Modulation of the organization of erythrocyte membrane phospholipids by cytoplasmic ATP. The susceptibility of isoionic human erythrocytes ghosts to attack by detergents and phospholipase C

Phospholipids in animal eukaryotic membranes: transverse asymmetry and movement

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Zinc deficiency in the rat alters the lipid composition of the erythrocyte membrane Triton shell

The effect of dietary zinc deficiency on the lipid composition of the erythrocyte membrane Triton shell was determined. Weanling male Wistar rats were fed an egg white-based diet containing < 1.0 mg Zn/kg diet ad libitum. Control rats were either pair-fed or ad libitum-fed the basal diet supplemented with 100 mg Zn/kg diet. A Zn refed group was fed the -Zn diet until day 18 and then pair-fed the +Zn diet until day 21. Dietary Zn deficiency caused an increased cholesterol/phospholipid ratio in Triton shells compared to those from pair-fed controls. Zn deficiency caused a decreased double bond index of fatty acids in phosphatidylinositol (PI) and phosphatidylcholine (PC); there was a decreased proportion of 18:2n-6 and 22:4n-6 in PC and 20:4n-6 in PI as compared to that found in pair-fed controls. All glycerophospholipids that were retained in the shell had a lower double bond index and increased content of 16:0 and/or 18:0 relative to the phospholipid in the intact membrane.

Transmembrane redistribution of phospholipids of the human red cell membrane during hypotonic hemolysis

The transmembrane distribution of spin-labeled phospholipids was measured in human erythrocytes before and after hypotonic hemolysis by electron paramagnetic resonance. With a first series of partially water soluble probes a complete randomization of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and sphingomyelin analogues was achieved when cells were resealed in the absence of Mg-ATP or when the aminophospholipid translocase was inhibited by vanadate or calcium. If the ghosts were resealed with Mg-ATP inside, the transmembrane asymmetry of the aminophospholipids was reestablished. With long chain insoluble spin-labeled lipids complete randomization was obtained with the phosphatidylcholine analogue but even in the presence of vanadate only a small percentage (approx. 15%) of the spin-labeled phosphatidylserine flopped to the outer monolayer and comparable percentage of the spin-labeled sphingomyelin flipped to the inner monolayer, indicating a hierarchy in the phospholipid redistribution for these water insoluble lipids during hemolysis. The mechanism by which a selective randomization takes place is not known. It may involve phosphatidylserine-protein interactions in the inner leaflet and sphingomyelin-cholesterol or sphingomyelin-sphingomyelin interaction in the outer leaflet.

Exogenous ATP antagonizes the actions of phospholipase A2, local anesthetics, Ca2+ ionophore A23187, and lithium on glucose-1,6-bisphosphate levels and the activities of phosphofructokinase and phosphoglucomutase in rat muscle

ATP, added externally to the incubation medium of rat diaphragm muscles, abolished the decrease in the levels of glucose-1,6-bisphosphate (Glc-1,6-P2), the powerful regulator of carbohydrate metabolism, induced by phospholipase A2, local anesthetics, Ca2+ ionophore A23187, or lithium. Concomitantly to the changes in Glc-1,6-P2, the potent activator of phosphofructokinase (the rate-limiting enzyme in glycolysis) and phosphoglucomutase, the activities of these enzymes were reduced by the myotoxic agents and restored by exogenous ATP, when assayed under conditions in which these enzymes are sensitive to regulation by Glc-1,6-P2. These findings suggest that ATP may have broad therapeutic action, as it may stimulate the impaired glycolysis in muscle induced by various drugs and conditions which cause muscle weakness or damage.

Influence of Ca2+ and Mg2+ on the turnover of the phosphomonoester group of phosphatidylinositol 4-phosphate in human erythrocyte membranes

In isolated erythrocyte membranes, increasing the free Mg2+ concentration from 0.5 to 10 mM progressively activates the membrane-bound phosphatidylinositol (PtdIns) kinase and leads to the establishment of a new equilibrium with higher phosphatidylinositol 4-phosphate (PtdIns4P) and lower PtdIns concentrations. The steady-state turnover of the phosphomonoester group of PtdIns4P also increases at high Mg2+ concentrations, indicating a simultaneous activation of PtdIns4P phosphomonoesterase by Mg2+. Half-maximum inhibition of PtdIns kinase occurs at 10 microM free Ca2+ in the presence of physiological free Mg2+ concentrations. Increasing free Mg2+ concentrations overcome Ca2+ inhibition of PtdIns kinase. In the presence of Ca2+, calmodulin activates Ca2+-transporting ATPase 5-fold, but does not alter pool size and radiolabelling of PtdIns4P. In intact erythrocytes, adding EGTA or EGTA plus Mg2+ and the ionophore A23187 to the external medium does not exert significant effects on concentration and radiolabelling of polyphosphoinositides when compared with controls in the presence of 1.4 mM free Ca2+.

A sphingomyelinase-resistant pool of sphingomyelin in the nuclear membrane of hen erythrocytes

Experiments in which hen erythrocytes were exposed to the action of exogenous sphingomyelinase (Staphylococcus aureus) or to their endogenous plasma membrane sphingomyelinase showed that about 15% of the total sphingomyelin was resistant to breakdown either in intact or lysed cells. This resistant pool of sphingomyelin seems likely to reside in the nuclear membranes of the cells, so that essentially all the plasma membrane sphingomyelin can be broken down by exogenous sphingomyelinase acting on intact cells, suggesting that plasma membrane sphingomyelin is exclusively localised in the outer lipid leaflet. Paradoxically, introduction of Ca2+ into the intact cells using A23187 causes the breakdown of up to 30% of total cell sphingomyelin inside the cells but without apparently affecting the putative nuclear pool of sphingomyelin and this suggests that Ca2+ may alter the original disposition of sphingomyelin in the membrane so that originally outer leaflet sphingomyelin becomes accessible to the endogenous sphingomyelinase inside the cells. No differences were seen in the fatty acid compositions of sphingomyelin degradable by exogenous sphingomyelinase, sphingomyelin degradable in the presence of A23187/Ca2+ or the enzyme-resistant pool of sphingomyelin.

Lipid organization in erythrocyte membrane microvesicles

The aminophospholipids of microvesicles released from human erythrocytes on storage or prepared from erythrocyte ghosts by shearing under pressure are susceptible to the action of 2,4,6-trinitrobenzenesulphonic acid. The aminophospholipids of the former vesicles are also susceptible to attack by phospholipase A2. Under the same conditions, the aminophospholipids of erythrocytes undergo little reaction. This suggests that the phospholipids in microvesicle membranes are more randomly distributed than those in erythrocyte membranes. Measurements have also been made of the ability of filipin to react with the cholesterol of sealed and unsealed erythrocyte ghosts and of microvesicles prepared from them. From the initial rates of reaction, it was concluded that there is no preferential transfer of cholesterol molecules from one side of the bilayer to the other during the formation of the microvesicles.

Organic phosphates modulate anion self-exchange across the human erythrocyte membrane

Anion transport across the red cell membrane has been measured as sulfate self-exchange flux (Ja) in fresh and metabolically depleted human red cells. Depletion of metabolic stores by a starvation of the cells decreases Ja by 50%. A similar effect was observed when ATP was acutely and selectively depleted by iodoacetamide. This inhibition was independent of the presence of calcium and reversible after metabolic rejuvenation of the cells. Ghosts prepared from fresh red cells exhibited the same value of Ja as fresh red cells. By contrast, ghosts prepared from depleted red cells exhibited a decrease in Ja which was reverted by a physiological concentration of ATP. The effect of ATP was dependent on its concentration (Km approximately 40 microM) and on the duration of the metabolic depletion: complete restoration of Ja was obtained only in ghosts prepared from red cells acutely depleted of ATP by a 2 h incubation with iodoacetamide. After a 20 h starvation, Ja restoration was never more than 80%. We postulate that ATP acts primarily through the phosphorylation of band 3 protein, the anion exchanger; it acts also through the stabilization of the normal organization of the membrane. This latter effect may involve the phosphorylation of membrane components, but also a direct interaction, as shown by the influence of other organic phosphates (2,3-diphosphoglycerate and inositol hexaphosphate) on Ja in the absence of ATP.

Microvesiculation and sphingomyelinase activation in chicken erythrocytes treated with ionophore A23187 and Ca2+

Treatment of chicken erythrocytes with ionophore A23187 and Ca2+ leads to the disappearance of the marginal band of microtubules and to a release of the constraints which normally maintain the nucleus in a central position in the cells. The consequent close apposition of the nucleus to the plasma membrane may allow nuclear-plasma membrane fusion to occur and subsequently results in the release of microvesicles from the hybrid surface membrane. The remnant cells are spherical, and have nuclei which appear to be partly exocytosed. Concomitant with these morphological changes, there is a breakdown of 20-30% of the total cell sphingomyelin by an endogenous sphingomyelinase which does not require Ca2+ and which releases phosphorylcholine only into the cell interior. It is suggested that the pool of sphingomyelin which is broken down as a consequence of Ca2+ entry into the cells is present in the nuclear membrane and that it becomes available to the plasma membrane sphingomyelinase as a result of the close apposition of nucleus and plasma membrane induced by Ca2+.

Membrane asymmetry. A survey and critical appraisal of the methodology. II. Methods for assessing the unequal distribution of lipids

In the companion paper, I have reviewed the techniques employed for assessment of the asymmetric distribution and orientation of membrane proteins. This article deals with methods applicable to the investigation of the unequal distribution of lipids between the two membrane leaflets. Among the techniques I will discuss are the use of immunological techniques and lectins, chemical reagents, enzymatic isotopic labeling and degradation of membrane lipids, exchange proteins and physical techniques. Whenever appropriate, problems of crypticity and non-availability of lipids to interact with the appropriate ligands, reagents, modifying enzymes or exchange proteins have been envisaged. It appears that in many case, highly discordant results, sometimes with the same biological material, have been obtained. Some of the difficulties encountered presumably stem from the reported existence of non-bilayer arrangements and isotropic movement of lipids as evidenced by freeze-fracture and NMR studies. Other problems may be related to the induction of such arrangements, especially the inverted micellar arrangement, by the modifying agents, particularly degradation enzymes or exchange proteins when they cause severe unilateral modification of the lipids of the exposed leaflet. In addition, the situation is complicated by the role of the induced increase in the flip-flop rate under different experimental conditions and by modification of the rearrangement of lipid molecules as a result of the metabolic state of the cell or ghost preparation and of the reactivity of lipids as a consequence of temperature changes. Here, more so than with proteins, one must be cautious in interpreting experimental results. Moreover, it would appear that the use of different techniques in conjunction and the consequent comparison of results should be recommended. It has been emphasized that 'general rules' do not hold and that each new material should be assay again. To give one example, it is not pertinent to state that proteins enhance the flip-flop rate in lipid vesicles (and hence in membranes). This holds true for glycophorin from erythrocyte membrane, but could not be proved when mitochondrial cytochrome oxidase was used. There seems to be no rule for the distribution of lipids between the two leaflets of different membranes. For example, even for different strains of the same bacterial species, highly divergent results have been reported. It is generally (and probably under the influence of different studies with erythrocytes) believed that in mammalian plasma membranes, choline phospholipids are enriched in the outer leaflet and aminophospholipids in the inner leaflet. Though this contention may prove to be correct, different instances of contradictory results have been given in the text. This shows that if rules do exist, they remain to be discovered or established...

Lysis of erythrocytes from stored human blood by phospholipase C (Bacillus cereus)

The ability of phospholipase C (Bacillus cereus) to lyse erythrocytes from human blood that had been stored under Transfusion Service conditions for up to 16 weeks has been examined. When incubated at 20 degrees C with enzyme (0.03 mg/ml, 55 units/ml) for up to 1 h fresh erythrocytes were not lysed. After about 4 weeks of storage a population of very readily lysed erythrocytes appeared. The morphological changes in erythrocytes from blood stored up to 16 weeks were examined by scanning electron microscopy. The proportion of very readily lysed erythrocytes correlated well with the proportion of spheroechinocytes I. This morphological form was shown to be preferentially removed by phospholipase C and before lysis a transient appearance of smooth spheres occurred. The decrease in blood ATP concentrations on storage was measured and found to correlate with the disappearance of discoid erythrocyte forms, but not directly with the increased susceptibility of the erythrocytes to lysis by the enzyme. However, erythrocytes of up to at least 15 weeks of age could be made less susceptible to lysis by pre-incubation in a medium designed to cause intracellular regeneration of ATP. During the lysis of spheroechinocytes I by electrophoretically pure recrystallized phospholipase C a rapid degradation of phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine + phosphatidylinositol) occurred together with a slower degradation of sphingomyelin.

The decreased membrane fluidity of in vivo aged, human erythrocytes. A spin label study

The decreased membrane fluidity of the in vivo aged, human erythrocytes is found, by monitoring the electron paramagnetic resonance (EPR) spectra of fatty acid spin labels incorporated into the membrane. In addition, the decreased cell sizes and the decreased cholesterol and phospholipids contents, without significant changes of the quantity of the membrane proteins, also the decrease of ATP and 2,3-diphosphoglycerate and the increase of ADP and AMP, in the aged cells, were observed. Further the functional impairments of the aged cells, i.e. the increased oxygen affinity and the decreased deformability, were shown. On the basis of these quantitative data, the alteration of the protein-lipid organization, due to decreased lipid/protein ratio, the modified protein-lipid interaction and/or the influences of the diminished ATP content, is suggested to contribute towards the decreased membrane fluidity of the in vivo aged erythrocytes.