Activity coefficients of the peptide and the electrolyte in ternary systems water+glycylglycine+NaCl, +NaBr, +KCl and +KBr at 298.2 K

The activity coefficients of glycylglycine in four aqueous electrolyte solutions (+NaCl, +NaBr, +KCl and +KBr) were obtained at 298.2 K. The mean ionic activity coefficient of the electrolyte in aqueous solutions containing the peptide was determined from measurements of the potential differences of a cation and an anion ion-selective-electrode, each vs. a double junction reference electrode. The results show that the nature of the anion has a major effect on the activity coefficients of glycylglycine. Comparison of activity coefficient data for glycylglycine with literature data for glycine, both in aqueous NaCl solutions, indicates that the effect of the electrolyte is larger for the peptide than for the amino acid. For the peptide, in all cases, the effect of the electrolyte is more important at low molalities of the electrolyte. The Wilson equation was used to correlate the activity coefficient data obtained. The correlation results were satisfactory for the region of concentrated electrolyte.     

Effect of cation and anion of an electrolyte on apparent molar volume, isentropic compressibility and refractive index of glycine in aqueous solutions

Experiments at 298.15 K have been performed to measure the density, velocity of sound and refractive index in three water+glycine+electrolyte systems. The electrolytes studied were KCl, KNO3 and NaNO3. The values of apparent molar volume and isentropic compressibility of glycine in aqueous electrolyte solutions were calculated from the measured data. The results obtained in this study and those reported previously for water+glycine+NaCl system have been comparatively studied. The results show that the nature of both the cation and the anion of an electrolyte influence the behaviour of glycine in aqueous solutions. For all four electrolytes studied, the comparison shows a positive volume transfer for glycine from an electrolyte solution to a more concentrated solution of the same electrolyte. The results also show a negative apparent isentropic compressibility for glycine in the presence of the electrolytes studied. These effects indicate that the volume of a glycine molecule is larger in solutions with higher electrolyte concentration and the water molecules around the glycine molecules are less compressible than the water molecules in the bulk solution. These effects were attributed to the doubly charged behaviour of glycine and to the formation of physically bonded ion-pairs between the charged groups of glycine and the cation and the anion of the electrolyte.     

Activity coefficients of NaCl in trehalose-water and maltose-water mixtures at 298.15 K

The ionic mean activity coefficients of NaCl in trehalose-water and maltose-water mixtures have been experimentally determined at 298.15 K from emf measurements by electrochemical cell containing ion selective electrodes (ISE): Na-ISE/NaCl (m), sugar (Y%), H(2)O (100-Y%)/Cl-ISE. The electrolyte molality (m) ranged between 0.01 and 3 mol kg(-1), approximately, and the weight percent (Y%) of sugar in the mixture of solvents between 0, 10, 20, 30 and 40%. The system is considered as an electrolyte in a mixture of solvents and the data have been analysed by using the Debye-Hückel and Pitzer equations. The results obtained with both equations are in quite agreement with each other. The variation of the fit parameter from both equations were studied relative to the change in the dielectric properties of the mixture of solvents. Standard free energy of transference were comparatively discussed in terms of ion-solvent, ion-ion interactions and the hydration of both the electrolyte and the sugar.     

Effect of the cation and the anion of an electrolyte on the solubility of DL-aminobutyric acid in aqueous solutions: measurement and modelling

The solubilities at 298.2 K of dl-aminobutyric acid in aqueous solutions of NaCl, KCl, NaNO(3) and KNO(3) were measured. The solubility of DL-aminobutyric acid was found to be influenced by the concentration and by the nature of both the cation and the anion of the electrolyte. Comparison of the results obtained in this study and those for other amino acids reported in the literature, indicates that the structure of the hydrocarbon backbone of an amino acid plays an important role in the interactions of an amino acid with an electrolyte. A thermodynamic model has been used to correlate the solubilities of DL-aminobutyric acid in aqueous electrolyte solutions. The activity coefficients of the amino acid in the electrolyte solutions, were represented by a model proposed by Khoshkbarchi and Vera [M.K. Khoshkbarchi, J.H. Vera, AIChE J. 42 (1996) 2354; M.K. Khoshkbarchi, J.H. Vera, Ind. Eng. Chem. Res. 35 (1996) 4755]. This model, which considers a combination of both long- and short-range interactions, contains only two adjustable parameters. All other parameters are available in the literature. The model can accurately correlate the solubility of dl-aminobutyric acid in aqueous solutions of electrolytes.     

Activity coefficients for NaCl-monosaccharide (D-glucose, D-galactose, D-xylose, D-arabinose)-water systems at 298.15 K

Electrochemical cells with a sodium ion selectivity electrode (Na-ISE) versus a chloride ion selectivity electrode (Cl-ISE) as a reference electrode were used to determine the activity coefficients for NaCl-monosaccharide (D-glucose, D-galactose, D-xylose, and D-arabinose) systems in water at 298.15 K. A comparison of the results thus obtained was made with those determined by another electromotive force (emf) method. It is shown that agreement is excellent. The Gibbs free energy parameters of the interactions between these sugars and NaCl in water were evaluated together with the parameter C1(CHOH, exo), indicating the interaction of the exocyclic CHOH group of saccharide molecules and NaCl. The results suggested that the interactions of these monosaccharides with NaCl are controlled mostly by the dominant conformer of their molecules in water.     

Experimental data and modelling of apparent molar volumes, isentropic compressibilities and refractive indices in aqueous solutions of glycine + NaCl

Experiments have been performed at 298.15 K to measure the density, sound velocity and refractive index of glycine in aqueous solutions of NaCl over a wide range of both glycine and NaCl concentrations. The values of apparent molar volume and isentropic compressibility of glycine were calculated from the measured data. The results show a positive transfer volume of glycine from an NaCl solution to a more concentrated NaCl solution. This indicates that the size of a glycine molecule is larger in a solution with higher NaCl concentration. The negative values of apparent isentropic compressibility imply that the water molecules around the glycine molecules are less compressible than the water molecules in the bulk solution. These effects are attributed to the doubly charged behaviour of glycine and to the formation of physically bonded ion-pairs between the charged groups of glycine and sodium and chloride ions. The formation of ion-pairs, whose extents of binding reactions depend on the concentrations of both NaCl and glycine, alter the hydration number of glycine. This also explains the reason for the increase in the size of glycine with an increase in the NaCl concentration. A model based on the Pitzer formalism has been developed to correlate the activity coefficient, apparent molar volume and isentropic compressibility of glycine in aqueous solutions of NaCl. The results show that the model can accurately correlate the interactions in aqueous solutions of glycine and NaCl.     

THE INFLUENCE OF ELECTROLYTES ON THE ELECTRIFICATION AND THE RATE OF DIFFUSION OF WATER THROUGH COLLODION MEMBRANES

1. When pure water is separated by a collodion membrane from a watery solution of an electrolyte the rate of diffusion of water is influenced not only by the forces of gas pressure but also by electrical forces. 2. Water is in this case attracted by the solute as if the molecules of water were charged electrically, the sign of the charge of the water particles as well as the strength of the attractive force finding expression in the following two rules, (a) Solutions of neutral salts possessing a univalent or bivalent cation influence the rate of diffusion of water through a collodion membrane, as if the water particles were charged positively and were attracted by the anion and repelled by the cation of the electrolyte; the attractive and repulsive action increasing with the number of charges of the ion and diminishing inversely with a quantity which we will designate arbitrarily as the "radius" of the ion. The same rule applies to solutions of alkalies. (b) Solutions of neutral or acid salts possessing a trivalent or tetravalent cation influence the rate of diffusion of water through a collodion membrane as if the particles of water were charged negatively and were attracted by the cation and repelled by the anion of the electrolyte. Solutions of acids obey the same rule, the high electrostatic effect of the hydrogen ion being probably due to its small "ionic radius." 3. The correctness of the assumption made in these rules concerning the sign of the charge of the water particles is proved by experiments on electrical osmose. 4. A method is given by which the strength of the attractive electric force of electrolytes on the molecules of water can be roughly estimated and the results of these measurements are in agreement with the two rules. 5. The electric attraction of water caused by the electrolyte increases with an increase in the concentration of the electrolyte, but at low concentrations more rapidly than at high concentrations. A tentative explanation for this phenomenon is offered. 6. The rate of diffusion of an electrolyte from a solution to pure solvent through a collodion membrane seems to obey largely the kinetic theory inasmuch as the number of molecules of solute diffusing through the unit of area of the membrane in unit time is (as long as the concentration is not too low) approximately proportional to the concentration of the electrolyte and is the same for the same concentrations of LiCl, NaCl, MgCl₂, and CaCl₂.     

Synthesis and potent ionophoric activity of a squaramide-linked bis(choloyl) conjugate

A squaramide-linked bis(choloyl) conjugate was synthesized and fully characterized on the basis of NMR (¹H and ¹³C) and ESI MS (LR and HR) data. Fluorescence and chloride ion selective electrode assays indicate that this compound exhibits potent ionophoric activity across egg-yolk l-α-phosphatidylcholine-based liposomal membranes, presumably via an anion-modulating anion–cation co-transport/symport process. A Hill analysis reveals that three molecules of this compound are assembled into the transport-active species.     

Calculation of self-diffusion coefficients of the [BMIM][TFSA]/water system by molecular dynamics simulation

We performed a molecular dynamics simulation to calculate the self-diffusion coefficients of 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and water in a water–ionic liquid mixture. We then compared the simulated self-diffusion coefficients of cation, anion and water molecules with experimental data and with simulated data from the literature. Although the simulation overestimated the self-diffusion coefficients of ions, the simulated results qualitatively reproduced the enhancement of the self-diffusion coefficients of water as the water molar fraction increased. We also calculated the radial distribution functions to investigate the solution structure, i.e. the clustering of water molecules. The clustering of water in ionic liquid was found to play an important role in the enhancement of the diffusion of water molecules in the ionic liquid.     

Ion solvation and water structure in potassium halide aqueous solutions

The structure of water and the nature of ionic hydration is explored in aqueous solutions of potassium fluoride, chloride, bromide and iodide over a range of concentrations up to 4.8 ion pairs per 100 water molecules, using the combined techniques of neutron diffraction with hydrogen isotope substitution. The diffraction data are interpreted using the method of empirical potential structure refinement, which attempts to build a three-dimensional model of the scattering system consistent with the diffraction data. The water structure is strongly perturbed in the first hydration shells of both anion and cation, but is found to be only mildly perturbed outside of this region, with the largest effects occurring with the smallest anion and highest concentrations. For the potassium ion there are strong orientational correlations in the first hydration shell, with the water molecules lying with their dipole moments pointing almost directly away from the cation on average, but with an angular spread of approximately +/-60 degrees which is mildly dependent on the anion type present. For all the anions the water molecules in the first shell are strongly oriented with one O-H vector pointing directly towards the anion on average, with an angular spread of approximately +/-10 degrees for F(-), increasing to approximately +/-22 degrees for I(-). For both anions and cations the second hydration shell is much more disordered than the first, but there is a weak pattern of orientational correlation which becomes more pronounced with the larger anions. There is some evidence that the fluoride ion structures water significantly in its first hydration shell, but not beyond. The findings throw further light on recent findings that the orientational relaxation time for water outside the first shell of dissolved ions is the same as in the bulk liquid.     

Ion coordination in the amphotericin B channel.

The antifungal polyene antibiotic amphotericin B forms channels in lipid membranes that are permeable to ions, water, and nonelectrolytes. Anion, cation, and ion pair coordination in the water-filled pore of the "barrel" unit of the channels was studied by molecular dynamics simulations. Unlike the case of the gramicidin A channel, the water molecules do not create a single-file configuration in the pore, and some cross sections of the channel contain three or four water molecules. Both the anion and cation are strongly bound to ligand groups and water molecules located in the channel. The coordination number of the ions is about six. The chloride has two binding sites in the pore. The binding with water is dominant; more than four water molecules are localized in the anion coordination sphere. Three motifs of the ion coordination were monitored. The dominant motif occurs when the anion is bound to one ligand group. The ion is bound to two or three ligand groups in the less favorable configurations. The strong affinity of cations to the channel is determined by the negatively charged ligand oxygens, whose electrostatic field dominates over the field of the hydrogens. The ligand contribution to the coordination number of the sodium ion is noticeably higher than in the case of the anion. As in the case of the anion, there are three motifs of the cation coordination. The favorable one occurs when the cation is bound to two ligand oxygens. In the less favorable cases, the cation is bound to three or four oxygens. In the contact ion pair, the cation and anion are bound to two ligand oxygens and one ligand hydrogen, respectively. There exist intermediate solvent-shared states of the ion pair. The average distances between ions in these states are twice as large as that of the contact ion pair. The stability of the solvent-shared state is defined by the water molecule oriented along the electrostatic field of both ions.     

Effects of voltage perturbation of the lingual receptive field on chorda tympani responses to Na+ and K+ salts in the rat: implications for gustatory transduction

Taste sensory responses from the chorda tympani nerve of the rat were recorded with the lingual receptive field under current or voltage clamp. Consistent with previous results (Ye, Q., G. L. Heck, and J. A. DeSimone. 1993. Journal of Neurophysiology. 70:167-178), responses to NaCl were highly sensitive to lingual voltage clamp condition. This can be attributed to changes in the electrochemical driving force for Na+ ions through apical membrane transducer channels in taste cells. In contrast, responses to KCl over the concentration range 50-500 mM were insensitive to the voltage clamp condition of the receptive field. These results indicate the absence of K+ conductances comparable to those for Na+ in the apical membranes of taste cells. This was supported by the strong anion dependence of K salt responses. At zero current clamp, the potassium gluconate (KGlu) threshold was > 250 mM, and onset kinetics were slow (12 s to reach half-maximal response). Faster onset kinetics and larger responses to KGlu occurred at negative voltage clamp (-50 mV). This indicates that when K+ ion is transported as a current, and thereby uncoupled from gluconate mobility, its rate of delivery to the K+ taste transducer increases. Analysis of conductances shows that the paracellular pathway in the lingual epithelium is 28 times more permeable to KCl than to KGlu. Responses to KGlu under negative voltage clamp were not affected by agents that are K+ channel blockers in other systems. The results indicate that K salt taste transduction is under paracellular diffusion control, which limits chemoreception efficiency. We conclude that rat K salt taste occurs by means of a subtight junctional transducer for K+ ions with access limited by anion mobility. The data suggest that this transducer is not cation selective which also accounts for the voltage and amiloride insensitive part of the response to NaCl.     

The binding of divalent metal ions to polyelectrolytes in mixed counterion systems. I. The dye spectrophotometric method

The dye spectrophotometric method for the measurement of the activity of divalent metal ions in polyelectrolyte solutions containing added electrolytes is discussed.The method is applied to mixtures containing the dextransulfate polyanion, NaCl, and MgCl2 or Ca2. A two wavelength ratio method as applied to polyelectrolyte solutions is compared to the standard method which makes use of the previous determination of the dye-metal ion formation constant. The ratio method is found to be a convenient and reliable method which is not influenced by decomposition of the dye or by statistical errors in the extrapolation procedure. The activity coefficients as determined by the two wavelength dye spectrophotometric method are compared to results of Donnan exclusion measurements, and of EMF measurements using a calcium ion selective electrode. The results of the spectrophotometric method are equal to those of the two other methods within the limits of error in the latter. The spectrophotometric measurements can extend to much lower ion activaties than the other two methods, and can be done in the presence of a large excess of added electrolyte, yielding results of considerably improved precision when compared to Donnan and EMF methods.     

Energy profile of Cs+ in gramicidin A in the presence of water. Problem of the ion selectivity of the channel

The effect of water present at the mouth and inside the channel of Gramicidin A on the energy profile calculated for a caesium ion is determined. The total optimal interaction energy computed for the system GA-Cs+-(22 waters) leads to an energy profile characterized by a deep minimum at 11 A followed by an entrance energy barrier of 7 Kcal/mol expanding until 9 A from the center. After this point, a second minimum less deep than the previous one is observed, itself followed by a central barrier. The shape of the profile at the entrance is governed by the balance between the progressive desolvation process of the ion and the increase of favorable hydrogen bond interactions implying both the water molecules and GA. The comparison of this energy profile with that obtained in vacuo shows that the presence of water molecules does not modify the pathway of the ion which, owing to its size, is constrained essentially to remain on the channel axis. The comparison Na+ versus Cs+ indicates that although the phenomena involved are globally the same, differences between the two profiles appear due firstly to the difference in the affinity of the two ions for water and secondly to their respective size. This last difference implies that the number of water molecules present in the interior of the channel during the cation progression is reduced roughly by one in the case of caesium. The desolvation barrier computed for Cs+ is half the corresponding value for Na+, a result in agreement with the observed selectivity.     

Mean activity coefficients for the simple electrolyte in aqueous mixtures of polyelectrolyte and simple electrolyte. V. common counterion mixtures of alkali-metal dextransulfates with alkali metal chlorides

Mean molal activity coefficients of simple electrolyte in aqueous solutions of Li, Na, K or Cs salts of dextransulfate (DS) with added LjCl, NaCl, KCl or CsCl are reported. The measurements were carried out by means of an electrochemical cell method using a cation exchange membrane as cation selective electrode and Ag/AgCl electrodes. For LiDS-LiCl, NaDS-NaCl and CsDS-CsCl systems the polymer concentration, m_p, was varied from 0.0088 to 0.113 m and at a given m_p the ratio X of the polymer to salt concentration was varied from 0.5 to 16. Due to the insolubility of KDS in high concentration of KCl, the measurements on KDS-KCl system were performed in the m_p range of 0.0088–0.089 m and some of the smaller X values were omitted. The activity coefficient results are compared to Manning's limiting laws, the additivity rule, and to new limiting laws. The additivity rule can give an excellent representation of the data for all m_p values when γ_p is used as an adjustable parameter.     

Reconstitution of sarcoplasmic reticulum Ca2+-ATPase vesicles lacking ion channels and demonstration of electrogenicity of Ca2+-pump

Sarcoplasmic reticulum (SR) vesicles were reconstituted by the salting out method in the presence of excess phospholipids: the lipid-to-protein ratio ranged from 10 to 100. It was found that the reconstituted vesicles could be separated by KC1 density gradient centrifugation into four types: those having both cation and anion channels (CASR), those having only cation channels (CSR), those having only anion channels (ASR), and those having no ion channels (PSR). From the yield of these vesicles, it was estimated that one native SR vesicle contains 19 cation channels and 1.4 anion channels on average; the amount of cation channels is 14 times larger than that of anion channels. Although all vesicles thus prepared are considered to contain the Ca2+-ATPase protein, the PSR vesicles alone did not take up Ca2+, but they did do so in the presence of valinomycin. This result indicates that the Ca2+-ATPase takes up Ca2+ in an electrogenic manner. The electromotive force was estimated to be about 50 mV.     

Independence of apical membrane Na+ and Cl- entry in Necturus gallbladder epithelium

Transepithelial fluid transport (Jv) and intracellular Na+ and Cl- activities (aNai, aCli) were measured in isolated Necturus gallbladders to establish the contribution of different proposed apical membrane entry mechanisms to transepithelial salt transport. In 10 mM HCO3- Ringer's, Jv was 13.5 +/- 1.1 microliter X cm-2 X h-1, and was significantly reduced by a low bicarbonate medium and by addition of amiloride (10(-3)M) or SITS (0.5 X 10(-3)M) to the mucosal bathing solution. Bumetanide (10(-5)M) was ineffective. Bilateral Na+ removal abolished Jv. The hypothesis of NaCl cotransport was rejected on the basis of the following results, all obtained during mucosal bathing solution changes: during Na+ removal, aNai fell 4.3 times faster than aCli; during Cl- removal, aCli fell 7.5 times faster than aNai; amiloride (10(-3) M) reduced aNai at a rate of 2.4 +/- 0.3 mM/min, whereas aCli was not changed; bumetanide (10(-5) M) had no significant effects on Jv or aCli. The hypothesis of Na-K-Cl cotransport was rejected for the same reasons; in addition, K+ removal from the mucosal bathing solution (with concomitant Ba2+ addition) did not alter aNai or aCli. The average rate of NaCl entry under normal transporting conditions, estimated from Jv, assuming that the transported fluid is an isosmotic NaCl solution, was 22.5 nmol X cm-2 X min-1. Upon sudden cessation of NaCl entry, assuming no cell volume changes, aNai and aCli should fall at an average rate of 4.8 mM/min. To compare this rate with the rates of Na+ and Cl- entry by ion exchange, the Na+ or Cl- concentration in the mucosal bathing solution was reduced rapidly to levels such that electroneutral cation or anion exchange, respectively, should cease. The rate of Na+ or Cl- entry before this maneuver was estimated from the initial rate of fall of the respective intracellular ionic activity upon the mucosal solution substitution. aNai and aCli decreased at initial rates of 3.7 +/- 0.4 and 5.9 +/- 0.8 mM/min, respectively. The rate of fall of aNai upon reduction of external [Na] was not affected by amiloride (10(-3) M), and the rate of fall of aCli upon reduction of external [Cl] was unchanged by SITS (0.5 X 10(-3) M), which indicates that net cation or anion exchange was, in fact, abolished by the changes in Na+ and Cl- gradients, respectively. I conclude that double exchange (Na+/H+ and Cl-/HCO-3) is the predominant or sole mechanism of apical membrane NaCl entry in this epithelium.     

TNF-alpha down-regulates the Na+-K+ ATPase and the Na+-K+-2Cl-cotransporter in the rat colon via PGE2

TNF-alpha is believed to play a pivotal role in the pathogenesis of inflammatory bowel diseases which have diarrhea as one of their symptoms. This work studies the effect of the cytokine on electrolyte and water movements in the rat distal colon using an intestinal perfusion technique and attempts to determine its underlying mechanism of action. TNF-alpha inhibited net water and chloride absorption, down-regulated in both surface and crypt colonocytes the Na+-K+-2Cl- cotransporter, and reduced the protein expression and activity of the Na+-K+ ATPase. Indomethacin up-regulated the pump and the cotransporter in surface cells but not in crypt cells, and in its presence, TNF-alpha could not exert its effect, suggesting an involvement of PGE2 in the cytokine action. The effect of TNF-alpha on the pump and symporter was studied also in cultured Caco-2 cells in isolation of the effect of other cells and tissues, to test whether the cytokine acts directly on intestinal cells. In these cells, TNF-alpha and PGE2 had a similar effect on the pump expression and activity as that observed in crypt cells but were without any effect on the Na+-K+-2Cl- cotransporter. It was concluded that the effect of the cytokine on colonocytes is mediated via PGE2. By inhibiting the Na+-K+ ATPase, it reduces the Na+ gradient needed for NaCl absorption, and by down-regulating the expression of the Na+-K+-2Cl- symporter, it reduces basolateral Cl- entry and luminal Cl- secretion. The inhibitory effect on absorption is more significant than the inhibitory effect on secretion resulting in a decrease in net electrolyte uptake and consequently in more water retention in the lumen.     

Ion disturbance and clustering in the NaCl water solutions

Ion clustering and the solvation properties in the NaCl solutions are explored by molecular dynamics simulations with several popular force fields. The existence of ions has a negligible disturbance to the hydrogen bond structures and rotational mobility of water beyond the first ion solvation shells, which is suggested by the local hydrogen bond structures and the rotation times of water. The potential of mean force (PMF) of ion pair in the dilute solution presents a consistent view with the populations of ion clusters in the electrolyte solutions. The aggregation level of ions is sensitive to the force field used in the simulations. The ion-ion interaction potential plays an important role in the forming of the contact ion pair. The entropy of water increases as the ion pair approaches each other and the association of ion pair is driven by the increment of water entropy according to the results from the selected force fields. The kinetic transition from the single solvent separated state to the contact ion pair is controlled by the enthalpy loss of solution.

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

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