Properties and structural basis of simple diffusion pathways in the erythrocyte membrane

Insights on the Mechanisms of the Protective Action of Naringenin, Naringin and Naringin Dihydrochalcone on Blood Cells in Terms of Their Potential Anti-Atherosclerotic Activity

Atherosclerosis is caused by injury to the blood arteries and progressive oxidative stress. Blood cells play an important role in its development; thus, their protection is important. Naringenin (N) is documented to possess a protective action against atherosclerosis, and we hypothesize that its derivatives, naringin (Nr) and naringin dihydrochalcone (Nd), with slightly different structures, possess similar or better activity. Therefore, this research aimed to find the mechanism of protective action of N, Nr and Nd in relation to erythrocytes, peripheral blood mononuclear cells (PBMCs) and platelets in terms of their potential anti-atherosclerotic effect. Moreover, their physicochemical properties and the interaction of flavonoids with liposomes were studied. All flavonoids protected erythrocytes from AAPH- and H2O2-induced oxidation to varying degrees. None of them had a destructive effect on erythrocyte membrane, and they did not impact the metabolic activity of PBMC and platelets. Nr and Nd inhibited collagen-induced platelet aggregation better in tested concentrations than N. Studied compounds did not induce liposome aggregation, but N and Nd changed their dipole potential. Obtained results show that Nd possesses slightly better activity than N and may have a better potential health effect on blood cells, which is very important in the design of anti-atherosclerotic therapeutics.

Protective Effect of Polyphenolic Extracts from Hippophae rhamnoides L. and Reynoutria japonica Houtt. on Erythrocyte Membrane

Sea buckthorn and Japanese knotweed are known in many traditional medicine systems to be a great source of bioactive substances. This research aims to compare the bioactivity and protective effects of the phenolic extracts of leaves from sea buckthorn and roots and leaves from the Japanese knotweed on erythrocytes. The polyphenol composition of the extract was analyzed using UPLC-PDA-ESI-MS/MS. The extracts' toxicity and impact on the erythrocytes' osmotic fragility were measured spectrophotometrically. The antioxidant activity was determined based on the inhibition of oxidation of erythrocytes and their membrane induced by 2,2'-Azobis(2-methylpropionamidine) dihydrochloride (AAPH),measured spectrophotometrically and using fluorimetry. To find the possible mechanism of the extracts' action, extract-modified cells were observed under a microscope, and the potential localization of the extract's phytochemical composition was checked using fluorescent probes. The results showed that the used extracts are not toxic to erythrocytes, increase their osmotic resistance, and successfully protect them against free radicals. Extract components localize on the outer part of the membrane, where they can scavenge the free radicals from the environment. Altogether, the presented extracts can greatly protect living organisms against free radicals and can be used to support the treatment of diseases caused by excess free radicals.

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

Efflux transporter breast cancer resistance protein dominantly expresses on the membrane of red blood cells, hinders partitioning of its substrates into the cells, and alters drug-drug interaction profiles

1. Red blood cell (RBC) partitioning is important in determining pharmacokinetic and pharmacodynamic properties of a compound; however, active transport across RBC membranes is not well understood, particularly without transporter-related cell membrane proteomics data. 2. In this study, we quantified breast cancer resistance protein (BCRP/Bcrp) and MDR1/P-glycoprotein (P-gp) protein expression in RBCs from humans, monkeys, dogs, rats and mice using nanoLC/MS/MS, and evaluated their effect on RBC partitioning and plasma exposure of their substrates. BCRP-specific substrate Cpd-1 and MDR1-specific substrate Cpd-2 were characterized using Caco-2 Transwell® system and then administered to Bcrp or P-gp knockout mice. 3. The quantification revealed BCRP/Bcrp but not MDR1/P-gp to be highly expressed on RBC membranes. The knockout mouse study indicated BCRP/Bcrp pumps the substrate out of RBCs, lowering its partitioning and thus preventing binding to intracellular targets. This result was supported by a Cpd-1 and Bcrp inhibitor ML753286 drug-drug interaction (DDI) study in mice. Because of enhanced partitioning of Cpd-1 into RBCs after BCRP/Bcrp inhibition, Cpd-1 plasma concentration changed much less extent with genetic or chemical knockout of Bcrp albeit marked blood concentration increase, suggesting less DDI effect. 4. This finding is fundamentally meaningful to RBC partitioning, pharmacokinetics and DDI studies of BCRP-specific substrates.

Concentration dependence of the cell membrane permeability to cryoprotectant and water and implications for design of methods for post-thaw washing of human erythrocytes

For more than fifty years the human red blood cell (RBC) has been a widely studied model for transmembrane mass transport. Existing literature spans myriad experimental designs with varying results and physiologic interpretations. In this review, we examine the kinetics and mechanisms of membrane transport in the context of RBC cryopreservation. We include a discussion of the pathways for water and glycerol permeation through the cell membrane and the implications for mathematical modeling of the membrane transport process. In particular, we examine the concentration dependence of water and glycerol transport and provide equations for estimating permeability parameters as a function of concentration based on a synthesis of literature data. This concentration-dependent transport model may allow for design of improved methods for post-thaw removal of glycerol from cryopreserved blood. More broadly, the consideration of the concentration dependence of membrane permeability parameters may be important for other cell types as well, especially for design of methods for equilibration with the highly concentrated solutions used for vitrification.

Pharmacokinetic profile and partitioning in red blood cells of romifidine after single intravenous administration in the horse

The aims of this study were to assess the plasma concentrations of romifidine in horses after intravenous injection, to evaluate the red blood cell (RBC) partitioning of the anaesthetic drug, and to improve knowledge regarding its sedative effect in horses describing the pharmacokinetic model. Eight adult Standardbred horses received a single bolus of romifidine at a dosage of 100 μg/kg. Blood samples (5 mL) were collected immediately before romifidine administration (t0), and at 2, 5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 90, 105, 120, 150 and 180 min after injection. A sedation score was recorded at the same time. The romifidine concentrations in plasma and red blood cells were determined by high performance liquid chromatography (HPLC). The plasma and red blood cell concentrations were correlated with the sedation at each time point. Romifidine produced a satisfactory level of sedation in all animals. The sedation was detectable in all horses for up to 105 min. All the animals returned to normal without any behavioural changes at 180 min. The romifidine concentrations in the red blood cells were significantly higher (P < 0.01) at all time points than those in the plasma. The T1/2β was 148.67 ± 61.59 min and body clearance was 22.55 ± 6.67 mL/kg per min. The results showed that after a single bolus administration of romifidine, a partitioning in the RBCs was detected.

Biological Activity of Japanese Quince Extract and Its Interactions with Lipids, Erythrocyte Membrane, and Human Albumin

The aim of the study was to determine in vitro biological activity of fruit ethanol extract from Chaenomeles speciosa (Sweet) Nakai (Japanese quince, JQ) and its important constituents (−)-epicatechin (EC) and chlorogenic acid (CA). The study also investigated the structural changes in phosphatidylcholine (PC) liposomes, dipalmitoylphosphatidylcholine liposomes, and erythrocyte membranes (RBC) induced by the extract. It was found that the extract effectively inhibits oxidation of RBC, induced by 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH), and PC liposomes, induced by UVB radiation and AAPH. Furthermore, JQ extract to a significant degree inhibited the activity of the enzymes COX-1 and COX-2, involved in inflammatory reactions. The extract has more than 2 times greater activity in relation to COX-2 than COX-1 (selectivity ratio 0.48). JQ extract stimulated growth of the beneficial intestinal bacteria Lactobacillus casei and Lactobacillus plantarum. In the fluorimetric method by means of the probes Laurdan, DPH and TMA-DPH, and ¹H-NMR, we examined the structural changes induced by JQ and its EC and CA components. The results show that JQ and its components induce a considerable increase of the packing order of the polar heads of lipids with a slight decrease in mobility of the acyl chains. Lipid membrane rigidification could hinder the diffusion of free radicals, resulting in inhibition of oxidative damage induced by physicochemical agents. JQ extract has the ability to quench the intrinsic fluorescence of human serum albumin through static quenching. This report thus could be of huge significance in the food industry, pharmacology, and clinical medicine.

Synergistic and additive effects of chromosomal and plasmid-encoded hemolysins contribute to hemolysis and virulence in Photobacterium damselae subsp. damselae

Photobacterium damselae subsp. damselae causes infections and fatal disease in marine animals and in humans. Highly hemolytic strains produce damselysin (Dly) and plasmid-encoded HlyA (HlyA(pl)). These hemolysins are encoded by plasmid pPHDD1 and contribute to hemolysis and virulence for fish and mice. In this study, we report that all the hemolytic strains produce a hitherto uncharacterized chromosome-encoded HlyA (HlyAch). Hemolysis was completely abolished in a single hlyAch mutant of a plasmidless strain and in a dly hlyApl hlyAch triple mutant. We found that Dly, HlyA(pl), and HlyAch are needed for full hemolytic values in strains harboring pPHDD1, and these values are the result of the additive effects between HlyApl and HlyAch, on the one hand, and of the synergistic effect of Dly with HlyApl and HlyAch, on the other hand. Interestingly, Dly-producing strains produced synergistic effects with strains lacking Dly production but secreting HlyA, constituting a case of the CAMP (Christie, Atkins, and Munch-Petersen) reaction. Environmental factors such as iron starvation and salt concentration were found to regulate the expression of the three hemolysins. We found that the contributions, in terms of the individual and combined effects, of the three hemolysins to hemolysis and virulence varied depending on the animal species tested. While Dly and HlyApl were found to be main contributors in the virulence for mice, we observed that the contribution of hemolysins to virulence for fish was mainly based on the synergistic effects between Dly and either of the two HlyA hemolysins rather than on their individual effects.

Allometric dependence of the life span of mammal erythrocytes on thermal stability and sphingomyelin content of plasma membranes

Thermal stability of erythrocyte membrane is a measure for its ability to maintain permeability barrier at deleterious conditions. Hence, it could impact the resistance of erythrocytes against detrimental factors in circulation. In this study the thermostability of erythrocyte membranes was expressed by the temperature, T(go), at which the transmembrane gradient of ion concentration rapidly dissipated during transient heating. T(go) is the inducing temperature of the membrane transition that activated passive ion permeability at hyperthermia causing thermal hemolysis. A good allometric correlation of T(go) to the resistance against thermal hemolysis and the life span of erythrocytes were found for 13 mammals; sheep, cow, goat, dog, horse, man, rabbit, pig, cat, hamster, guinea pig, rat, and mouse. For the same group, the values of T(go) were strictly related to the sphingomyelin content of erythrocyte membranes. The residual ion permeability, P, was temperature activated from 38 to 57 degrees C with activation energy of 250+/-15 kJ/mol that strongly differed from that below 37 degrees C. The projected value of P at 37 degrees C was about half that of residual physiological permeability for Na+ and K+ that build ground for possible explanation of the life span vs membrane thermostability allometric correlation.

Kinetics of organic transformations under mild aqueous conditions: implications for the origin of life and its metabolism

The rates of thermal transformation of organic molecules containing carbon, hydrogen, and oxygen were systematically examined in order to identify the kinetic constraints that governed origin-of-life organic chemistry under mild aqueous conditions. Arrhenius plots of the kinetic data were used to estimate the reaction of half-lifes at 50 degrees C. This survey showed that hydrocarbons and organic substances containing a single oxygenated group were kinetically the most stable; whereas organic substances containing two oxygenated groups in which one group was an alpha- or beta-positioned carbonyl group were the most reactive. Compounds with an alpha- or beta-positioned carbonyl group (aldehyde or ketone) had rates of reaction that were up to 10(24)-times faster than rates of similar molecules lacking the carbonyl group. This survey of organic reactivity, together with estimates of the molecular containment properties of lipid vesicles and liquid spherules, indicates that an origins process in a small domain that used C,H,O-intermediates had to be catalytic and use the most reactive organic molecules to prevent escape of its reaction intermediates.

Lipid polarity in brain capillary endothelial cells

Brain capillary endothelial cells (BCEC) represent an epithelial like cell type with continuous tight junctions and polar distributed proteins. In this paper we investigated whether cultured BCEC show a polar distribution of membrane lipids as this was demonstrated for many epithelial cell types. Therefore we applied a high yield membrane fractionation method to isolate pure fractions of the apical and the basolateral plasma membrane (PM) domains. Using a set of methods for lipid analysis we were able to determine the total lipid composition of the whole cells and the PM fractions. Both membrane domains showed a unique lipid composition with clear differences to each other and to the whole cell composition. Three lipid species were polar distributed between the two PM domains. Phosphatidylcholine was enriched in the apical membrane whereas sphingomyelin and glucosylceramide were enriched in the basolateral membrane. The possible function of this lipid polarity for the blood-brain barrier mechanism is the generation of a suitable lipid environment for polar distributed membrane proteins and the generation of two PM domains with different biophysical properties and permeabilities.

Nitric oxide and peroxynitrite anion modulate liver plasma membrane fluidity and Na(+)/K(+)-ATPase activity

Free radicals attack membranes and frequently alter their fluidity and function. The aim of the present work was to study the effect of nitric oxide (NO) radical and peroxynitrite anion on basolateral liver plasma membrane fluidity and on the activity of Na(+)/K(+)-ATPase. Basolateral membranes (BM) were isolated by ultracentrifugation in sucrose gradients and characterized enzymatically. BM were incubated with SNAP (a NO donor) or SIN-1 (a peroxynitrite donor). The release of NO or peroxynitrite was monitored by measuring NO(-)(2) + NO(-)(3). Relative fluidity was measured by polarization of fluorescence. NO increased membrane fluidity while peroxynitrite decreased it in a concentration-dependent manner. Na(+)/K(+)-ATPase activity was reduced by NO or peroxynitrite. Peroxynitrite anion inhibits ATPase activity in part by decreasing fluidity. However, it is very likely that both compounds inhibit ATPase activity by oxidation of the thiol groups of the enzyme. Our results suggest that NO may exert part of its biological effects by modulating membrane fluidity and function.

Low pH-induced hemolysis of erythrocytes is related to the entry of the acid into cytosole and oxidative stress on cellular membranes

HCl-induced lysis of mammalian erythrocytes, pretreated with DIDS, which is a specific inhibitor of the anion transport in their membranes, was markedly delayed. After acidification of a suspension of DIDS-inhibited cells, hemolysis was initiated by addition of a protonophore (Na-salicylate) at any moment chosen by will. These findings revealed that low-pH hemolysis depended on the rate of the transfer of acid equivalents into cytosole. Erythrocyte acid resistance was studied in a group of mammals and found to be inversely related to the rate of monovalent anion exchange in membranes which supported the above observations. In human erythrocytes, the critical level of cytosole acidification was found to be about pH 5.7 by measuring the acid equivalent absorbed by cells prior to hemolysis. HCl-induced hemolysis was also studied in human erythrocyte ghosts resealed with one-sixth of the initial hemoglobin content of cells. During the prelytic interval the ghosts suspended in isotonic NaCl/sucrose media shrunk, indicating an increase in ion permeability. The increase in prelytic permeability and hemolysis were strongly delayed in ghosts prepared from DIDS-treated cells, suggesting a uniform mechanism of lysing in cells and their ghosts. The prelytic increase in ion permeability was measured by the corresponding rate of ghost shrinkage and was found to be pH-dependent, with a high value below pH 3.4 and a very low one above pH 4.0. Compared to cells, the prelytic barrier impairment in ghosts had more mild character although it required greater concentration of cytosolic H+. While finally complete, hemolysis of cells was strongly delayed in the presence of catalase (500-1500 U/ml) and superoxide dismutase (200-600 U/ml) in hemolytic media. In conclusion, the acid-induced hemolysis could be associated with an oxidative injury of membranes, mainly triggered by the entry of acid equivalents into the cytosole.

Differential resistance to calcium-induced bilirubin-dependent hemolysis in mammalian erythrocytes

Washed erythrocytes from human, buffalo, sheep and goat preincubated with different concentrations of calcium chloride (16.7-1830 microM) showed significantly different rates of hemolysis (up to 62%) after addition of bilirubin (72 microM). Goat erythrocytes displayed marked resistance to hemolysis with only 11% hemolysis observed at the highest calcium concentration. Similar trend in hemolysis was also observed when the concentration of CaCl2 was fixed (330 microM) and bilirubin concentration varied (0-72 microM). (Ca(2+)-Mg(2+)-ATPase levels were found significantly lower in goat and sheep erythrocyte membranes compared to human and buffalo erythrocyte membranes. This was correlated well with the observed hemolysis in various mammalian erythrocytes.

Biophysical aspects of P-glycoprotein-mediated multidrug resistance

In the 45 years since Burchenal's observation of chemotherapeutic drug resistance in tumor cells, many investigators have studied the molecular basis of tumor drug resistance and the phenomenon of tumor multidrug resistance (tumor MDR). Examples of MDR in microorganisms have also become topics of intensive study (e.g., Plasmodium falciparum MDR and various types of bacterial MDR) and these emerging fields have, in some cases, borrowed language, techniques, and theories from the tumor MDR field. Serendipitously, the cloning of MDR genes overexpressed in MDR tumor cells has led to elucidation of a large family of membrane proteins [the ATP-binding cassette (ABC) proteins], an important subset of which confer drug resistance in many different cells and microorganisms. In trying to decipher how ABC proteins confer various forms of drug resistance, studies on the structure and function of both murine and human MDR1 protein (also called P-glycoprotein or P-gp) have often led the way. Although various theories of P-gp function have become popular, there is still no precise molecular-level description for how P-gp overexpression lowers intracellular accumulation of chemotherapeutic drugs. In recent years, controversy has developed over whether the protein protects cells by translocating drugs directly (as some type of drug pump) or indirectly (through modulating biophysical parameters of the cell). In this ongoing debate over P-gp function, detailed consideration of biophysical issues is critical but has often been neglected in considering cell biological and pharmacological issues. In particular, P-gp overexpression also changes plasma membrane electrical potential (delta psi zero) and intracellular pH (pHi), and these changes will greatly affect the cellular flux of a large number of compounds to which P-gp overexpression confers resistance. In this chapter, we highlight these biophysical issues and describe how delta psi zero and pHi may in fact be responsible for many MDR-related phenomena that have often been hypothesized to be due to direct drug translocation (e.g., drug pumping) by P-gp.

Uptake of azithromycin by various cells and its intracellular activity under in vivo conditions

The concentrations of azithromycin in polymorphonuclear leukocytes (PMNLs), monocytes, erythrocytes, and plasma were measured in six healthy volunteers after the last treatment of a 3-day regimen of 500 mg once daily. Marked enrichment of azithromycin was found in PMNLs and monocytes. The drug concentrations after the last dose amounted to 114 +/- 43 (mean +/- standard deviation) mg/liter at 12 h in PMNLs and 34 +/- 17 mg/liter at 6 h in monocytes. Fourteen days thereafter, azithromycin was still detectable in the PMNLs at 53 +/- 34 mg/liter and in the monocytes at 1 +/- 2 mg/liter, although the drug was no longer detectable in plasma (< 0.02 mg/liter). Maximum drug concentrations for azithromycin in plasma (0.40 +/- 0.30 mg/liter) and erythrocytes (0.15 +/- 0.05 mg/liter) at 3 h after the last administration were much lower and occurred earlier than those observed in the phagocytic cells. The mean enrichment factors (cellular/extracellular ratios) of azithromycin in phagocytes relative to plasma came to 231 +/- 150 and 3,924 +/- 584 at 3 and 120 h, respectively, for PMNLs and 83 +/- 55 and 523 +/- 285 at 3 and 120 h for monocytes, respectively, after the last dose. The phagocytosis tests with PMNLs separated from the blood of volunteers at various times after the last treatment confirmed the enhanced intracellular activity of these cells against staphylococci.

Characterization of the stilbenedisulfonate binding site on band 3

Stilbenedisulfonates are potent inhibitors of Band 3 mediated anion exchange. They bind tightly to the protein and form a 1-to-1 reversible complex. Those stilbenedisulfonates which contain isothocyanato groups such as DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonate) and H2DIDS (4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate) can also react rapidly with lysine residues within the binding pocket to yield an irreversible covalent adduct. The reactive lysine residue is known as lysine-A, and is thought to have an unusually low pKa. In this report, we characterize the kinetics of DIDS adduct formation with respect to the effect of substrate anions, competitive inhibitory anions, and pH on the rate of covalent adduct formation. We investigate the following: (a) whether stilbenedisulfonates bind to or block access of substrate anions to the transport site; (b) whether the rapidity of the covalent reaction of DIDS at neutral pH is due to a low pKa for lysine-A within the binding pocket; and (c) whether once bound, DIDS and H2DIDS isothiocyanato groups are accessible to reagents. For this latter experiment, we have utilized a newly discovered reaction of the DIDS isothiocyanato groups with azide to test for accessibility. Our results show that substrate anions, DIDS, and Band 3 form a ternary complex. Significantly, the binding of large substrate anions, such as iodide, is not weakened by DIDS to any greater extent than is the binding of smaller substrates such as chloride or fluoride. These results are not consistent with a "partial blockade" hypothesis for the relationship between the stilbenedisulfonate and transport sites. Rather, they support an allosteric site-site interaction hypothesis. Our pH dependence results show that the apparent pKa for the DIDS/lysine-A reaction is greater than 9.26. This is consistent with typical lysine pKa values, and indicates that lysine-A does not have an unusually low pKa. Finally, we show that azide can react with the isothiocyanato groups of DIDS and H2DIDS within their Band 3 complexes, indicating that the stilbenedisulfonate binding site is accessible to solute. These results support a view which suggests that the stilbenedisulfonate site is a superficial inhibitory site on Band 3 which inhibits transport by allosteric interactions within the protein, rather than by either direct or partial blockade of the transport site.

Interaction of two sulfhydryl reagents with a cation recognition site on the neuronal dopamine carrier evidences small differences between [3H]GBR 12783 and [3H]cocaine binding sites

We have compared the effect of treating rat striatal cell membranes with ionic hydrophilic sulfhydryl reagents on the specific bindings of [3H]cocaine and of [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1-3H]propenyl)-piperaz ine) to the neuronal transporter of dopamine. Treatment with 1 mmol/l 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) resulted in similar time- and concentration-dependent reductions of the specific binding of both radioligands. None of the uptake blockers tested afforded any protection against 1 mmol/l DTNB. Addition of (sub)millimolar concentrations of CaCl2 or MgCl2, or 250 mmol/l KCl to a treatment medium containing 10 mmol/l Na+ significantly increased the DTNB-induced reduction of the specific binding of both radioligands. Cations were likely to be responsible for this effect since ions in combination with DTNB induced similar reductions in binding when either 1 mmol/l CaCl2 or 50-250 mmol/NaCl were added. Effects of cations on the DTNB-induced inhibition of binding were generally more marked on [3H]GBR 12783 than on [3H]cocaine binding. When added to a medium containing 10 mmol/l Na+ 1 mmol/l DTNB induced a reduction in the Bmax of the specific binding of both radioligands. Addition of 1 mmol/l Ca2+ maintained or increased this Bmax reduction and elicited a decrease in affinity which was significant for [3H]GBR 12783 binding. Treatment of membranes with the sodium salt of p-hydroxymercurybenzenesulfonate (pHMBS) induced time- and concentration-dependent decreases in [3H]GBR 12783 binding which were significantly greater than decreases in [3H]cocaine binding. However, 50 mumol/l pHMBS produced a similar decrease in the Bmax of the specific binding of both radioligands. The pHMBS-induced reduction of [3H]GBR 12783 binding was not reversed by drugs whose action is purely that of uptake inhibition or by substrates of the dopamine carrier. Some of these drugs (100 mumol/l dopamine, 1 mumol/l mazindol or 100 mumol/l cocaine) protected the specific binding of [3H]cocaine against the effects of pHMBS, whereas 1 mmol/l p-tyramine, 10 mumol/l nomifensine and 10 nmol/l GBR 12783 were ineffective. Addition of 120 mmol/l Na+, 1 mmol/l Ca2+ or 10 mmol/l Mg2+ to a treatment medium containing 10 mmol/l Na+ significantly reduced the effects of pHMBS on the specific binding of both radioligands. When striatal cell membranes were treated in a medium containing 130 mmol/l Na+, there was a general decrease in the effects of ions on the reductions of specific binding produced by DTNB or pHMBS.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the chloride carrier in the plasmalemma of the alga Valonia utricularis: the inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid

The effect of the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) on the Cl(-)-transport system located in the plasmalemma of cells of the giant marine alga Valonia utricularis was studied by using the charge pulse relaxation technique. Analysis of the biphasic relaxation patterns in terms of the kinetic model published previously (Wang, J., Wehner, G., Benz, R. and Zimmermann, U. (1991) Biophys. J. 59, 235-248) demonstrated that extracellular DIDS dramatically reduced the translocation rate, KAS, of the Cl(-)-carrier complex (maximal inhibition 79%). The translocation rate of the free carrier molecules, KS, as well as the total surface concentration of the carrier, No, were not affected. A Hill-plot of DIDS inhibition on KAS yielded an half-maximal inhibition concentration (IC50) of 3.9 x 10(-5) M and Hill-coefficient of 1.61, suggesting a co-operative binding of the inhibitors to the Cl(-)-carrier. The maximal inhibition of DIDS was dependent on the extracellular Cl(-)-concentration. This inhibition was not competitive to chloride, since it increased and did not decrease with increasing chloride concentration. The DIDS effect decreased with increasing pH-value (investigated pH range between 6.5 and 10). Intravascular DIDS or SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid) had no effect on the biphasic voltage relaxation pattern. These results showed that the binding sites of DIDS must be located on the outer surface of the plasmalemma of V. utricularis and, in turn, supported previous conclusions that the Cl(-)-carrier (which is assumed to be part of the turgor-pressure-sensing mechanism) is only located in the outer membrane.

Reduction of erythrocyte membrane permeability and protein binding of low-molecular-weight drugs following glycoside derivitization

The permeability of glycosides (salicin, arbutin, glycyrritin, p-nitrophenyl-beta-D-glucopyranoside, p-nitrophenyl-beta-D-galactopyranoside, p-nitrophenyl-beta-D-lactopyranoside, and p-nitrophenyl-beta-D-maltopyranoside) and their aglycons through human erythrocyte membrane was investigated. The transport rate of the glycosides through human erythrocyte membrane was slower than that of their aglycons. Glycosides with a disaccharide did not permeate the erythrocyte membrane; this observation suggests that the introduction of disaccharide to drugs gives rise to a significant decrease in the leakage of drugs through the erythrocyte membrane. The derivatives of glycosides encapsulated in erythrocytes were not released from these erythrocytes into the outer medium. The transport capacity of the glycosides was not influenced by the kind of suspending medium, but that of the aglycons was influenced by the medium. The glycosides bound to human serum albumin more weakly than their aglycons. Particularly, the glycosides were more difficult to displace from 7-anilinocoumarin-4-acetic acid (site III drug) than their aglycons, except for glycyrritin.

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.

Substrate and inhibitor specificity of the lactate carrier of human neutrophils

The substrate and inhibitor specificity of the lactic acid (Lac) transport system of human neutrophils was investigated. The ability of a variety of compounds to inhibit the influx of [14C]lactate, presumably reflecting competition by substrate analogues for binding at the external translocation site, was taken as an index of affinity for the Lac carrier. pH-state techniques were utilized to assess transportability. Results indicate a relatively low order of selectivity, the neutrophil H+(+)lactate- cotransport system demonstrating a broad acceptance of short-chain unsubstituted and substituted alkyl monocarboxylates as well as aromatic monocarboxylates. There was a slight preference for oxo, Cl, and OH substituents over other groups at the two-position of short chain alkyl fatty acids: all were readily transported across the plasma membrane at rates approaching that of L-lactate itself. Aromatic acids were not transported inward by the carrier although these compounds did permeate via simple nonionic diffusion. The neutrophil Lac carrier can be blocked by a number of cyanocinnamate derivatives, the classical inhibitors of monocarboxylate transport in mitochondria, and by dithiol compounds and sulfhydryl-reactive agents. This constellation of biochemical properties is similar to the features that characterize other well described H+(+)lactate- cotransport systems in red blood cells, Ehrlich ascites tumor cells, hepatocytes, and cardiac sarcolemmal vesicles, although significant differences exist when comparisons are made to the Na(+)-dependent lactate transporter of the kidney proximal tubule.

Effects of brefeldin A on sphingomyelin transport and lipid synthesis in BHK21 cells

1. Addition of brefeldin A (BFA) to BHK cells incubated for 4 h with [3H]acetate led to a 3-4-fold increase in incorporation of label into sphingomyelin, monoglucosylceramide and cholesterol ester compared with untreated controls. There was a similar increase in incorporation of [3H]choline into sphingomyelin. The level of cholesterol ester increased 3-fold when BFA was added to cells labelled to equilibrium with [3H]acetate, but no statistically significant changes in the levels of other lipids were seen. 2. BFA appeared to act by diverting incorporation of acetate into sphingolipids and cholesterol ester at the expense of phosphatidylcholine (decreased by up to 15%), cholesterol (decreased by 30-40%) and triacylglycerol (decreased by 35-50%). 3. Forskolin (100 microM) prevented the changes in labelling induced by 0.25 micrograms of BFA/ml, but in the presence of 1 micrograms of BFA/ml it had no effect on sphingomyelin and triacylglycerol labelling and only partly blocked the effects of BFA on labelling of cholesterol and cholesterol ester. 4. None of the labelled sphingomyelin was degraded in BFA-treated cells which were subsequently exposed to an extracellular sphingomyelinase, showing that all the newly synthesized sphingomyelin remained inside the cells. Determinations of phospholipid phosphorus in unlabelled cells confirmed that, in the presence of BFA, no newly synthesized sphingomyelin was able to reach the cell surface, supporting the idea that sphingomyelin normally depends on vesicular transport for its passage to the plasma membrane. 5. The results are consistent with the hypothesis that cholesterol synthesis and esterification processes in BHK cells are sensitive to the plasma-membrane deficit of sphingomyelin caused by BFA.

Alcohols produce reversible and irreversible acceleration of phospholipid flip-flop in the human erythrocyte membrane

The slow, non-mediated transmembrane movement of the lipid probes lysophosphatidylcholine, NBD-phosphatidylcholine and NBD-phosphatidylserine in human erythrocytes becomes highly enhanced in the presence of 1-alkanols (C2-C8) and 1,2-alkane diols (C4-C8). Above a threshold concentration characteristic for each alcohol, flip rates increase exponentially with the alcohol concentration. The equieffective concentrations of the alcohols decrease about 3-fold per methylene added. All 1-alkanols studied are equieffective at comparable calculated membrane concentrations. This is also observed or the 1,2-alkane diols, albeit at a 5-fold lower membrane concentration. At low alcohol concentrations, flip enhancement is reversible to a major extent upon removal of the alcohol. In contrast, a residual irreversible flip acceleration is observed following removal of the alcohol after a treatment at higher concentrations. The threshold concentrations to produce irreversible flip acceleration by 1-alkanols and 1,2-alkane diols are 1.5- and 3-fold higher than those for flip acceleration in the presence of the corresponding alcohols. A causal role in reversible flip-acceleration of a global increase of membrane fluidity or membrane polarity seems to be unlikely. Alcohols may act by increasing the probability of formation of transient structural defects in the hydrophobic barrier that already occur in the native membrane. Membrane defects responsible for irreversible flip-acceleration may result from alterations of membrane skeletal proteins by alcohols.

Transport of drugs through human erythrocyte membranes: pH dependence of drug transport through labeled human erythrocytes in the presence of band 3 protein inhibitor

To reveal the role of the band 3 anion transport protein of the erythrocyte membrane in drug transport through the membrane, the possible effects of inhibitors of anion transport on the permeability of some anionic drugs were examined. The amounts of these drugs that permeated varied markedly with the pH of the outer medium around human erythrocytes that contained the band 3 protein inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS). In the pH range 7.0-8.0, the membrane permeability of drugs through DIDS-treated erythrocytes was affected by a slight pH change (0.2), whereas that through the intact erythrocytes was not pH dependent. These results suggest that the band 3 protein acts not only as a channel for the transport of anions or some anionic drugs but also as protection for the transport system from changes in the pH of the outer medium.

Involvement of cytoskeletal proteins in the barrier function of the human erythrocyte membrane. III. Permeability of spectrin-depleted inside-out membrane vesicles to hydrophilic nonelectrolytes. Formation of leaks by chemical or enzymatic modification of membrane proteins

Spectrin-depleted inside-out vesicles (IOV's) prepared from human erythrocyte membranes were characterized in terms of size, ground permeability to hydrophilic nonelectrolytes and their sensitivity to modification by SH reagents, DIDS and trypsin. IOV's proved to have the same permeability of their lipid domain to erythritol as native erythrocytes, in contrast to resealed ghosts (Klonk, S. and Deuticke, B. (1992) Biochim. Biophys. Acta 1106, 126-136 (Part I in this series)), which have a residual leak. On the other hand, IOV's have a slightly elevated permeability for mannitol and sucrose, nonelectrolytes which are almost (mannitol) or fully (sucrose) impermeant in the native membrane. These increased fluxes, which have a high activation energy and can be stimulated by phloretin, are, however, also much smaller than the corresponding leak fluxes observed in resealed ghosts. In view of these differences, formation of IOV's can be concluded to go along with partial annealing of barrier defects persisting in the erythrocyte membrane after preparation of resealed ghosts. Oxidation of SH groups of the IOV membrane by diamide produces an enhancement of permeability for hydrophilic nonelectrolytes which is much less pronounced than that induced by a similar treatment of erythrocytes or ghosts (Klonk, S. and Deuticke, B. (1992) Biochim. Biophys. Acta 1106, 126-136 (Part I in this series)). Moreover, proteolytic treatment of the vesicle membrane, although leading to a marked digestion of integral membrane proteins, only induces a minor, saturating increase of permeability, much lower than that in trypsinized resealed ghosts (Klonk, S. and Deuticke, B. (1992) Biochim. Biophys. Acta 1106, 137-142 (Part II of this series)). Since absence of the cytoskeletal proteins, spectrin and actin, is the major difference between IOV's and resealed ghosts, these results may be taken as further evidence for a dependence of the barrier properties of the erythrocyte membrane bilayer domain on its interaction with cytoskeletal elements. In contrast, these barrier properties seem to be rather insensitive to perturbations of integral proteins.

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.

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.

Palmitate binding to and efflux kinetics from human erythrocyte ghost

At 0 degrees C, pH 7.3, palmitate (PA) binds to human erythrocyte ghosts suspended in 0.2% bovine serum albumin (BSA) solution with molar ratios of PA to BSA, v, between 0.2 and 1.3. The binding depends on the water phase PA concentration, measured in equilibrium experiments, using BSA-filled ghosts as semipermeable bags. The saturable binding has a capacity of 19.4 +/- 7.5 nmol g-1 packed ghosts (7.2 x 10(9) cells) and Kd = 13.5 +/- 5 nM. PA exchange efflux kinetics to 0.2% BSA is recorded from ghosts without and with 0.2% BSA with a resolution time of about 1 s. Data are analyzed in terms of compartmental models. Using BSA-free ghosts the kinetics is essentially monoexponential. The rate constant is 0.0287 +/- 0.0022 s-1. Using ghosts with BSA, the kinetics is biexponential with widely different rate constants. Extrapolated zero-time values reflect, according to additional investigations, 'instantaneous' release of PA from the outer surface of the ghosts. Analyses of the biexponential curve up to about 55% tracer efflux assign unequivocally values to three model parameters. (1) k1, the dissociation rate constant of the PA-BSA complex is (1.47 +/- 0.03) x 10(-3) s-1 and (2.56 +/- 0.08) x 10(-3) s-1 and (4.08 +/- 0.13) x 10(-3) s-1 at v = 0.2, 0.6 and 1.4, respectively. (2) k3*, the overall rate constant of PA transport from the inside of the ghost membrane to the medium is 0.0269 +/- 0.0020 s-1 independent of v. (3) Qkin, the ratio of PA on the inside of the membrane to PA on BSA within the ghosts is v dependent and smaller than a corresponding ratio Qeq measured in equilibrium by a value corresponding to PA on the outer surface. This fraction is released with a rate constant, k5, which is of the order of 1 s-1. The data suggest a maximum PA transport capacity, Jmax, of 2 pmol min-1 cm-2, 0 degrees C, pH 7.3.

Okadaic acid inhibition of KCl cotransport. Evidence that protein dephosphorylation is necessary for activation of transport by either cell swelling or N-ethylmaleimide

The mechanism of activation of KCl cotransport has been examined in rabbit red blood cells. Previous work has provided evidence that a net dephosphorylation is required for activation of transport by cell swelling. In the present study okadaic acid, an inhibitor of protein phosphatases, was used to test this idea in more detail. We find that okadaic acid strongly inhibits swelling-stimulated KCl cotransport. The IC50 for okadaic acid is approximately 40 nM, consistent with the involvement of type 1 protein phosphatase in transport activation. N-Ethylmaleimide (NEM) is well known to activate KCl cotransport in cells of normal volume. Okadaic acid, added before NEM, inhibits the activation of transport by NEM, indicating that a dephosphorylation is necessary for the NEM effect. Okadaic acid added after NEM inhibits transport only very slightly. After a brief exposure to NEM and rapid removal of unreacted NEM, KCl cotransport activates with a time delay that is similar to that for swelling activation. Okadaic acid causes a slight increase in the delay time. These findings are all consistent with the idea that NEM activates transport not by a direct action on the transport protein but by altering a phosphorylation-dephosphorylation cycle. The simplest hypothesis that is consistent with the data is that both cell swelling and NEM cause inhibition of a protein kinase. Kinase inhibition causes net dephosphorylation of some key substrate (not necessarily the transport protein); dephosphorylation of this substrate, probably by type 1 protein phosphatase, causes transport activation.

Asymmetry of membrane fluidity in the lipid bilayer of blood platelets: fluorescence study with diphenylhexatriene and analogs

Membrane fluidity of bovine platelets was examined with diphenylhexatriene (DPH), its cationic trimethylammonium derivative (TMA-DPH) and anionic propionic acid derivative (DPH-PA). After addition of these probes to platelet suspensions at 37 degrees C, the fluorescence intensity of DPH-PA reached equilibrium within 2 min, whereas those of DPH and TMA-DPH increased gradually. With increase in the fluorescence intensity of TMA-DPH, its fluorescence anisotropy decreased significantly, but the fluorescence anisotropies of DPH-PA and DPH did not change during incubation. The gradual increase of fluorescence intensity of TMA-DPH was due to its penetration into the cytoplasmic side of the platelet membrane, as shown quantitatively by monitoring decrease in its extractability with albumin. Transbilayer movement of TMA-DPH was markedly temperature-dependent, and was scarcely observed at 15 degrees C. The fluorescence intensity of TMA-DPH was much higher in platelet membranes and vesicles of extracted membrane lipids than the initial intensity in intact platelets. Moreover, the fluorescence anisotropy of TMA-DPH was much lower in the former preparations than the initial value in intact platelets. These results suggest that binding sites for TMA-DPH in the cytoplasmic side of the platelet membrane are more fluid than those in the outer leaflet of the plasma membrane. Platelet activation by ionomycin induced specific change in the fluorescence properties of TMA-DPH without causing transbilayer incorporation of the probe.

Factors contributing to the distribution of free fatty acids among phospholipid vesicles

The distribution of free fatty acids at equilibrium after incubation of small sonicated unilamellar vesicles (SUV) with large unilamellar vesicles (LUV) of different lipid composition has been determined. Stearic acid (SA) and oleic acid (OA) showed similar preferences for SUV and LUV of egg yolk phosphatidylcholine (EYPC). Both ionized and protonated forms of the free fatty acids (FFAs) behaved similarly with respect to the equilibrium distribution between EYPC of different size. The charge of the vesicles was found, however, to be important, since both FFAs in their ionized form preferentially associated to vesicles of phosphatidylcholine (PC) as compared with vesicles of phosphatidylglycerol (PC). While SA preferred membranes in the gel state, OA showed preference for the membrane in fluid state. The insertion of both OA and SA in phosphatidylethanolamine (PE)/phosphatidylcholine vesicles is less favourable than in vesicles of pure PC. All these data suggest that membrane lipid content may play a role in determining the distribution of free fatty acids among the membranes of a cell.

The role of membrane composition in ATPase activities of cirrhotic rat liver: effect of silymarin

The activities of Ca2(+)- and Na+, K(+)-ATPases were studied in liver plasma membranes from CCl4-cirrhotic rats and from livers of rats treated with silymarin in addition to CCl4. CCl4 chronic treatment produced significant decreases in Na+, K(+)- and Ca2(+)-ATPase activities; however, the animals treated with silymarin along with CCl4 showed no differences in ATPase activities as compared to controls. The lipid analysis performed in plasma membranes revealed increases in the cholesterol/phospholipid (CH/PL) and sphingomyelin/phosphatidylcholine (SM/PC) ratios in the cirrhotic group. Again, the membranes isolated from rats receiving CCl4 + silymarin showed normal CH/PL and SM/PC values. Considering that CH/PL and SM/PC ratios are related to membrane microviscosity, this study suggests that a lower fluidity of the membrane may be responsible for the observed decreases in ATPase activities in the cirrhotic group. Additionally, the role of silymarin to improve liver function in CCl4-cirrhosis can be attributed partially to its action at membrane level by preventing the increases in CH/PL and SM/PC ratios.

Binding of a radiolabeled sea anemone cytolysin to erythrocyte membranes

Stichodactyla helianthus cytolysin III, a 17 kDa basic polypeptide isolated from a Caribbean sea anemone, is one of the most potent hemolysins yet found in a living organism. This toxin has been reported to form new ion channels in artificial lipid bilayer membranes. The ability of this toxin to attack cell membranes is greatly enhanced by the presence of sphingomyelin. In order to investigate the mechanism by which the cytolysin causes cell lysis, we have prepared a highly active [3H]cytolysin derivative by reductive methylation with sodium cyanoborohydride and [3H]formaldehyde. A dimethylated toxin derivative was used to investigate the basis for the differential lytic activity of this polypeptide upon erythrocytes from six mammalian species. Using both direct [3H]toxin binding and indirect (Thron method) binding techniques, we found that the interspecies differences are due to variable membrane susceptibilities toward the bound toxin, rather than to differences in membrane affinity for the toxin. Similarly, we showed the enhanced lytic activity of the toxin for rat erythrocytes at elevated pH to be caused by enhanced activity of the bound toxin.

Nonmediated flip-flop of phospholipid analogues in the erythrocyte membrane as probed by palmitoylcarnitine: basic properties and influence of membrane modification

The rules governing the transbilayer reorientation (flip-flop) of long-chain amphiphilic components in biological membranes were further elucidated by studying the flip-flop of palmitoylcarnitine in human erythrocytes. Flip rates were derived from the time-dependent decrease of extractability of palmitoylcarnitine by albumin after primary insertion of trace amounts of the labeled probe into the outer membrane layer. The flip rate (half time 2.6 hr at 37 degrees C in human erythrocytes) is fast enough to be measurable also in membranes exhibiting low flip rates such as that of ox erythrocytes. Flip rate constants for the inward and outward reorientation are similar and the probe equilibrates at a 1:1 ratio between the two layers. The flip is a simple, diffusion-like process. It is not inhibited but even enhanced by chemical modification of membrane proteins. It is also enhanced by insertion of channel-forming antibiotics into the membrane and by pre-exposure of the cells to temperatures exceeding 42 degrees C. The extent of this enhancement increases with the duration and the temperature of the pre-exposure. Since spectrin is denatured in this range of temperatures, the finding constitutes a new piece of evidence that the membrane skeleton is involved in the maintenance of bilayer stability and that a decrease of bilayer stability goes along with the formation of local defects acting as flip sites for phospholipids and related compounds. As a particularity, the flip is enhanced by lowering the pH and exhibits interindividual variability, phenomena not observed for the flip-flop of lysophosphatidylcholine. This suggests that generalizations on the kinetics of nonmediated flip-flop of membrane-intercalated amphiphiles may not be justified.

Characteristics of sorbitol uptake in rat glial primary cultures

Uptake of [U-14C]sorbitol was studied in astrogliarich rat primary cultures. Initial rate of sorbitol uptake is proportional to sorbitol concentration between 20 microM and 400 mM. Sorbitol transport is not inhibited by glucose, fructose, and a variety of structurally related polyols, or by cytochalasin B, an inhibitor of glucose transport. Phloretin, phlorizin, filipin, and n-hexanol, all compounds that alter the properties of biological membranes, and the sulfhydryl reagent p-chloromercuribenzoate inhibit sorbitol uptake to various degrees. Variation in the concentrations of extracellular Na+ and K+ does not affect transfer of sorbitol across the cell membrane. It is concluded that sorbitol is taken up into glial cells by a diffusion process, not involving a carrier and probably not through the lipid bilayer, but through a proteinaceous channel-like structure.

Erythrocyte bisulfite transport

The wide range of transport rates for anions of differing chemical structure by the human erythrocyte anion transport protein (Band 3 protein) suggests that this protein is highly selective for anions that chemically resemble its natural substrate bicarbonate. To test this hypothesis, the influx of bisulfite (HSO3-), a bicarbonate analog, was compared to influxes of chloride, sulfate, and bicarbonate, as measured by the technique of colloid osmotic lysis in isotonic ammonium salt solution. The lysis time induced in chloride solution (much greater than 10 min) was markedly accelerated to 0.6 min by the addition of small amounts (5 mM) of bicarbonate, an effect characteristic of colloid osmotic lysis induced by the anion transport pathway. Lysis in bicarbonate solution was extremely rapid (0.09 min), and was markedly inhibited by acetazolamide (2.9 min). Lysis in bisulfite solution occurred spontaneously (2.2 min) but was markedly accelerated to a time similar to that of chloride (0.56 min) by addition of 5 mM bicarbonate. In contrast, sulfate induced lysis was extremely slow (less than 10% lysis at 40 min in the presence of bicarbonate). Preincubation of erythrocytes with SITS, an inhibitor of anion exchange, prevented lysis by chloride, but had no effect on lysis by bicarbonate, indicating that lysis by bicarbonate was predominantly through diffusion and not anion transport. SITS treatment of erythrocytes eliminated the catalytic effect of bicarbonate during lysis by bisulfite, indicating that anion transport of bisulfite and diffusion of the conjugate acid in the form of SO2 both contribute to the total membrane flux. When the contribution of diffusion is taken into account, the rate of bisulfite influx through the anion exchange pathway is at least 100-fold faster than that for sulfate.

Functional carboxyl groups in the red cell anion exchange protein. Modification with an impermeant carbodiimide

Anion exchange in human red blood cell membranes was inactivated using the impermeant carbodiimide 1-ethyl-3-(4-azonia-4,4-dimethylpentyl)-carbodiimide (EAC). The inactivation time course was biphasic: at 30 mM EAC, approximately 50% of the exchange capacity was inactivated within approximately 15 min; this was followed by a phase in which irreversible exchange inactivation was approximately 100-fold slower. The rate and extent of inactivation was enhanced in the presence of the nucleophile tyrosine ethyl ester (TEE), suggesting that the inactivation is the result of carboxyl group modification. Inactivation (to a maximum of 10% residual exchange activity) was also enhanced by the reversible inhibitor of anion exchange 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS) at concentrations that were 10(3)-10(4) times higher than those necessary for inhibition of anion exchange. The extracellular binding site for stilbenedisulfonates is essentially intact after carbodiimide modification: the irreversible inhibitor of anion exchange 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) eliminated (most of) the residual exchange activity: DNDS inhibited the residual (DIDS-sensitive) Cl- at concentrations similar to those that inhibit Cl- exchange of unmodified membranes: and Cl- efflux is activated by extracellular Cl-, with half-maximal activation at approximately 3 mM Cl-, which is similar to the value for unmodified membranes. But the residual anion exchange function after maximum inactivation is insensitive to changes of extra- and intracellular pH between pH 5 and 7. The titratable group with a pKa of approximately 5.4, which must be deprotonated for normal function of the native anion exchanger, thus appears to be lost after EAC modification.

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.

The mechanisms of inhibition of anion exchange in human erythrocytes by 1-ethyl-3-[3-(trimethylammonio)propyl]carbodiimide

Treatment of human erythrocytes with the membrane-impermeant carbodiimide 1-ethyl-3-[3-(trimethylammonio)propyl]carbodiimide (ETC) in citrate-buffered sucrose leads to irreversible inhibition of phosphate-chloride exchange. The level of transport inhibition produced was dependent on the concentration of citrate present during treatment, with a maximum of approx. 60% inhibition. [14C]Citric acid was incorporated into Band 3 (Mr = 95,000) in proportion to the level of transport inhibition, reaching a maximum stoichiometry of 0.7 mol citrate per mol Band 3. The citrate label was localized to a 17 kDa transmembrane fragment of the Band 3 polypeptide. Citrate incorporation was prevented by the transport inhibitors 4,4'-diisothiocyano- and 4,4'-dinitrostilbene-2,2'-disulfonate. ETC plus citrate treatment also dramatically reduced the covalent labeling of Band 3 by [3H]4,4'-diisothiocyano-2,2'-dihydrostilbene disulfonate (3H2DIDS). Noncovalent binding of stilbene disulfonates to modified Band 3 was retained, but with reduced affinity. We propose that the inhibition of anion exchange in this case is due to carbodiimide-activated citrate modification of a lysine residue in the stilbenedisulfonate binding site, forming a citrate-lysine adduct that has altered transport function. The evidence is consistent with the hypothesis that the modified residue may be Lys a, the lysine residue involved in the covalent reaction with H2DIDS. Treatment of erythrocytes with ETC in the absence of citrate resulted in inhibition of anion exchange that reversed upon prolonged incubation. This reversal was prevented by treatment in the presence of hydrophobic nucleophiles, including phenylalanine ethyl ester. Thus, inhibition of anion exchange by ETC in the absence of citrate appears to involve modification of a protein carboxyl residue(s) such that both the carbodiimide- and the nucleophile-adduct result in inhibition.

Transport of glucose, fructose and sucrose by Streptanthus tortuosus suspension cells : II. Uptake at high sugar concentration

In the concentration range above 1 mM a linear diffusion-like component of sugar uptake by Streptanthus suspension cells is observed. The rate of permeation is the same for sucrose, glucose, fructose and sorbitol, despite the very different uptake features of these sugars at low concentrations, where sorbitol and sucrose are not taken up at all and where different affinities for glucose and fructose are seen. The linear uptake component is responsible for 80% of sugar uptake at 100 mM, and it is an efficient permeation path for sucrose and fructose, which show poor permeation compared to glucose in the low concentration range. The mechanistic nature of the linear uptake component remains obscure: it is not directly dependent on metabolic energy (uncoupler does not inhibit it) and it is neither saturable up to 100 mM nor is it sugar-specific, but it is changeable, for instance, by plasmolysis or by protoplast generation. The permeation rates are very similar to those found in other plants for the linear component, but are much higher than in artificial membranes. These features are neither fully compatible with diffusion through a lipid phase nor with catalysed transport, and it is therefore suggested that this linear uptake proceeds through hydrophilic domains of the membrane. The linear uptake component will have consequences for apoplastic sugar concentration, sugar-accumulation factors and cell metabolism.