Ion selectivity of aqueous leaks induced in the erythrocyte membrane by crosslinking of membrane proteins

The involvement of cation leaks in the storage lesion of red blood cells

Stored blood components are a critical life-saving tool provided to patients by health services worldwide. Red cells may be stored for up to 42 days, allowing for efficient blood bank inventory management, but with prolonged storage comes an unwanted side-effect known as the "storage lesion", which has been implicated in poorer patient outcomes. This lesion is comprised of a number of processes that are inter-dependent. Metabolic changes include a reduction in glycolysis and ATP production after the first week of storage. This leads to an accumulation of lactate and drop in pH. Longer term damage may be done by the consequent reduction in anti-oxidant enzymes, which contributes to protein and lipid oxidation via reactive oxygen species. The oxidative damage to the cytoskeleton and membrane is involved in increased vesiculation and loss of cation gradients across the membrane. The irreversible damage caused by extensive membrane loss via vesiculation alongside dehydration is likely to result in immediate splenic sequestration of these dense, spherocytic cells. Although often overlooked in the literature, the loss of the cation gradient in stored cells will be considered in more depth in this review as well as the possible effects it may have on other elements of the storage lesion. It has now become clear that blood donors can exhibit quite large variations in the properties of their red cells, including microvesicle production and the rate of cation leak. The implications for the quality of stored red cells from such donors is discussed.

Disease-associated glycosylated molecular variants of human C-reactive protein activate complement-mediated hemolysis of erythrocytes in tuberculosis and Indian visceral leishmaniasis

Human C-reactive protein (CRP), as a mediator of innate immunity, removed damaged cells by activating the classical complement pathway. Previous studies have successfully demonstrated that CRPs are differentially induced as glycosylated molecular variants in certain pathological conditions. Affinity-purified CRPs from two most prevalent diseases in India viz. tuberculosis (TB) and visceral leishmaniasis (VL) have differential glycosylation in their sugar composition and linkages. As anemia is a common manifestation in TB and VL, we assessed the contributory role of glycosylated CRPs to influence hemolysis via CRP-complement-pathway as compared to healthy control subjects. Accordingly, the specific binding of glycosylated CRPs with erythrocytes was established by flow-cytometry and ELISA. Significantly, deglycosylated CRPs showed a 7-8-fold reduced binding with erythrocytes confirming the role of glycosylated moieties. Scatchard analysis revealed striking differences in the apparent binding constants (10(4)-10(5) M(-1)) and number of binding sites (10(6)-10(7)sites/erythrocyte) for CRP on patients' erythrocytes as compared to normal. Western blotting along with immunoprecipitation analysis revealed the presence of distinct molecular determinants on TB and VL erythrocytes specific to disease-associated CRP. Increased fragility, hydrophobicity and decreased rigidity of diseased-erythrocytes upon binding with glycosylated CRP suggested membrane damage. Finally, the erythrocyte-CRP binding was shown to activate the CRP-complement-cascade causing hemolysis, even at physiological concentration of CRP (10 microg/ml). Thus, it may be postulated that CRP have a protective role towards the clearance of damaged-erythrocytes in these two diseases.

Chemical and physical in vitro alterations of the erythrocyte membrane: a model for its pathophysiological states?

Plasmodia induce conspicuous structural and functional changes in the erythrocyte membrane. Besides the insertion and apposition of 'xenoproteins', and alterations of lipid composition (fatty acid pattern) and dynamics (transbilayer mobility and disposition of phospholipids, or related probes), new permeation pathways (NPP) are formed, which are still ill-defined in terms of their molecular origin. A remarkable ion selectivity and a high and complete sensitivity of the NPP to inhibitors indicate a rather specific nature. On the other hand, numerous experimental perturbations of the erythrocyte membrane structure induce unspecific alterations of its barrier function. In view of the apparent similarities--in simple physicochemical terms--between the experimentally and the plasmodially induced structural perturbations, one would expect, in Plasmodium-invaded cells, unspecific alterations of permeability and phospholipid dynamics of the type observed after in vitro modification, in contrast to much of the experimental evidence. In order to highlight this puzzling discrepancy, this chapter outlines techniques of producing and analysing experimental barrier defects in erythrocytes, and summarizes the properties of the defects induced by electroporation and oxidative damage, in terms of solute permeability, transbilayer mobility of phospholipid probes and the disposition of native phospholipids. The possible absence of comparable unspecific defects in Plasmodium-modified cells may provide an interesting example for the evolutionary adaptation of the parasite.

Carbohydrate induced modulation of cell membrane VII. Binding of exogenous lectin increases osmofragility of erythrocytes

Due to their multivalent binding character, lectins when added exogenously will cross-link membrane surface receptors leading to lateral molecular reorganizations in the plane of the bilayer. This study reports for the first time that agglutination of rabbit erythrocytes by lentil lectin and concanavalin A increases their osmofragility. Increase in osmofragility was detected by measuring the hemolysis of erythrocytes in hypotonic as well as in isotonic solutions. It was also found that agglutination per se does not increase osmofragility but the binding of legume lectin is essential since human Rh+ cells agglutinated by a monoclonal antibody do not exhibit hemolysis.

Mechanism of hemolysis of human erythrocytes exposed to monosodium urate monohydrate crystals. Preliminary characterization of membrane pores

Microcrystals of monosodium urate monohydrate (MSUM) have the ability to cause rapid hemolysis of erythrocytes. The nature of the initial MSUM crystal-erythrocyte membrane binding interaction was investigated over a range of different ionic strength media. There was negligible binding of MSUM to erythrocyte ghost membranes in low ionic strength media such as isotonic mannitol but binding was dramatically increased in isotonic NaCl/mannitol solutions or isotonic mannitol containing 1 mM Ca2+. Hemolysis induced by MSUM crystals was preceded by the leakage of K+ from the cells suggesting a colloid-osmotic mechanism of hemolysis. The inclusion of large (oligosaccharide) molecules in the extracellular media or the modulation of the extracellular solution tonicity inhibited both the rate and extent of hemolysis supporting the concept of MSUM-induced pores followed by colloid osmotic hemolysis.

Identification of the structural elements of amphotericin B and other polyene macrolide antibiotics of the hepteane group influencing the ionic selectivity of the permeability pathways formed in the red cell membrane

The selectivity of the transmembrane permeability induced by polyene antibiotics was studied in human erythrocytes and related to the hemolytic potency of the drugs. The selectivity induced was differently, dependent on the antibiotic structure in aromatic (vacidin A, gedamycin) and nonaromatic heptaenes (amphotericin B, candidin). Aromatic heptaenes were more effective than nonaromatic in inducing permeability to K+. For both groups of antibiotics, permeability to K+ was not affected by substitution at the carboxyl group but important differences in the induction of permeability to H+, OH- and Cl- were found. The strongly hemolytic aromatic heptaenes vacidin A and gedamycin exhibited much higher protonophoric activity than the nonaromatic ones: amphotericin B, and candidin. The protonophoric properties of aromatic heptaenes were related to the presence of a free carboxyl group in the antibiotic molecule. Indeed the esterification or amidation of the carboxyl group of vacidin A or gedamycin eliminated the ability of the antibiotic to increase H+ conductance and consequently diminished their hemolytic activity to an important extent. Both groups of antibiotics differed also in the efficiency of anion permeability induction. Only unsubstituted aromatic heptaenes, at high concentration, induced Cl-/OH- exchange and conductive flux of Cl- in a concentration-dependent manner. Substitution at the carboxyl group of vacidin A or gedamycin eliminated this property. Amphotericin B as well as its carboxyl-substituted derivatives formed a pathway characterized by low K+ over Cl- selectivity, whatever the concentration. The hemolytic activity, related to K+ permeability increased by heptaenes was dependent on simultaneous increase of the permeability to anions, and net KCl influx. Carboxyl-substituted derivatives of aromatic heptaenes presenting a remarkably high selectivity for K+, had consequently a very poor hemolytic activity.

The effect of protoporphyrin on the susceptibility of human erythrocytes to oxidative stress: exposure to hydrogen peroxide

Binding of protoporphyrin caused a perturbation of the erythrocyte membrane, as reflected by a change in cell shape from discoid to echinocyte, and a concomitant increase in mean cellular volume and K(+)-loss. Protoporphyrin-induced changes could be prevented by the presence of BaCl2, whereas binding of protoporphyrin was not affected. Exposure of erythrocytes to hydrogen peroxide leads to K(+)-leakage and lipid peroxidation. In de presence of protoporphyrin, H2O2-induced K(+)-leakage was enhanced, whereas lipid peroxidation was inhibited. The increase in H2O2-induced K(+)-leakage by protoporphyrin was not affected by diamide or various K+ channel blockers, but could be prevented by the addition of BaCl2. The inhibition of lipid peroxidation, on the other hand, was not affected by BaCl2. These results indicate that the enhancement of H2O2-induced K(+)-leakage was most likely caused by the change in cell shape. Addition of chlorpromazine and promethazine, positively charged molecules that induce stomatocytosis, did not cause an enhancement of H2O2-induced K(+)-leakage.

Na+, K+ and Cl- selectivity of the permeability pathways induced through sterol-containing membrane vesicles by amphotericin B and other polyene antibiotics

Membrane diffusion potentials induced by amphotericin B (AmB), amphotericin B methyl ester (AmE), N-fructosyl AmB (N FruAmB) and vacidin, an aromatic polyene antibiotic, in ergosterol- or cholesterol-containing egg yolk phosphatidylcholine large unilamellar vesicles (LUV), were measured in various media, in order to determine the relative selectivity of Na+, K+, Cl- and other ions in these environments. Changes in the membrane potential were followed by fluorescence changes of 3,3'-dipropylthiadicarbocyanine (diS-C3-(5)). Subtle changes in intercationic selectivity were monitored by measuring biionic potentials, using the fluorescent pH sensitive probe pyranine. In all the cases studied, the intercationic selectivity of the permeability pathways induced by the four antibiotics was weak compared to that of specific biological channels, though distinct differences were noted. With AmB the selectivity appeared to be concentration dependent. Above 5 x 10(-7) M, the sequence determined for sterol-free small unilamellar vesicles (SUV) and cholesterol-containing SUV and LUV, Na+ > K+ > Rb+ > or = Cs+ > Li+ (sulfate salts), corresponded closely to Eisenman selectivity sequence number VII. At 5 x 10(-7) M and below the selectivity switched from Na+ > K+ to K+ > Na+. In contrast, Li+ was the most permeant ion for AmB channels in the presence of ergosterol. The selectivity between Na+ or K+ vs. Cl- varied with the antibiotic. It was very strong with vacidin at concentrations below 5 x 10(-7) M, smaller with AmB, nil with AmE and N FruAmB. The selectivities observed were antibiotic, concentration and time dependent, which confirms the existence of different types of channels.

Tellurite-induced damage of the erythrocyte membrane. Manifestations and mechanisms

Chemical and biophysical mechanisms underlying the thiol-dependent lytic action of tellurite (and selenite) on human erythrocytes were investigated using native and GSH-depleted cells. Exposure of GSH-depleted cells to tellurite alone produces oxidative cross-linking of membrane thiols paralleled by a moderate membrane leakiness comparable in its extent to that induced by other SH-oxidizing agents (diamide, periodate). Exposure to tellurite in presence of endogenous or exogenous GSH produces marked leakiness which stems from the formation of aqueous leaks permeant to ions and nonelectrolytes and sensitive to inhibition by phloretin. Apparent pore radii, derived from exclusion limits for polar non-electrolytes, range from 0.3 to at least 1.3 nm. Leak size increases with increasing exposure time and concentration of the modifier. Leak formation is paralleled by membrane rigidification based on the cross-linking of spectrin. Thiol-dependent leak formation by tellurite in GSH-depleted cells can be sustained not only by exogenous GSH but also by other thiols. Progress of leak formation by tellurite/thiol can not be reliably quenched by procedures such as removal of tellurite from the medium, inhibition of anion transport via band-3 protein, washing of the cells or low temperature. The reaction can, however, be terminated, even in the presence of tellurite, by addition of N-ethylmaleimide, presumably due to the blockage of thiols or thiol-analogous tellurium compounds. N-ethylmaleimide even brings about a partial reversal of leakiness, suggesting the contribution of a reversible and an irreversible component of tellurite damage. Membrane perturbation by tellurite/thiol involves the formation of a membrane permeant tellurium species, possibly HTe-, which is likely to induce progressive damage of membrane proteins by a redox shuttle going along with a formation of elemental tellurium and its reduction by thiols.

Investigation of monovalent cation influxes of diamide-treated human erythrocytes in solutions of different ionic strength

Total and residual i.e. (ouabain + bumetanide + EGTA)-insensitive K+ as well as Na+ influxes were investigated in human erythrocytes before and after treatment with diamide (5 mM). In physiological and in low ionic strength solution these influxes were increased after diamide treatment. Diamide-treated cells do not exhibit significant differences between the total and residual influxes for both Na+ and K+. The diamide-induced cation fluxes in low ionic strength solution are significantly higher compared with the fluxes in physiological ionic strength solution. The diamide-induced K+ influx is not chloride-dependent, and replacement of NaCl by sodium methylsulfate does not significantly reduce this flux. A subsequent incubation of diamide-treated erythrocytes with dithioerythritol which restores the cellular glutathione level to its original value only partly decreases the enhanced K+ influx. From these results it can be concluded that electrodiffusion and K/Cl cotransport are not involved in the diamide-induced stimulation of the residual K+ influx of human erythrocytes.

Influence of phloretin and alcohols on barrier defects in the erythrocyte membrane caused by oxidative injury and electroporation

Oxidative damage by diamide, periodate and oxygen-derived reactive species, but also exposure to electroporation induce in the erythrocyte membrane dynamic, presumably fluctuating, defects having the properties of aqueous holes with definable radii and selectivities. These leaks, which can be quantified by measuring tracer fluxes or rates of colloid-osmotic lysis, are here shown to be inhibited by phloretin and a small number of related phenol compounds (phenolphthalein, hydroxyacetophenones, nitrophenol), while a host of other 'membrane-active' agents is not effective in this respect. I50 values range from about 200 microM for phloretin and phenolphthalein to about 10 mM for 4-nitrophenol. Inhibition by phloretin is reversible, not competitive and not related in its extent to the extent of leakiness. In contrast, the enhancement of transbilayer mobility of amphiphilic lipid probes, which invariably goes along with leak formation of the type described, is not affected by phloretin. Aliphatic alcohols (hexanol, butanol) have an amplifying effect on leaks induced by oxidative damage but do not affect leaks induced by electroporation. The alcohol-amplified leaks maintain the properties of aqueous holes as indicated by a low activation energy of leak fluxes. Since both, inhibition and stimulation of leak fluxes do not go along with appreciable changes of the apparent radii of the aqueous holes, changes in the dynamics (opening and closing) of the defects are proposed to underly the effects of phloretin and alkanols. The membrane lipid domain is likely to be the site of the leaks and of their modulation.

Electroporation of cell membranes

Electric pulses of intensity in kilovolts per centimeter and of duration in microseconds to milliseconds cause a temporary loss of the semipermeability of cell membranes, thus leading to ion leakage, escape of metabolites, and increased uptake by cells of drugs, molecular probes, and DNA. A generally accepted term describing this phenomenon is "electroporation." Other effects of a high-intensity electric field on cell membranes include membrane fusions, bleb formation, cell lysis... etc. Electroporation and its related phenomena reflect the basic bioelectrochemistry of cell membranes and are thus important for the study of membrane structure and function. These phenomena also occur in such events as electric injury, electrocution, and cardiac procedures involving electric shocks. Electroporation has found applications in: (a) introduction of plasmids or foreign DNA into living cells for gene transfections, (b) fusion of cells to prepare heterokaryons, hybridoma, hybrid embryos... etc., (c) insertion of proteins into cell membranes, (d) improving drug delivery and hence effectiveness in chemotherapy of cancerous cells, (e) constructing animal model by fusing human cells with animal tissues, (f) activation of membrane transporters and enzymes, and (g) alteration of genetic expression in living cells. A brief review of mechanistic studies of electroporation is given.

Leakage of internal markers from erythrocytes and lipid vesicles induced by melittin, gramicidin S and alamethicin: a comparative study

The membrane-disruptive capacities of melittin, derivatised melittins, alamethicin and gramicidin S have been compared for the human erythrocyte membrane and lipid vesicles of three different compositions (phosphatidylcholine, 85% phosphatidylcholine/15% phosphatidylserine, and a lipid analogue of the outer leaflet of the human erythrocyte membrane). The sensitivity to ionic strength, divalent metal ions and polylysine of release of fluorescent markers from liposomes and of haemoglobin from intact erythrocytes has been assayed. Acetyl melittin was found to he more effective than melittin in lysing phosphatidylcholine and phosphatidylcholine/phosphatidylserine vesicles, somewhat less effective in the lipid analogue and markedly less effective in lysing erythrocytes. Succinyl melittin was non-haemolytic, but was able to lyse lipid vesicles at a high concentration. Ca2+ inhibited melittin haemolysis at high ionic strength (150 mM NaCl), but produced a more complex response of stimulation followed by inhibition at low ionic strength. In lipid vesicles, Ca2+ either stimulated melittin lysis or was ineffective. Zn2+ exerted effects similar to Ca2+ with lipid vesicles at approx. 10-fold lower concentration except that a weak inhibition was observed for the erythrocyte membrane lipid analogue at high ionic strength. Polylysine strongly inhibited haemolysis by melittin at low ionic strength, but was ineffective or stimulatory in lipid vesicle lysis. High phosphate concentration also inhibited melittin haemolysis, but again no corresponding effect could he found in any of the lipid vesicle systems. These disparities between effects of melittin on erythrocytes and lipid vesicles support the proposal that melittin-protein interactions are of consequence to its haemolytic action. Similar experiments were performed with gramicidin S and alamethicin in order to compare their lytic properties with those of melittin. It was found that each lysin exhibited its own individual pattern of sensitivity to lipid composition, ionic strength and inhibition by cations. It thus appears likely that the detailed molecular interactions responsible for lysis are significantly different for each of these three agents.

The accumulation of malonyldialdehyde, an end product of membrane lipid peroxidation, can cause potassium leak in normal and sickle red blood cells

This study has examined the effect of malonyldialdehyde (MDA), an end product of lipid peroxidation, on the K+ leak in normal (AA) and sickle (SS) red blood cells (RBCs). In vitro MDA accumulation in human RBCs was accomplished by treating them with exogenous standard MDA. MDA accumulation assessed by the thiobarbituric acid reactivity of in vitro MDA-treated RBCs was comparable to the RBCs in hemolytic anemias. There was a significant K+ leak in AA RBCs after in vitro treatment with MDA. The effect of MDA on the K+ leak was greater in SS RBCs. The increase in cellular K+ leak was significantly positively correlated with the extent of MDA accumulation as assessed by thiobarbituric acid reactivity.

Enhancement of transbilayer mobility of a membrane lipid probe accompanies formation of membrane leaks during photodynamic treatment of erythrocytes

In order to further characterize membrane alterations in human erythrocytes subjected to photodynamic treatment the passive transbilayer mobility of a phospholipid analogue was studied in cells illuminated for various lengths of time in the presence of the photosensitizer, aluminum chlorotetrasulfophthalocyanine. These measurements were combined with the characterization of the membrane leaks for polar solutes occurring under the same conditions with respect to their apparent size, number and ion selectivity. The time-dependent photodynamic enhancement of leaks for K+ as well as choline or erythritol was paralleled by a marked increase of the transbilayer reorientation rate of the amphiphilic lipid probe, palmitoyllysophosphatidylcholine from 0.05% min-1 in native cells to 0.32% min-1 after 60 min illumination. The asymmetric orientation of native phospholipids was not affected by this treatment. The leak permeability proved to be due to the formation of pores with apparent radii of about 0.45 nm after 60 min illumination, and of 0.75 nm after 90 min. The number of pores per cell was calculated to be less than 1, the pores are slightly cation-selective (PK/PCl approximately 3:1). Since photodynamic treatment did not induce lipid peroxidation under the prevailing experimental conditions, protein modification must be the primary cause of both, leak permeability and flip enhancement. Since it is also likely that the leak permeability arises from oxidation of intrinsic membrane proteins, the results raise the interesting possibility that oxidative alteration of intrinsic membrane proteins may lead to enhanced transbilayer mobility of lipids.

Relationship between ionophoric and haemolytic activities of perimycin A and vacidin A, two polyene macrolide antifungal antibiotics

The ionophoric and hemolytic activities of two antifungal aromatic heptaenes: vacidin A and perimycin A, were studied on human red blood cells. Measurements of hemolysis, K+ influx and efflux, H+ movement and potential difference across the cell membrane, show that the hemolytic activity, being related to the K+ permeability induced by the polyene, is strongly dependent on the ability of this polyene to induce H+ movement. It was shown that: (1) both antibiotics have approximately the same efficiency in inducing K+ permeability, but a 100-fold difference in their hemolytic activity; (2) their hemolytic activity is related to their ability to induce H+ movement; (3) the protonophoric activity requires the existence of a free carboxyl group in the macrolide ring, as in vacidin A. The hemolytic activity is determined by the intrinsic efficiency of a K+/H+ exchange induced by this polyene. With perimycin A, which lacks the free carboxyl group, the hemolytic activity is dependent on the Cl- conductive flux which slows down the K+ flux.

Chemo-mechanical leak formation in human erythrocytes upon exposure to a water-soluble carbodiimide followed by very mild shear stress. I. Basic characteristics of the process

Human erythrocytes treated with low concentrations (1-5 mM) of the carboxyl group-modifying reagent 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) lose their native deformability in parallel with extensive cross-linking of the membrane skeleton. After treatment with higher (5-40 mM) concentrations of the reagent the cells develop a hitherto undescribed property: when subjected to even very low shear stresses (resuspension after packing by centrifugation or viscometric shearing at up to 4 s-1) they become highly leaky to ions, lose their K+ with a half-time of about 5 min and subsequently undergo hemolysis. Lysis is not accompanied by cell fragmentation as occurs with mechanical hemolysis, but is colloid-osmotic, due to the formation of aqueous membrane leaks with an apparent radius of about 3 nm. Leakiness and lysis affect an increasing fraction of the cell population, in relation to (a) the concentration of EDC applied, (b) the shearing intensity, and (c) particularly, the hematocrit during shearing. The physical parameter determining the mechanical component of this 'chemo-mechanical' leak formation is not predominantly the shear stress. Rather, cell-cell interactions of as yet undefined nature seem to be involved. The analysis of chemo-mechanical leak formation may provide interesting insights into the influence of mechanical forces on membranes.

Diamide stimulates calcium-sodium exchange in dog red blood cells

Calcium influx can be stimulated in dog red blood cells by preexposure to diamide under certain conditions. Diamide-activated calcium influx resembles swelling-induced Ca2+-Na+ exchange in several respects. These include saturation of calcium influx at external calcium levels greater than 0.5 mM, suppression of calcium influx by external sodium, and inhibition by quinidine. The ability of diamide to stimulate this transport pathway depends critically on the ionic composition of the medium in which the cells are bathed at the time of diamide exposure. The effect is greatest if the diamide preincubation is conducted in a hypotonic lithium chloride medium containing at least 1 microM calcium. Stimulation of Ca2+-Na+ exchange is seen at diamide concentrations (0.10-0.33 mM) that are lower than those reported to cause major spectrin cross-linking, glutathione depletion, Ca2+-ATPase inhibition, or ion channel formation. The results suggest that dog red cells have a large latent capacity for Ca2+-Na+ exchange.

Peroxide-induced effects on lens cation transport following inhibition of glutathione reductase activity in vitro

Previous studies from this laboratory have shown that the normal lens can tolerate exposure to 0.05 mM H2O2 without apparent damage and that this is due in part to an active glutathione redox cycle. The present studies were designed to investigate the role of glutathione reductase in protecting cation transport systems in the lens against potentially damaging effects of peroxide. Pre-treatment of rabbit lenses with 0.5 mM 1.3-bis(2-chloroethyl)-1-nitrosourea (BCNU), a relatively specific inhibitor of glutathione reductase, brought about a 71% inhibition of the enzyme in the capsule-epithelia of the lenses. Subsequent exposure of the lenses for 3 hr to a constant level of 0.05 mM H2O2 in culture medium produced significant accumulation of oxidized glutathione (GSSG) in the lens epithelium and severe effects on the electrolyte balance in the lens, on the activity of Na, K-ATPase and on the accumulation and efflux of 86Rb. The effects included a 35% decrease in activity of Na, K-ATPase, a 10 mM increase in the concentration of Na+ and an 8 mM decrease in K+. BCNU-H2O2 treatment also resulted in loss of transparency of the lenses in the form of vacuoles present in the anterior, subcapsular region, encircling the entire periphery of the organ near the germinative zone of the epithelium. Treatment with either BCNU or 0.05 mM H2O2 alone had only minimal effects on accumulation of GSSG in the epithelium, on lens transparency and on the parameters of cation transport which were investigated. When lenses were treated with 0.05 mM H2O2 alone and then placed in normal medium to measure the accumulation of 86Rb it was found that the cation pump was stimulated 20% above the normal level of activity. Levels of H2O2 higher than 0.05 mM without BCNU pre-treatment produced significant inhibition of Na, K-ATPase and the effects of 0.3 mM H2O2 on cation transport and GSSG accumulation were comparable to those of BCNU-0.05 mM H2O2. While inhibition of the activities of glutathione reductase and Na, K-ATPase in the lenses was found to be irreversible, a partial recovery of the Na+ level and nearly complete recovery of the K+ level were observed when treated lenses were cultured in normal medium for an additional 6 hr. In addition, the rate of efflux of 86Rb which was significantly faster from the BCNU-H2O2-treated lenses compared with the controls, was found to return to the control value during the recovery period.(ABSTRACT TRUNCATED AT 400 WORDS)

Progressive oxidative membrane damage in erythrocytes after pulse treatment with t-butylhydroperoxide

Development of membrane damage in erythrocytes in the presence of the radical-forming oxidant t-butylhydroperoxide is a well established fact (see, for example, Deuticke et al. (1986) Biochim. Biophys. Acta 854, 169-183). We have now demonstrated that a mere pulse treatment of erythrocytes (5-15 min) with this agent leads to subsequent development of progressive oxidative membrane damage in spite of the absence of exogenous oxidant. Damage comprises the occurrence of ion leakiness and subsequent colloid-osmotic lysis, enhancement of the transbilayer mobility of phospholipid analogues, and lipid peroxidation. There is, however, only very little concomitant oxidation and precipitation of hemoglobin. Defect formation is not due to oxidation of SH-groups nor is it directly related to lipid peroxidation, since it can be suppressed by thiourea without concommitant inhibition of lipid peroxidation. This 'spontaneous' development of membrane damage can be antagonized by metabolic substrates and by desferrioxamine, indicating that lack of protective metabolic resources as well as the presence of catalytic metal (iron) complexes are required for the development of membrane damage. This progressive development of injury in cells only temporarily exposed to an exogenous oxidant may be regarded as a more appropriate model for oxidative membrane damage under pathophysiological conditions in vivo than cells exposed to continuous damage by exogenous oxidants.

Bacterial cytotoxins, amphotericin B and local anesthetics enhance transbilayer mobility of phospholipids in erythrocyte membranes. Consequences for phospholipid asymmetry

Incorporation of the channel-forming polyene antibiotic amphotericin B and of cytotoxins from Staphylococcus aureus (alpha-toxin) or Pseudomonas aeruginosa into erythrocyte membranes results in a concentration-dependent enhancement of the flip rates of exogenous lysophosphatidylcholine. The flip rate is also enhanced by incorporation of tetracaine and dibucaine. Removal of tetracaine and amphotericin B from the cells normalizes the flip rates. In parallel to the enhancement of flip rates, alpha-toxin produces a loss of transmembrane asymmetry of both phosphatidylethanolamine and phosphatidylserine. Pretreatment of cells with amphotericin or high concentrations (over 2.5 mmol . l-1) of tetracaine, followed by removal of the perturbing agent by washing, produces a selective loss of the asymmetric orientation of phosphatidylethanolamine to the inner membrane layer, as evaluated by the accessibility of the lipid towards cleavage by phospholipase A2. The extent to which asymmetry is lost depends on the time of pretreatment with amphotericin or tetracaine, indicating a limitation by the rate of reorientation of phosphatidylethanolamine to the outer membrane surface. Evaluation of the accessibility of phosphatidylethanolamine towards cleavage by phospholipase A2 in the presence of local anesthetics indicates accessible fractions much higher than those obtained after removal of the perturbant. In the presence of tetracaine, endofacial phosphatidylethanolamine seems somehow to become accessible to phospholipase A2. Phosphatidylserine does not exhibit this peculiarity. The results indicate that various types of perturbation of the lipid domain of the erythrocyte membrane may enhance the transbilayer mobility of phospholipids as well as destabilize the asymmetric distribution of aminophospholipids. However, as in other instances reported previously (Haest, C.W.M., Erusalimsky, J., Dressler, V., Kunze, I. and Deuticke B. (1983) Biomed. Biochim. Acta 42, 17-21), there is no tight coupling between transbilayer mobility and destabilization of asymmetry of the transbilayer distribution of phospholipids.

Leak formation in human erythrocytes by the radical-forming oxidant t-butylhydroperoxide

Oxidative damage of human erythrocytes by the lipoperoxide analogue, t-butylhydroperoxide, has been characterized with regard to ion-permeable leaks formed in the membrane. The formation of these leaks is not correlated with oxidative denaturation of hemoglobin and its precipitation at the membrane. It is also not related to the oxidation of membrane protein SH-groups. A close, although not simply proportional correlation could be demonstrated between leak formation and phospholipid peroxidation as monitored by occurrence of malondialdehyde. The two processes showed similar dependences on exposure time, concentration and temperature. Both were stimulated by the addition of azide as a ligand of ferric heme iron, and suppressed by the anti-oxidant, butylated hydroxytoluene. The leak pathway permits solute permeation with a temperature dependency of bulk diffusion in water and discriminates nonelectrolytes according to size. Discrimination among alkali chlorides corresponds to their free solution mobility; sodium halides are discriminated more effectively. Apparent radii of about 0.5-0.7 nm can be assigned to the defects, while apparent numbers of defects per cell as low as 0.1-0.2 suggest that the defects are dynamic in nature.

Properties of the leak permeability induced by a cytotoxic protein from Pseudomonas aeruginosa (PACT) in rat erythrocytes and black lipid membranes

A cytotoxic protein, isolated from Pseudomonas aeruginosa (PACT), was tested on red blood cells of rats and on black lipid membranes for changes of membrane permeability. In rat erythrocytes PACT induces lysis indicative of the formation of a leak permeable to monovalent ions. The dose response curve for the PACT-induced hemolysis demonstrates that the rate of lysis as well as the fraction of lytic cells increases with increasing toxin concentration. Furthermore, the leak pathway discriminates hydrophilic non-electrolytes according to their molecular weight. The findings indicate formation by PACT of a pore with an apparent radius of about 1.2 nm. In pure lipid membranes PACT forms hydrophilic pathways with moderate selectivity for small cations over small anions. The presence of cholesterol is a prerequisite for the occurrence of these PACT-induced permeability changes.

Effects of N-ethylmaleimide on ouabain-insensitive cation fluxes in human red cell ghosts

In red cells of several species, the sulfhydryl reagent N-ethylmaleimide activates a Cl- -dependent, ouabain-resistant K+ transport pathway. Here we report our attempts to demonstrate ouabain-resistant Cl- -dependent K+ fluxes stimulated by N-ethylmaleimide in resealed human red cell ghosts using Rb+ as a K+ analogue. In contrast to intact cells, the rate constants of the base level Rb+ efflux in ghosts were similar in NaNO3 and NaCl (okRb = 0.535 +/- 0.079 h-1 and 0.534 +/- 0.085 h-1, respectively), while 1 mM N-ethylmaleimide stimulated Rb+ efflux strongly in NaNO3 (okRb = 14.26 +/- 1.32 h-1) and moderately in NaCl (okRb = 2.73 +/- 0.54 h-1). This effect was dependent on the presence of internal ATP. Stimulation of Rb+ efflux was observed in the presence of greater than or equal to 0.2 mM N-ethylmaleimide and increased at pH values approaching 8.0, consistent with titration of SH groups. N-Ethylmaleimide-stimulated Rb+ efflux was approx. 50% inhibited by 100 microM quinine sulfate whereas 1 microM bumetanide had no effect. In NaCl the N-ethylmaleimide-stimulated efflux saturated with initial internal ghost Rb+ concentration, but rates increased linearly in NaNO3. Replacement of external Na+ with glucamine or choline decreased the N-ethylmaleimide-stimulated Rb+ efflux, suggesting a role for external Na+. N-Ethylmaleimide-stimulated Rb+ efflux was greater in buffers with lipophilic anions such as SCN- or NO3- than in solutions with Cl- or acetate. However, the cation selectivity of the pathway studied was low, as Li+ efflux was also stimulated by N-ethylmaleimide. We conclude that the effect of N-ethylmaleimide on ouabain-resistant cation effluxes of human red cell ghosts is very different from the selective action of N-ethylmaleimide on Rb+ influxes in intact red cells.

Peroxide-induced membrane damage in human erythrocytes

Erythrocytes exposed to H2O2 or t-butyl hydroperoxide (tBHP) exhibited lipid peroxidation and increased passive cation permeability. In the case of tBHP a virtually complete inhibition of both processes was caused by butylated hydroxytoluene (BHT), whereas pretreatment of the cells with CO increased both lipid peroxidation and K+ leakage. In the experiments with H2O2, on the other hand, both BHT and CO strongly inhibited lipid peroxidation, without affecting the increased passive cation permeability. These observations indicate different mechanisms of oxidative damage, induced by H2O2 and tBHP, respectively. The SH-reagent diamide strongly inhibited H2O2-induced K+ leakage, indicating the involvement of SH oxidation in this process. With tBHP, on the contrary, K+ leakage was not significantly influenced by diamide. Thiourea inhibited tBHP-induced K+ leakage, without affecting lipid peroxidation. Together with other experimental evidence this contradicts a rigorous interdependence of tBHP-induced lipid peroxidation and K+ leakage.

Formation and properties of aqueous leaks induced in human erythrocytes by electrical breakdown

Leaks were induced in human erythrocytes by brief (tau = 1-40 microseconds) discharges of high electric fields (3-20 kV/cm). Leak permeabilities were characterized by measuring (a) net and tracer fluxes of K+ and nonelectrolytes under protection of the cells against colloid-osmotic lysis, or (b) rates of colloid osmotic lysis in various salt solutions. The induced permeabilities are essentially stable for hours at 0-2 degrees C. Leak permeability P increases exponentially with the breakdown voltage ED according to a function of the general type P = bED. The basis b varies with the pulse length. A log-linear presentation reveals a biphasic linear relationship with a break at which the slope (= log b) decreases markedly. Elevated ionic strengths of the suspension medium during the electric discharge enhance leak formation. Leak permeability exhibits an apparent activation energy of 29 +/- 5 kJ/mol, indicative of diffusion through aqueous pathways. Somewhat differing equivalent pore radii emerge from measurements with different probes: 0.6-0.8 nm from tracer fluxes of polyols (Mr = 3600, ED = 4-7 kV/cm) and 0.8-1.9 nm from osmotic protection studies with polyethylene glycols (Mr = 200-3300, ED = 6-10 kV/cm). These numbers and the non-monoexponential increase of leak permeability with the field strength suggest a dual mechanism for the increase of leak permeability: an increase of the number of pores at low breakdown voltage and an additional increase of pore size at higher voltage. Estimated numbers of pores range from 1 to 10 per cell, which suggests dynamic fluctuating structural defects to be involved. The leaks discriminate small monovalent inorganic ions in the sequence of free solution mobility. Organic anions are discriminated according to size and charge. Common properties of these electrically induced defects and of chemically induced leaks (diamide, periodate, t-butylhydroperoxide) in the erythrocyte membrane suggest close similarities in the molecular organization.

Oxidative stress of human erythrocytes by iodate and periodate. Reversible formation of aqueous membrane pores due to SH-group oxidation

Human erythrocytes were exposed to oxidative stress by iodate and periodate. Oxidation causes a time- and concentration-dependent increase in membrane permeability for hydrophilic molecules and ions. The induced leak discriminates nonelectrolytes on the basis of molecular size and exhibits a very low activation energy (Ea = 1-4 kcal.mol-1). These results are reconcilable with the formation of aqueous pores. The pore size was approximated to be between 0.45 and 0.6 nm. This increase in permeability is reversible upon treatment with dithioerythritol. Blocking of membrane thiol groups with N-ethylmaleimide protects the membranes against leak formation. The oxidation causes dithioerythritol-reversible modification of membrane proteins as indicated by the gel electrophoretic behavior. These modifications can also be suppressed by blocking the membrane thiol groups with N-ethylmaleimide. About half of the membrane methionine is oxidized to acid hydrolysis-stable derivatives. A fast saturating increase in diene conjugation was observed in whole cells but not in isolated membranes, with only minor degradation of fatty acid chains. The oxidation of cell membrane lipids as well as oxidation of cell surface carbohydrates are not involved in leak formation. Taken together with earlier data (Deuticke, B., Poser, B., Lütkemeier, P. and Haest, C.W.M. (1983) Biochim. Biophys. Acta 731, 196-210), these findings indicate that formation of disulfide bonds by different oxidative mechanisms results in leaks with similar properties.