Rearrangement of aminophospholipids in bilayers from sheep platelet plasma membranes and platelet liposomes by increasing their cholesterol levels

Effect of cholesterol content on affinity and stability of factor VIII and annexin V binding to a liposomal bilayer membrane

To investigate the effect of cholesterol composition on the binding of factor VIII (FVIII) and annexin V (AV) to membranes, liposomal membranes with phospholipid bilayers of various compositions of phosphatidylcholine (PC), phosphatidylserine (PS), and cholesterol were constructed. A surface plasmon resonance (SPR) biosensor system was employed to measure the equilibrium and rate constants of the bindings. As expected, PS was found to play a dominant role in the binding of AV; its binding level was directly proportional to the PS composition in a liposome. The binding levels of FVIII and AV to liposome increased with an increase in cholesterol composition in liposome. It seemed to suggest that cholesterol in liposome acts as a 'phospholipid arrangement' factor by inducing the formation of PS-rich microdomains. However, in the absence of PS (20% on a mole basis), cholesterol could not exert the binding enhancement effect, which again confirmed the critical role of PS in the bindings. Stability of the AV binding was significantly improved by the increase in cholesterol content; for AV, the dissociation rate constant was decreased approximately fivefold, from 1.7 x 10(-3)s(-1) in the absence of cholesterol to 3.3 x 10(-4)s(-1) in the presence of only 10% cholesterol. But, for FVIII the binding stability was not so much influenced by the cholesterol addition (up to 50% on a mole basis). In summary, by using liposomes on an SPR system, we were able to demonstrate quantitatively the apparent effects of cholesterol on the binding affinity and stability of the membrane-binding proteins.

Increase in vulnerability to oxidative damage in cholesterol-modified erythrocytes exposed to t-BuOOH

During the course of radical oxidation, cholesterol may exert seemingly contradictory effects. In order to gain a better understanding of the relationship between cholesterol levels and membrane susceptibility to oxidative damage induced by reactive oxygen species (ROS), here we analyze the integrity and structural stability of cholesterol-modified (enriched or depleted) and unmodified (control) erythrocytes exposed to tert-butyl hydroperoxide. The oxidant significantly increased ROS production, with almost complete oxidation of hemoglobin and a reduction in GSH content in the different erythrocyte groups at 2 mM concentration. These changes were accompanied by losses of cholesterol and total phospholipids, the main decreases being in phosphatidylethanolamine and phosphatidylcholine. The highest lipid loss was found in the cholesterol-depleted group. Fatty acid analyses revealed changes only in peroxidized cholesterol-modified erythrocytes, with decreases in linoleic and arachidonic acids. Fluorescence anisotropy studies showed an increase in the fluidity of the negatively charged surface of peroxidized control erythrocytes. Increased hemolysis and a positive correlation between cellular osmotic fragility and malondialdehyde contents were found in all peroxidized groups. These findings provide evidence that the modification of cholesterol levels in the erythrocyte membrane has provoking effects on peroxidation, with corresponding increases in oxidative damage in the treated cell, possibly as a consequence of lipid bilayer destabilization.

Na pump and plasma membrane structure in L-cell fibroblasts expressing rat liver fatty acid binding protein

Although the intracellular fatty acid binding proteins have been investigated for nearly two decades and purified proteins are now available, little is known regarding the function of these proteins in intact cells. Therefore, L-cell fibroblasts transfected with cDNA encoding for rat liver fatty acid binding protein (L-FABP) were examined as to whether L-FABP expression in intact cells modifies plasma membrane enzyme activities, fluidity, and lipids. Plasma membrane Na/K-ATPase activity was 65.9 +/- 18.7 and 38.6 +/- 22.8 (P less than 0.001) nmol/mg protein x min for control and high-expression transfected cells, respectively. Consistent with this observation, [3H] ouabain binding to whole cells was significantly decreased from 3.7 +/- 0.3 to 2.0 +/- 0.8 pmol ouabain bound/mg cell protein in control and high-expression cells, respectively, whereas the cell's affinity for ouabain was not significantly altered. Unexpectedly, Western blot analysis indicated that transfected cells had higher levels of Na+, K(+)-ATPase protein; in contrast, the activities of 5'-nucleotidase and Mg-ATPase were unaltered. The effects of L-FABP expression on plasma membrane Na/K-ATPase function appeared to be mediated through alterations in plasma membrane lipids and/or structure. The plasma membrane cholesterol/phospholipid ratio decreased and the bulk plasma membrane fluidity increased in the high-expression cells. In conclusion, plasma membrane Na/K-ATPase activity in L cells may be regulated in part through expression of cytosolic L-FABP.

Fatty acid composition of subcellular particles from sheep platelets and topological distribution of phosphatidylethanolamine fatty acids in the plasma membrane

The fatty acid composition of individual phospholipids in subcellular fractions of sheep platelets and the asymmetrical distribution of phosphatidylethanolamine (PE) fatty acyl chains across the plasma membrane were examined. The main fatty acids of total lipid extracts were oleic (18:1; 32-41%), linoleic (18:2, 10-17%), stearic (18:0; 13-15%), palmitic (16:0; 11-15%) and arachidonic (20:4; 8-12%) acids, with a saturated/unsaturated ratio of about 0.4. Each phospholipid class had a distinct fatty acid pattern. Sphingomyelin (SM) showed the highest degree of saturation (50%), with large proportions of behenic (22:0), 18:0 and 16:0 acids. The main fatty acid in PE, phosphatidylserine (PS) and phosphatidylcholine (PC) was 18:1n-9. Our findings suggest that fatty acids are asymmetrically distributed between the choline versus the non-choline phospholipids, and also between plasma membranes and intracellular membranes. The transbilayer distribution of PE fatty acids in plasma membranes from non-stimulated sheep platelets was investigated using trinitrobenzene-sulfonic acid (TNBS). A significant degree of asymmetry was found, which is a new observation in a non-polar cell. The PE molecules from the inner monolayer contained higher amounts of 18:2 and significantly less 18:1 and 20:5 than those found in the outer monolayer, although no major differences were detected in the transbilayer distribution of total unsaturated versus saturated PE acyl chains.

Changes in the phospholipid and fatty acid composition in normal erythrocytes from sheep of different ages. Aminophospholipid organization in the membrane bilayer

Development and aging processes in mammals are associated with changes in several physiological parameters. The aim of the present study was to investigate the changes in erythrocyte lipid composition during sheep development. In all the age groups studied, cholesterol/phospholipid ratios remained constant, at close to unity, while phospholipid patterns (sphingomyelin: 45-51%, phosphatidylethanolamine: 26-33%, phosphatidylserine: 13-19% and phosphatidylcholine: less than 2%) changed during development, with a statistically significant decrease (P less than 0.01) in phosphatidylserine and an increase in sphingomyelin content. These data suggest an increase in the rigidity of the erythrocyte lipid bilayer in adult sheep when compared with 1-month-old animals due to a decrease in the phosphatidylserine/sphingomyelin ratio. Fatty acid profiles consistently showed 5 main acids: oleic (52-54%), stearic (17-18%), linoleic (9-15%), palmitic (8.5-11%) and arachidonic acid (2-3%), mainly with significant variations (P less than 0.01) in palmitic and linoleic acid contents, respectively reaching the highest and lowest percentages in the youngest sheep. However, the developmental process seems to have no influence on the aminophospholipid topology of erythrocytes. This study suggests that the animals' developmental process has a marked effect on the lipid composition of erythrocyte membranes, which could affect cell functions.