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

Phloretin-induced cytoprotective effects on mammalian cells: A mechanistic view and future directions

Phloretin (C15 H14 O5 ), a dihydrochalcone flavonoid, is mainly found in fruit, leaves, and roots of apple tree. Phloretin exerts antioxidant, anti-inflammatory, and anti-tumor activities in mammalian cells through mechanisms that have been partially elucidated throughout the years. Phloretin bioavailability is well known in humans, but still remains to be better studied in experimental animals, such as mouse and rat. The focus of the present review is to gather information regarding the mechanisms involved in the phloretin-elicited effects in different in vitro and in vivo experimental models. Several manuscripts were analyzed and data raised by authors were described and discussed here in a mechanistic manner. Comparisons between the effects elicited by phloretin and phloridzin were made whenever possible, as well as with other polyphenols, clarifying questions about the use of phloretin as a potential therapeutic agent. Toxicological aspects associated to phloretin exposure were also discussed here. Furthermore, a special section containing future directions was created as a suggestive guide towards the elucidation of phloretin-related actions in mammalian cells and tissues.

Peroxidation of polyunsaturated phosphatidyl-choline lipids during electroformation

Giant unilamellar vesicles (GUVs) have been utilized both as model systems to study the physico-chemical properties of biomembranes and as host materials for investigating biological processes in microbioreactors. GUVs are commonly formed by an electroformation technique. However, there is a concern that the electric fields applied during electroformation can peroxidize lipid acyl chains, thereby altering the phospholipid composition and material properties of the synthesized vesicles. Here in this paper, we report the effect of electroformation on the extent of peroxidation of a number of polyunsaturated phosphatidyl-choline lipids (PULs). Specifically, we detected peroxidation byproducts (malonaldehydes and conjugated dienes) of the following lipids utilizing UV/Vis spectroscopy: dilinoleoyl phosphatidyl-choline (DLPC) (di-18:2 PC), dilinolenoyl phosphatidyl-choline (DNPC) (di-18:3 PC), diarachidonoyl phosphatidyl-choline (DAPC) (di-20:4 PC), and didocosaheexaenoyl phosphatidyl-choline (DHA) (di-22:6 PC). The results indicate that PC PULs lipids are prone to peroxidation, with increasing unsaturation levels leading to higher levels of peroxidation byproducts. The levels of peroxidation byproducts of DAPC were found to depend linearly on the strength of the electric field, indicating that the observed effects were due to the applied electric field. Lipid peroxidation can affect a number of important membrane properties, including domain formation and mechanical stability. Thus, alteration of the chemical composition of polyunsaturated lipids (PULs) by the electroformation technique can potentially complicate the interpretation of experimental studies that utilize GUVs composed of PULs.

Fatty acid transport and metabolism in HepG2 cells

The mechanism(s) of fatty acid uptake by liver cells is not fully understood. We applied new approaches to address long-standing controversies of fatty acid uptake and to distinguish diffusion and protein-based mechanisms. Using HepG2 cells containing an entrapped pH-sensing fluorescence dye, we showed that the addition of oleate (unbound or bound to cyclodextrin) to the external buffer caused a rapid (seconds) and dose-dependent decrease in intracellular pH (pH(in)), indicating diffusion of fatty acids across the plasma membrane. pH(in) returned to its initial value with a time course (in min) that paralleled the metabolism of radiolabeled oleate. Preincubation of cells with the inhibitors phloretin or triacsin C had no effect on the rapid pH(in) drop after the addition of oleate but greatly suppressed pH(in) recovery. Using radiolabeled oleate, we showed that its esterification was almost completely inhibited by phloretin or triacsin C, supporting the correlation between pH(in) recovery and metabolism. We then used a dual-fluorescence assay to study the interaction between HepG2 cells and cis-parinaric acid (PA), a naturally fluorescent but slowly metabolized fatty acid. The fluorescence of PA increased rapidly upon its addition to cells, indicating rapid binding to the plasma membrane; pH(in) decreased rapidly and simultaneously but did not recover within 5 min. Phloretin had no effect on the PA-mediated pH(in) drop or its slow recovery but decreased the absolute fluorescence of membrane-bound PA. Our results show that natural fatty acids rapidly bind to, and diffuse through, the plasma membrane without hindrance by metabolic inhibitors or by an inhibitor of putative membrane-bound fatty acid transporters.

Partial oxidative-stress perturbs membrane permeability and fluidity of fish nucleated red blood cells

We investigated the influence of partial oxidative stress on permeability and fluidity of nucleated fish red blood cells for simulating nucleated somatic cells. Peroxide value indicating lipid hydroperoxide level in nucleated red blood cells of common carp (Cyprinus carpio) increased with increasing body size. We detected that oxidation of nucleated red blood cells led to the degraded PUFA compositions and accelerated the permeability of calcein and ATP in the nucleated red blood cells restrictedly oxidized with 1 mM AAPH treatment for 30 min at 21 degrees C in the dark. Using fluorescence probes, PC3P, we found that oxidative stress reduced the membrane fluidity of nucleated red blood cells. It was also observed that AAPH had no significant influence on the osmotic fragility and electrophoretic profiles of red blood cell proteins. These results suggest that partial oxidative-stress, even if failure to fragment the membrane, may affect membrane permeability of fish nucleated red blood cells for an important energy molecule, ATP.

Trehalose improves survival of electrotransfected mammalian cells

BACKGROUND

Electropermeabilization is widely used for introduction of DNA and other foreign molecules into eukaryotic cells. However, conditions yielding the greatest molecule uptake and gene expression can result in low cell survival. In this study, we assessed the efficiency of trehalose for enhancing cell viability after excessive electropermeabilization. This disaccharide was chosen because of its capability of stabilizing cell membranes under various stressful conditions, such as dehydration and freezing.

MATERIALS AND METHODS

Various mammalian cell lines were electropermeabilized by single exponentially decaying electric pulses of few kV/cm strength and of several-microsecond duration. Propidium iodide (PI) and a plasmid encoding green fluorescent protein (GFP), respectively, served as reporter molecules. The effects of trehalose on PI-uptake, GFP gene expression, transfection yield, and short- and long-term viability were analyzed by flow cytometry and electronic cell counting.

RESULTS

The substitution of inositol by trehalose in pulse media protected cells against field-induced cell lysis. The protection effect saturated at about 40-50 mM trehalose. Transfection yield and gene expression were not significantly affected by trehalose. But the transfection efficiency was generally higher in the presence of trehalose, mainly because of the increased cell survival.

CONCLUSIONS

We demonstrated that trehalose-substituted media are superior to standard trehalose-free pulse media for improving cell survival and achieving higher electrotransfection efficiency.

Phloretin-induced changes of lipophilic ion transport across the plasma membrane of mammalian cells

The adsorption of the hydrophobic anion [W(CO)(5)CN](-) to human lymphoid Jurkat cells gave rise to an additional anti-field peak in the rotational spectra of single cells, indicating that the cell membrane displayed a strong dielectric dispersion in the kilohertz to megahertz frequency range. The surface concentration of the adsorbed anion and its translocation rate constant between the two membrane boundaries could be evaluated from the rotation spectra of cells by applying the previously proposed mobile charge model. Similar single-cell electrorotation experiments were performed to examine the effect of phloretin, a dipolar molecule known to influence the dipole potential of membranes, on the transport of [W(CO)(5)CN](-) across the plasma membrane of mammalian cells. The adsorption of [W(CO)(5)CN](-) was significantly reduced by phloretin, which is in reasonable agreement with the known phloretin-induced effects on artificial and biological membranes. The IC(50) for the effect of phloretin on the transport parameters of the lipophilic ion was approximately 10 microM. The results of this study are consistent with the assumption that the binding of phloretin reduces the intrinsic dipole potential of the plasma membrane. The experimental approach developed here allows the quantification of intrinsic dipole potential changes within the plasma membrane of living cells.

Permeability and channel-mediated transport of boric acid across membrane vesicles isolated from squash roots

Boron is an essential micronutrient for plant growth and the boron content of plants differs greatly, but the mechanism(s) of its uptake into cells is not known. Boron is present in the soil solution as boric acid and it is in this form that it enters the roots. We determined the boron permeability coefficient of purified plasma membrane vesicles obtained from squash (Cucurbita pepo) roots and found it to be 3 x 10(-7) +/-1.4 x 10(-8) cm s(-1), six times higher than the permeability of microsomal vesicles. Boric acid permeation of the plasma membrane vesicles was partially inhibited (30%-39%) by mercuric chloride and phloretin, a non-specific channel blocker. The inhibition by mercuric chloride was readily reversible by 2-mercaptoethanol. The energy of activation for boron transport into the plasma membrane vesicles was 10.2 kcal mol(-1). Together these data indicate that boron enters plant cells in part by passive diffusion through the lipid bilayer of the plasma membrane and in part through proteinaceous channels. Expression of the major intrinsic protein (MIP) PIP1 in Xenopus laevis oocytes resulted in a 30% increase in the boron permeability of the oocytes. Other MIPs tested (PIP3, MLM1, and GlpF) did not have this effect. We postulate that certain MIPs, like those that have recently been shown to transport small neutral solutes, may also be the channels through which boron enters plant cells.

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.

Inhibition of the electrostatic interaction between beta-amyloid peptide and membranes prevents beta-amyloid-induced toxicity

The accumulation of beta-amyloid peptides (Abeta) into senile plaques is one of the hallmarks of Alzheimer disease. Aggregated Abeta is toxic to cells in culture and this has been considered to be the cause of neurodegeneration that occurs in the Alzheimer disease brain. The discovery of compounds that prevent Abeta toxicity may lead to a better understanding of the processes involved and ultimately to possible therapeutic drugs. Low nanomolar concentrations of Abeta1-42 and the toxic fragment Abeta25-35 have been demonstrated to render cells more sensitive to subsequent insults as manifested by an increased sensitivity to formazan crystals following MTT (3-[4,5-dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide) reduction. Formation of the toxic beta-sheet conformation by Abeta peptides is increased by negatively charged membranes. Here we demonstrate that phloretin and exifone, dipolar compounds that decrease the effective negative charge of membranes, prevent association of Abeta1-40 and Abeta25-35 to negatively charged lipid vesicles and Abeta induced cell toxicity. These results suggest that Abeta toxicity is mediated through a nonspecific physicochemical interaction with cell membranes.

The effect of hypotonicity, glutamine, and glycine on red cell preservation

BACKGROUND

Red cells (RBCs) stored in hypo-osmolar additive solutions with the same concentrations of adenine, dextrose, mannitol, and sodium chloride and varied amounts of ammonium, phosphate, glycerol, and glutamine were better preserved than RBCs in the standard additive solution (Adsol). Cell swelling occurred in all the experimental additives. This observation prompted the evaluation of glutamine and glycine alone, as well as a combination of glutamine and glycine, all of which have been described as producing swelling of rat liver cells.

STUDY DESIGN AND METHODS

Aliquots of RBCs were stored at 4 degrees C in Adsol or experimental additive solutions (EASs) all containing adenine, 2 mM; dextrose, 110 mM; mannitol, 55 mM; and sodium chloride, 50 mM. EAS 42 had, in addition, glutamine, 10 mM; glycine 5 mM, and phosphate, 20 mM. EAS 43 had glutamine, 10 mM; glycine, 10 mM; and phosphate 20 mM. EAS 44 had glutamine, 10 mM; EAS 45 had glutamine, 10 mM, and phosphate, 20 mM, and EAS 46 had only glycine, 10 mM. At intervals, measurements were made of mean corpuscular volume, mean corpuscular hemoglobin concentration, morphology, ATP, hemolysis, supernatant potassium, ammonia, pH, and microvesicles shed.

RESULTS

The initial mean corpuscular volumes were larger in all EASs than in Adsol, but the greatest difference was between EASs 44 and 46 (108 fL) and Adsol (86 fL) (p < 0.001). The morphology scores were significantly better in all the EASs (p < 0.04). The ATPs were significantly greater in all the EASs (p < 0.001), and highest in those with phosphate. potassium leakage and hemolysis were less in the EASs (p < 0.001). The ammonia levels higher in all the EASs than in Adsol, with the exception of EAS 46. During storage, the extracorpuscular and intracorpuscular pH levels were essentially identical. The shedding of microvesicles was greatly reduced in all the EASs.

CONCLUSION

Cell swelling induced in RBCs after collection appears to improve preservation. Ammonia and phosphate enhance RBC ATP maintenance. Glycine decrease the formation of ammonia by RBCs stored in a hypotonic medium.

The effects of tert-butyl hydroperoxide on human erythrocyte membrane ion transport and the protective actions of antioxidants

The oxidising actions of tert-butyl hydroperoxide (tBH) (0-3 mmol/l) on human erythrocyte membrane ion transport have been studied using measurements of 86Rb+ influx. Ouabain and bumetanide were used to distinguish Rb+ flux via the sodium pump (Na,K-ATPase), Na,K,2Cl cotransporter and through residual membrane permeability. The protective actions of antioxidants and related molecules (vitamin E, vitamin E acetate, Trolox, butylated hydroxytoluene (BHT) and dithioerythritol (DTE) were studied. The effects of tBH were concentration dependent and the mean residual (ouabain and bumetanide insensitive) Rb+ permeability was increased by a factor of 8.5 (S.E.M. 2.2, n = 15) by a 5-min exposure to 2 mmol/l tBH. This action was almost completely prevented by co-incubation with Trolox or BHT, and partially prevented by the presence of vitamin E or DTE. Incubation with 2 mmol/l tBH for 5 min increased intracellular Na+ by a factor of 1.8 (S.E.M. 0.1, n = 8) and reduced intracellular K+ by a factor of 0.93 (S.E.M. 0.03, n = 8). These effects were prevented by Trolox and partially prevented by vitamin E, whereas DTE and vitamin E acetate were ineffective. Incubation with 2 mmol/l tBH for 5 min reduced the mean apparent sodium pump Vmax by 43% (S.E.M. 4, n = 8). This effect was completely prevented by Trolox and partially prevented by vitamin E. Vitamin E acetate had no effect. The mean bumetanide-sensitive Rb+ influx via the Na,K,2Cl cotransporter was reduced by 30% (S.E.M. 8.7, n = 25) by a 5-min exposure to 2 mmol/l tBH. This action was variable and no significant actions of the antioxidants studied could be demonstrated. This study suggests that tBH-mediated oxidative damage occurs from a hydrophilic site and involves increased non-selective membrane cation permeability and inhibition of specific transport systems.

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.

Characterization of phloretin-sensitive urea export from the perfused rat liver

In single pass perfused rat liver, rapid osmotic water shifts across the plasma membrane in response to hyperosmolar urea were followed by monitoring liver mass and transient concentrating or diluting effects on Na+ concentration in effluent perfusate. Sudden addition or removal of hyperosmolar urea (200mM, resulting in a step change of the perfusate osmolarity from 305 to 505 mosmol/l) had little effect on liver mass or Na+ activity in the effluent perfusate, suggesting that urea equilibrated at a rate similar to that of water across the liver plasma membrane. When, however, phloretin (0.2mM) was present, sudden addition (removal) of urea (200mM) induced within seconds a marked and transient decrease (increase) of both liver mass and effluent Na+ concentration, suggestive of transient osmotic water shifts out of/into the cells. Although to a lesser extent, comparable effects were induced when urea was added/removed in the presence of the phloretin-related phenol compounds 2,4,6-trihydroxyacetophenone (5mM) and 2,4,5-trihydroxybutyrophenone (5mM). Phloretin-induced inhibition of urea export from livers preloaded with [14C]urea was reversible, and no saturation of urea transport was found at concentrations up to 200mM. In contrast to [14C]urea transport, [3H]water transport across the plasma membrane was not affected by phloretin. The data indicate that urea export across the hepatocyte plasma membrane is almost as fast as water export. The urea transport mechanism is sensitive to phloretin and other phenol compounds, works with high capacity and is distinct from the water-transporting system. The regulation of this putative transport mechanism and its relevance for hepatic nitrogen metabolism remain to be established.

Studies in red blood cell preservation 10. 51Cr recovery of red cells after liquid storage in a glycerol-containing additive solution

The purpose of the present study was to compare the 24-hour recovery of red blood cells stored for 9 weeks in a hypoosmolar additive solution containing 150 mM glycerol to cells stored in Adsol. Seven units of packed red cells were split into 2 aliquots. To one sample, 100 ml of the experimental additive solution (EAS 25) was added, and to the other, 50 ml of Adsol. At the end of the storage period the cells were labeled with 51Cr. A double chromium technique was used to make it possible to perform comparative autologous studies in the same donor. The 24-hour 51Cr recovery value for EAS 25 was 73.0 +/- (SD) 4.2% and for Adsol 60.9 +/- 7.1. At 9 weeks the adenosine triphosphate levels were not significantly better compared to Adsol but the other in vitro measurements were better. New approaches to the study of red cell preservation are suggested.

Morphology and volume alterations of human erythrocytes caused by the anion transporter inhibitors, DIDS and p-azidobenzylphlorizin

p-Azidobenzylphlorizin (p-AzBPhz) is a potential photoaffinity labeling agent for the anion and glucose transporters in human RBCs. In the absence of light and at the same low concentrations which block these transport processes (only 1-2 million molecules bound/cell), this impermeable membrane probe produces rapid morphological and volume alterations. This high-affinity activity, called phase 1, can be rapidly and completely reversed by simply diluting the azide-treated cell suspension. Phase 2 effects, including formation of cells with multiple, long spicules (stage 3/4 echinocytes), followed by an influx of salt and water with eventual lysis, occur at two log units higher concentration by a different mechanism, probably by intercalating into and selectively expanding the outer lipid monolayer. Light scattering, electronic cell sizing, microhematocrit measurements and scanning electron microscopy have been employed to compare the effects of the azide and the anion transport inhibitor, DIDS (4,4'-diisothiocyano-2,2'-stilbene disulfonate), on red cells. DIDS produced only those changes analogous to the azide's low dose phase 1 action; cells swell, lose the ability to scatter 800 nm light and undergo a limited shape change (comparable to stage 1 echinocytosis). The mechanism by which the two ligands perturb the membrane is additive, suggesting that a Band 3-mediated transmembrane signaling is involved which leads to altered cytoskeleton dynamics.

Cell electropermeabilization: a new tool for biochemical and pharmacological studies

Cell electropermeabilization is the transient permeabilization of the plasma membrane by means of short and intense electric pulses. Under optimized conditions, electropermeabilization is compatible with cell survival. It provides a direct access into the cytosol to ions, small molecules, exogenous drugs and macromolecules. As cells remain functional, a large variety of cell biology questions can be addressed. Such 'in situ biochemistry' opens new possibilities beside the more classical studies dealing with unpermeabilized cells or subcellular extracts. Electropermeabilization also allows pharmacological studies with cells, cultured monolayers and in vivo tissues as well as the design of drug controlled-release systems.

The membrane defect in hereditary stomatocytosis

Hereditary stomatocytosis and allied conditions represent a series of diseases in which abnormal movements of univalent cations across the plasma membrane play an important part in cellular disease. The primary problem lies not in the active transporters but in the basal permeability of the membrane, which is always increased, and the extent of the increase correlates with the cellular dysfunction. A number of structural abnormalities have been described in these membranes, but the most consistent and convincing is the deficiency of a hitherto uncharacterized integral membrane protein of molecular weight 31 kDa in the severe, 'overhydrated' form of the disease. The true function of this protein remains enigmatic, but its deficiency in this condition indicates that it may have a role in the regulation of cation transport.

Divalent cations, phospholipid asymmetry and osmotic swelling in electrically-induced lysis, cell fusion and giant cell formation with human erythrocytes

We have previously reported that acidic phospholipids are exposed at the surface of human erythrocytes when the cells are subjected to electrical breakdown. It has now been shown that the prothrombinase assay, which was used previously for the determination of acidic phospholipids, is specific for phosphatidylserine under the conditions of our experiments. In the light of this finding, we have investigated and characterised factors that govern cell lysis, cell fusion, and the formation of giant cells induced by electrical breakdown with human erythrocytes in media of low ionic strength. Divalent cations (1.1 mM) protected the cells against haemolysis, in the order Mn2+ > Ca2+ > Ba2+ > Mg2+ >> Zn2+, whereas about 99% of the cells lysed immediately on breakdown in the presence of Na+ or K+ (2.1 mM), or Al3+ (0.95 mM). The lengths of pearl chains of fused erythrocytes formed was similarly greatest with Mn2+ and decreased progressively with Ba2+, Zn2+, Ca2+ and Mg2+. No cell fusion occurred with Na+, K+, or Al3+. It is suggested that interactions with phosphatidylserine, which is exposed at the cell surface by electrical breakdown, may enable Mn2+, Ba2+ and Ca2+ ions to inhibit cell lysis (via membrane resealing) and facilitate cell fusion. Following electrically-induced cell fusion, erythrocytes round-up into giant cells. It has previously been proposed that Ca2+ ions accelerate the rounding-up process. However, data are presented which show that, as with erythrocytes treated with Sendai virus, the formation of rounded, giant cells following cell fusion depends on the osmotic swelling properties of permeabilised erythrocytes. Osmotic swelling may also have induced any hemi-fused cells present to fuse completely. Zn2+ ions anomalously enabled erythrocytes to round-up very rapidly into giant cells following electrical breakdown. This phenomenon may result from an interaction of Zn2+ ions with cysteine groups in membrane proteins, which decreases the immediate loss of ions that occurs when erythrocytes are subjected to electrical breakdown in low-ionic-strength media.

Characterization of the inhibition by stilbene disulphonates and phloretin of lactate and pyruvate transport into rat and guinea-pig cardiac myocytes suggests the presence of two kinetically distinct carriers in heart cells

1. The kinetics of transport of pyruvate (Km 0.20 mM), L-lactate (Km 2.2 mM) and D-lactate (Ki 10.2 mM) into rat cardiac myocytes were studied and compared with those for guinea-pig heart cells [Poole, Halestrap, Price and Levi (1989) Biochem. J. 264, 409-418] whose equivalent values were 0.07, 2.3 and 6.6 mM respectively. Maximal rates of transport were about 5-fold higher in the rat heart cells. 2. 4,4'-Dibenzamidostilbene-2,2'-disulphonate (DBDS), a powerful inhibitor of monocarboxylate transport into erythrocytes [Poole & Halestrap (1991) Biochem. J. 275, 307-312], was found to be a potent but apparently partial inhibitor of lactate and pyruvate transport, with an apparent Ki value at 0.5 mM L-lactate of about 16 microM in both species. Maximal inhibition was 50% and 80% in rat and guinea-pig cells respectively. 3. The maximal extent of inhibition and apparent Ki values were dependent on both the substrate transported and its concentration. Maximum inhibition was less and the Ki was greater at higher substrate concentrations. 4. A variety of other stilbene disulphonates were studied which showed different Ki values and maximal extents of inhibition. 5. Phloretin was a significantly less potent inhibitor of transport into both rat (Ki 25 microM) and guinea-pig (Ki 16 microM) heart cells than into rat erythrocytes (Ki 1.4 microM). In the rat but not the guinea-pig heart cells, inhibition appeared partial (maximal inhibition 84%). 6. We demonstrate that our results can be explained by the presence of two monocarboxylate carriers in heart cells, both with Km values for L-lactate of about 2 mM and inhibited by alpha-cyano-4-hydroxycinnamate, but with different affinities for other substrates and inhibitors. One carrier is sensitive to inhibition by stilbene disulphonates and has lower Km values for pyruvate (0.05-0.10 mM) and D-lactate (5 mM), whereas the other has higher Km values for pyruvate (0.30 mM) and D-lactate (25 mM), and is relatively insensitive to stilbene disulphonates. Rat heart cells possess more of the latter carrier and guinea-pig heart cells more of the former. 7. The significance of these results for the study of lactate transport in the perfused heart is discussed.

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.

Involvement of cytoskeletal proteins in the barrier function of the human erythrocyte membrane. II. Formation of membrane leaks in ghost membranes after limited proteolysis of skeletal proteins by trypsin

Limited proteolysis of human erythrocyte ghost membranes by low levels of trypsin (10-240 ng/ml) added bilaterally at 0 degrees C together with the proteinase inhibitor, phenylmethylsulfonyl fluoride (PMSF) before resealing at 37 degrees C leads to a graded digestion of spectrin and ankyrin and the disappearance of band 4.1 protein, while band 3 is cleaved only to a very low extent. These alterations are accompanied by an increase of membrane permeability of the resealed ghosts to hydrophilic nonelectrolytes (erythritol to sucrose), taken to reflect impaired resealing. Moreover, the membrane begins to vesiculate. Shedding of vesicles during the efflux measurements can not be responsible for the increased release of test solutes, since the ghosts do not loose hemoglobin and discriminate the nonelectrolytes according to their size. Moreover, the vesiculation site itself does not seem to act as the leak site, since ghosts prepared from erythrocytes pretreated with a carbodiimide which induces membrane rigidification still exhibit a pronounced protein degradation and vesiculation while the permeability enhancement induced by trypsination is markedly suppressed. The trypsin-induced leak has the properties of an aqueous pore as indicated, besides size selectivity, by its inhibition by phloretin and the very low activation energy. In analogy with concepts developed in the preceding paper (Klonk, S. and Deuticke, B. (1992) Biochim. Biophys. Acta 1106, 126-136 (Part I in this series)) the impaired resealing after limited proteolysis is assumed to be related to a perturbation of interactions of membrane skeletal elements with themselves and/or with the bilayer domain constituting the permeability barrier.

Involvement of cytoskeletal proteins in the barrier function of the human erythrocyte membrane. I. Impairment of resealing and formation of aqueous pores in the ghost membrane after modification of SH groups

Resealed human erythrocyte ghosts prepared by a two-step procedure were shown to have small residual barrier defects with the properties of aqueous pores, such as size discrimination of hydrophilic nonelectrolytes (erythritol to sucrose), indicative of an apparent pore radius of about 0.7 nm, and a low activation energy (about 12-20 kJ/mol (mannitol, sucrose)) of the leak fluxes. As in other cases (Deuticke et al. (1991) Biochim. Biophys. Acta 1067, 111-122) these leak fluxes can be inhibited by phloretin. Treatment of such resealed ghosts with the mild SH oxidizing agent, diamide, induces additional membrane leaks to the same extent and with the same properties as in native erythrocytes (Deuticke et al. (1983) Biochim. Biophys. Acta 731, 196-210), including reversibility of the leak by SH reducing agents, inhibition by phloretin and stimulation by alkanols. In contrast, resealed ghosts prepared either from diamide-treated erythrocytes or by adding diamide to the 'open' membranes prior to reconstitution of high ionic strength and raising the temperature, exhibit a state of greater leakiness. This leakiness is somewhat different in its origin from the former class of leaks, since it can also be produced by N-ethylmaleimide, which is essentially ineffective when added to the membrane in its 'tight' state. The leaks induced in the 'open' state of the membrane, which can be regarded as a consequence of an impaired resealing, are nevertheless reversible by reducing agents added after resealing and are comparable in many, but not all their characteristics to leaks induced in the 'tight' state of the membrane. Resealing in the presence of the isothiocyanostilbenes DIDS or SITS mimicks the leak forming effect of diamide by modifying a small population of SH groups, while amino groups seem not to be involved. The findings indicate and substantiate an important role of the redox state of membrane skeletal protein sulfhydryls in the maintenance and the re-establishment of the barrier function of the erythrocyte membrane.