane vesicles. Cellular ATP levels are not affected during these incubations. From these results, it can be concluded that a decrease in cholesterol content of the erythrocyte membrane is essential for the DMPC-induced vesiculation of those cells.     

High density lipoprotein phospholipid composition is a major determinant of the bi-directional flux and net movement of cellular free cholesterol mediated by scavenger receptor BI

The role of high density lipoprotein (HDL) phospholipid in scavenger receptor BI (SR-BI)-mediated free cholesterol flux was examined by manipulating HDL(3) phosphatidylcholine and sphingomyelin content. Both phosphatidylcholine and sphingomyelin enrichment of HDL enhanced the net efflux of cholesterol from SR-BI-expressing COS-7 cells but by two different mechanisms. Phosphatidylcholine enrichment of HDL increased efflux, whereas sphingomyelin enrichment decreased influx of HDL cholesterol. Although similar trends were observed in control (vector-transfected) COS-7 cells, SR-BI overexpression amplified the effects of phosphatidylcholine and sphingomyelin enrichment of HDL 25- and 2.8-fold, respectively. By using both phosphatidylcholine-enriched and phospholipase A(2)-treated HDL to obtain HDL with a graded phosphatidylcholine content, we showed that SR-BI-mediated cholesterol efflux was highly correlated (r(2) = 0.985) with HDL phosphatidylcholine content. The effects of varying HDL phospholipid composition on SR-BI-mediated free cholesterol flux were not correlated with changes in either the K(d) or B(max) values for high affinity binding to SR-BI. We conclude that SR-BI-mediated free cholesterol flux is highly sensitive to HDL phospholipid composition. Thus, factors that regulate cellular SR-BI expression and the local modification of HDL phospholipid composition will have a large impact on reverse cholesterol transport.     

Phosphatidylcholine and cholesterol interactions in model membranes

Various phosphatidylcholines differing either in the stereochemistry around their chiral center or in the position of a cis double bond along the acyl chains were synthesized in order to study critical contact regions in the phospholipid molecule with adjacent cholesterol in model membranes. Microviscosities calculated from fluorescence depolarization of diphenylhexatriene and chain order from spin label studies were measured to monitor physical membrane properties. The enhancing effect of cholesterol on the microviscosity of membranes containing phosphatidylcholines with comparable acyl chain length was largest when the two acyl chains were saturated and smallest when both were unsaturated. Membranes prepared from phosphatidylcholines having a single cis double bond at different positions along the sn-2 acyl chain showed roughly the same changes of microviscosity or chain order upon incorporation of cholesterol. No discrimination was evident in the interaction between cholesterol and enantiomeric phosphatidylcholines or between the enantiomeric phosphatidylcholine molecules themselves. We conclude that the rigidifying effect of cholesterol in membranes does not depend on specific sites of interaction and that with respect to physical membrane properties phosphatidylcholine behaves as an achiral molecule.     

Nonsymmetrical polymethine dyes as fluorescent probes for the study of microviscosity of membrane phospholipid bilayers

Nonsymmetrical polymethine dyes are shown to be applied as a new approach in the studies of phospholipid membrane microviscosity. The method requires determination of the intensity ratio for the long-wave length (Ig) and short-wave length (Ik) bands of fluorescence spectra in the region of 730-770 nm at exitation 600 nm. It allows determination of microviscosity by comparative measurements of the fluorescence parameter Ig/Ik in the model medium of the known viscosity (glycerol) and the object under study. Microviscosity in egg phosphatidylcholine vesicules (liposomes) is found to be 0.6-1.2 P. It depends on the surface curvature (size of vesicle), cholesterol content and temperature. It the studies of dimiristoylphosphatidyl choline liposomes the changes in microviscosity at the phase transition temperature are shown to be from 4.5 to 1.1 P. The transition temperature is 24.5 degrees C, the transition range being 2.2 degrees C. The results of this work demonstrate the advantages of the suggested approach to study mobility in phospholipid membranes and confirm it to be promising to study natural membranes and whole cells.     

Interaction of cholesterol, phospholipid and apoprotein in high density lipoprotein recombinants.

To examine the effect of incorporation of cholesterol into high density lipoprotein (HDL) recombinants, multilamellar liposomes of 3H cholesterol/14C dimyristoyl phosphatidylcholine were incubated with the total apoprotein (apoHDL) and principal apoproteins (apoA-1 and apoA-2) of human plasma high density lipoprotein. Soluble recombinants were separated from unreacted liposomes by centrifugation and examined by differential scanning calorimetry and negative stain electron microscopy. At 27 degrees C, liposomes containing up to approx. 0.1 mol cholesterol/mol dimyristoyl phosphatidylcholine (DMPC) were readily solubilized by apoHDL, apoA-1 or apoA-2. However, the incorporation of DMPC and apoprotein into lipoprotein complexes was markedly reduced when liposomes containing a higher proportion of cholesterol were used. For recombinants prepared from apoHDL, apoA-1, or apoA-2, the equilibrium cholesterol content of complexes was approx. 45% that of the unreacted liposomes. Electron microscopy showed that for all cholesterol concentrations, HDL recombinants were predominantly lipid bilayer discs, approx. 160 X 55 A. Differential scanning calorimetry of cholesterol containing recombinants of DMPC/cholesterol/apoHDL or DMPC/cholesterol/apoA-1 showed, with increasing cholesterol content, a linear decrease in the enthalpy of the DMPC gel to liquid crystalline transition, extrapolating to zero enthalpy at 0.15 cholesterol/DMPC. The enthalpy values were markedly reduced compared to control liposomes, where the phospholipid transition extrapolated to zero enthalpy at approx. 0.45 cholesterol/DMPC. The calorimetric and solubility studies suggest that in high density lipoprotein recombinants cholesterol is excluded from 55% of DMPC molecules bound in a non-melting state by apoprotein.     

Phosphatidylcholine substrate specificity of lecithin: cholestrol acyltransferase-in high density lipoproteins and in lipids dispersions.

Lecithin: cholesterol acyltransferase (LCAT) was more highly activated by apolipoprotein A-I (apoA-I) with dimyristoyl phosphatidylcholine (DMPC) than with dilinoleoyl phosphatidylcholine (DLPC) when lipid dispersion of cholesterol and each phosphatidylcholine was used as a substrate. When the enzyme reactions were activated by whole apolipoproteins of high density lipoproteins (HDL), DLPC was more available to the LCAT reaction than DMPC with high concentrations of apoHDL in an incubation mixture. However, no detectable enzyme reaction was observed with dipalmitoyl phosphatidylcholine (DPPC) under both conditions. On the other hand, all of these phosphatidylcholines acted as substrates of LCAT when they were incorporated into HDL coupled to Sepharose. The order of their relative reactivities to cholesterol was DMPC, DPPC, AND DLPC under the conditions used.     

Thrombin receptors define responsiveness of cholesterol-modified platelets

The microviscosity of human platelet membranes was changed by incubating platelets with liposomes containing various ratios of cholesterol and lecithin. Binding of 125I-thrombin to the modified platelets was measured together with platelet aggregation and secretion. In cholesterol-normal platelets (mole ratio of cholesterol to phospholipid (C:PL) = 0.553; eta = 2.40 poise), weighted nonlinear least squares curve fitting indicated that a model involving two classes of sites was adequate to describe the binding isotherm (K1 = 8.3 X 10(8) M-1; R1 = 150 sites/platelet; K2 = 6.4 X 10(6) M-1; R2 = 16,000 sites/platelet). In cholesterol-enriched platelets (C:PL = 0.857; eta = 3.05 poise), the apparent affinities for the two classes of sites decreased to 55 and 53%, respectively, while the binding capacities increased to 170 and 160%, respectively. In contrast, in the cholesterol-depleted platelets (C:PL = 0.435; eta = 2.03 poise), the affinities increased to 220 and 180%, respectively, while the binding capacities decreased to 53 and 46%, respectively. In cholesterol-enriched, cholesterol-normal, and cholesterol-depleted platelets, the thrombin concentrations required for half-maximal aggregation were 0.17, 0.35, and 0.52 nM, respectively, while the values for half-maximal secretion of [14C]serotonin were 0.17, 0.40, and 0.55 nM, respectively. Plots of receptor occupancy versus biological response showed that maximum response in cholesterol-enriched, cholesterol-normal, and cholesterol-depleted platelets occurred with occupancy of 30, 50, and 70% of the high affinity sites, respectively. In all three treatment groups, occupancy of 40-50 high affinity sites results in 50% aggregation. These results show that (i) modification of platelet membrane microviscosity results in changes in the number and affinity of both high and low affinity thrombin receptors, (ii) the change in receptor number rather than affinity is the determinant for platelet responsiveness, and (iii) the changes in membrane microviscosity do not appear to alter the coupling between occupied receptor and subsequent bioresponse.     

Fluorescence depolarization studies and phase transition in human apoprotein . phospholipid complexes.

The microviscosity of unilamellar vesicles of dimyristoyl-3-sn-phosphatidylcholine and that of phosphatidylcholine . apoprotein complexes was followed by fluorescence depolarization after labeling with 1,6-diphenyl-1,3,5-hexatriene. The transition temperature from gel-crystalline to liquid-crystalline phase in 24 degrees C for the dimyristoyl-phosphatidylcholine vesicles and is shifted to around 30 degrees C in the complexes between phosphatidylcholine and apoA-I, apoA-II, apoC-I, apoC-III proteins while the cooperativity of the transition is decreased. At temperatures below the transition of the phospholipid, the microviscosity of the complexes of phosphatidylcholine with apoA-I, apoA-II and apoC-I proteins is lower than that of the phosphatidylcholine, while the opposite effect is observed above 30 degrees C. The phosphatidylcholine . apoprotein complexes isolated on a Sepharose 6B column have a molecular weight around 100 000 and a phosphatidylcholine/apoprotein ratio of 2--2.6 (w/w). The microviscosity measurments at 35 degrees C performed after elution of the column enable the complex to be detected. The size and microviscosity of the apoprotein . phosphatidylcholine complex is compatible with a model where the vesicular structure has disappeared and the amino acid side chains present hydrophobic interaction with the phosphatidylcholine acyl chains.     

Alteration of Synaptosomal Plasma Membrane Cholesterol Content: Membrane Physical Properties and Cation Transport Proteins

The level of nerve membrane cholesterol was altered by in vitro incubation of rat brain synaptosomal plasma membrane with liposomes having varying cholesterol contents. The normal plasma membrane cholesterol/phospholipid ratio of 0.3-0.4 (mol/mol) could be decreased by about one-half or increased more than 100%. Fluorescence polarization measurements were made using the probe 1,6-diphenyl-1,3,5-hexatriene. At temperatures below 35 percent C, lowering membrane cholesterol led to increased apparent microviscosity, while raising cholesterol content produced little change. However, at 45 percent C a continuous direct relationship existed between experimental membrane cholesterol/phospholipid ratio (ranging from 0.18 to 0.73) and apparent microviscosity. Under standard liposome-synaptosomal plasma membrane exchange conditions, 80% of the initial specific [(3)H]saxitoxin binding activity to the voltage-dependent sodium channel and at least 95% of the (Mg2+,K+)-p-nitrophenylphosphatase activity were preserved. Our results indicate that neither the characteristics of toxin binding nor the kinetics of this enzyme activity is dependent upon membrane cholesterol content.     

Treatment of familial hypercholesterolaemia. United Kingdom lipid clinics study of pravastatin and cholestyramine.

OBJECTIVE--To compare the efficacy and safety of cholestyramine, an anion exchange resin, and pravastatin, a new hydrophilic specific inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, in the treatment of heterozygous familial hypercholesterolaemia. DESIGN--Double blind, double dummy, placebo controlled study with three parallel groups. SETTING--Six specialist lipid clinics in the United Kingdom. PATIENTS--128 patients aged 18-70 with heterozygous familial hypercholesterolaemia diagnosed on strict biochemical and clinical findings. MAIN OUTCOME MEASURES--Total plasma cholesterol, triglyceride, and lipoprotein subfractions and biochemical and haematological safety parameters. RESULTS--Pravastatin (40 mg/day) led to a 25% reduction in total plasma cholesterol concentration and a reduction in low density lipoprotein cholesterol concentration of 30%. Cholestyramine (24 g/day) led to similar reductions in concentrations of total cholesterol (23%) and low density lipoprotein cholesterol (31%). No consistent changes occurred in high density lipoprotein cholesterol values with either compound. Plasma triglyceride concentrations showed a small rise (18%) on resin therapy. No serious adverse drug reactions occurred during the study. CONCLUSIONS--Pravastatin seems to be a highly effective, well tolerated drug for severe hypercholesterolaemia. Patients chosen for this study were recruited on the basis that they could tolerate a full dose of cholestyramine, and in this situation cholestyramine was also highly effective in lowering plasma low density lipoprotein cholesterol concentrations.     

Membrane lipid composition and the interaction of pardaxin: the role of cholesterol

Modulation by pardaxin of the phase transitions of dimyristoyl phosphatidylcholine, 1-stearoyl-2-oleoyl phosphatidylcholine or 1-stearoyl-2-oleoyl phosphatidylglycerol in the presence or absence of cholesterol was studied by differential scanning calorimetry. The transition enthalpy of each of the phospholipids was lowered by pardaxin and there was a small decrease in the transition temperature. Addition of cholesterol and pardaxin to dimyristoyl phosphatidylcholine resulted in a very marked lowering of the transition temperature. Although the peptide broadens the transition of the pure phospholipids, it sharpens the transition of mixtures of the phospholipids with cholesterol. This and the observation that pardaxin also causes the formation of crystallites of anhydrous cholesterol, suggest that the peptide promotes redistribution of cholesterol in the membrane.     

Hepatic lipase stimulates the uptake of high density lipoprotein cholesterol by hepatoma cells

The objective of this study was to determine whether high density lipoproteins (HDL) that have been treated with hepatic lipase have an enhanced ability to deliver cholesterol to cells. Human HDL was incubated with rat hepatic lipase, reisolated, and subjected to compositional analysis. Approximately 28% of the HDL phosphatidylcholine was hydrolyzed by the hepatic lipase but no change was detected in the cholesterol or apoprotein content of the HDL compared to HDL incubated with heat-inactivated hepatic lipase. Cultured rat hepatoma cells exposed to hepatic lipase-modified HDL showed an increased uptake of HDL free cholesterol relative to cells exposed to control HDL. This increased delivery of HDL free cholesterol was demonstrated by both isotopic and mass determinations and it contributed to a 1.6-fold increase in total cellular cholesterol content relative to cells treated with control HDL. The free cholesterol delivered by the HDL is functionally available to the cell as evidenced by the conversion of radiolabeled free cholesterol to cholesteryl ester. The stimulation of free cholesterol delivery was dose-dependent up to a level of 100 micrograms of HDL free cholesterol/ml of extracellular medium, and was directly related to the extent of phosphatidylcholine hydrolysis. The enhanced cellular accumulation of HDL free cholesterol observed with hepatic lipase appears to be due to the phospholipase activity of this enzyme, since similar results were obtained with HDL that had been modified by snake venom phospholipase A2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sex related differences in platelet TxA2 generation

Platelet lipid composition, c arachidonic acid (AA) metabolism by platelets (stimulated with thrombin), serum thromboxane (Tx)B2 production and plasma lipid composition were investigated in 53 healthy females (18-45 years) and 65 males (19-45 years) with similar dietary habits. In males, serum TxB2 production and cholesterol platelet membrane levels were found significantly higher (p less than 0.001 and p less than 0.05) than in females. No differences were observed between the two groups in the AA conversion through cyclo-oxygenase and lipoxygenase pathways or in the platelet phospholipid fatty acid composition. These findings indicate that in males the platelet proaggregatory capacity is greater than in females and the higher platelet TxB2 production does not depend on a larger AA availability or on enzyme activation for its conversion. The increased TxB2 production may be, at least in part, induced by functional differences such as a different membrane cholesterol content inducing, in its turn, an increased microviscosity and/or higher number of platelet receptors for thrombin.     

Apparent microviscosity of intact and post-lipolysis ("remnant") very low density lipoprotein particles.

The apparent microviscosity of intact rat plasma very low density lipoprotein (VLDL) and post-lipolysis very low density lipoprotein was determined by fluorescence depolarization measurements and flurorescence decay measurements using 1, 6-diphenylhexatriene. Post-lipolysis very low density lipoprotein was prepared in vitro after incubation of the intact lipoprotein with either purified bovine milk lipoprotein lipase or lipoprotein lipase rich (post-heparin) plasma. During lipolysis, an average of 88% of the triglycerides were hydrolyzed, and the lipoprotein became depleted in phospholipids, cholesterol and apolipoprotein C. The apparent microviscosity of the lipoprotein increased by three-fold from 0.63 to 1.88 poise. It is concluded that the compositional changes occurring during lipolysis affect the physical properties of the lipoprotein, as measured here by the fluidity (microviscosity) of the particles.