The kinetics of anion equilibrium exchange across the red blood cell membrane as measured by means of 35S thiocyanate

Anion binding characteristics of the band 3 / 4,4'-dibenzamidostilbene-2,2'-disulfonate binary complex: Evidence for both steric and allosteric interactions

A novel kinetic approach was used to measure monovalent anion binding to better define the mechanistic basis for competition between stilbenedisulfonates and transportable anions on band 3. An anion-induced acceleration in the release of 4,4prime-dibenzamidostilbene-2,2prime-disulfonate (DBDS) from its complex with band 3 was measured using monovalent anions of various size and relative affinity for the transport site. The K1/2 values for anion binding were determined and correlated with transport site affinity constants obtained from the literature and the dehydrated radius of each anion. The results show that anions with ionic radii of 120-200 pm fall on a well-defined correlation line where the ranking of the K1/2 values matched the ranking of the transport site affinity constants (thiocyanate < nitrate equivalent to bromide < chloride < fluoride). The K1/2 values for the anions on this line were about 4-fold larger than expected for anion binding to inhibitor-free band 3. Such a lowered affinity can be explained in terms of allosteric site-site interactions, since the K1/2 values decreased with increasing anionic size. In contrast, iodide, with an ionic radius of about 212 pm, had a 10-fold lower affinity than predicted by the correlation line established by the smaller monovalent anions. These results indicate that smaller monovalent anions have unobstructed access to the transport site within the band 3 / DBDS binary complex, while iodide experiences significant steric hindrance when binding. The observation of steric hindrance in iodide binding to the band 3 / DBDS binary complex, but not in the binding of smaller monovalent anions, suggests that the stilbenedisulfonate binding site is located at the outer surface of an access channel leading to the transport site.Key words: band 3, anion transport, membrane protein structure, red cell membrane.

CFTR: mechanism of anion conduction

CFTR: Mechanism of Anion Conduction. Physiol. Rev. 79, Suppl.: S47-S75, 1999. - The purpose of this review is to collect together the results of recent investigations of anion conductance by the cystic fibrosis transmembrane conductance regulator along with some of the basic background that is a prerequisite for developing some physical picture of the conduction process. The review begins with an introduction to the concepts of permeability and conductance and the Nernst-Planck and rate theory models that are used to interpret these parameters. Some of the physical forces that impinge on anion conductance are considered in the context of permeability selectivity and anion binding to proteins. Probes of the conduction process are considered, particularly permeant anions that bind tightly within the pore and block anion flow. Finally, structure-function studies are reviewed in the context of some predictions for the origin of pore properties.

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.

Cytotoxic properties of salivary oxidants

Salivary peroxidase and to a lesser extent myeloperoxidase are present in significant concentrations in saliva and catalyze the oxidation of thiocyanate anion (SCN-) by H2O2 to yield the potent oxidants hypothiocyanous acid (HOSCN) and its conjugate base hypothiocyanite anion (OSCN-). The objective of this study was to characterize the cytotoxic potential of peroxidase-generated HOSCN/OSCN- toward human erythrocytes. We found that HOSCN/OSCN- (0.25 mM) generated by the peroxidase-H2O2-SCN- system caused significant hemolysis at pH 6.0 but not at pH 6.5, 7.0, or 7.4. Erythrocyte hemoglobin (OxyHb) was oxidized to methemoglobin (MetHb) at all pH values tested; however, the rate of MetHb formation was dramatically increased at low pH and was not affected by inosine hexaphosphate, suggesting that hemoglobin was oxidized primarily by HOSCN. Concurrent with oxidation of hemoglobin (Hb), there was a pH-dependent consumption of HOSCN/OSCN- with more of the oxidant consumed at pH 6.0 compared with pH 6.5, 7.0, or 7.4. The enhanced oxidation of Hb at acidic pH was not due simply to increased membrane permeability by the uncharged species (HOSCN), since both erythrocyte lysate Hb and purified Hb were oxidized to the same extent at low pH as were intact erythrocytes. It is concluded that both OSCN- and HOSCN enter human erythrocytes where the protonated oxidant (HOSCN) mediates hemolysis and oxidizes OxyHb to MetHb, whereas both HOSCN and OSCN- oxidize glutathione (GSH). These data suggest that the extracellular pH may play an important role in modulating the cytotoxic properties of salivary oxidants.

Determination of membrane potential and cell volume by 19F NMR using trifluoroacetate and trifluoroacetamide probes

The distribution of ionic species between intra- and extracellular compartments forms one basis for the determination of cell membrane potential. It is shown that fluorine-19 NMR studies of erythrocytes in the presence of trifluoroacetate, a stable, relatively nontoxic anion with pK = -0.3, provide a sensitive probe of membrane potential. Since such measurements are based on ion concentrations, the parallel use of the neutral analogue trifluoroacetamide to provide information on intra/extracellular volume ratios was also explored. In both cases, separate 19F resonances corresponding to intra- and extracellular ions were observed, with the intracellular resonance shifted downfield by approximately 0.2 ppm and the intracellular peak typically somewhat broader than the extracellular resonance. Studies with the band 3 anion-exchange inhibitor 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) indicate that both transmembrane diffusion and flux involving the band 3 anion exchanger contribute to the observed transport of the trifluoroacetate anion. Intra/extracellular volume ratios determined on the basis of trifluoroacetamide intensity ratios were in good agreement with determinations based on measured hematocrits. On the basis of the high sensitivity of 19F NMR and the capability of monitoring volume changes simultaneously, the time resolution for these measurements can approach the lifetime of intracellular trifluoroacetate ions and hence be limited by the trifluoroacetate flux rate.

Cholinergic-induced electrolyte transport in rat parotid acini

Secretory responses of parotid acini occurring within 10 sec following cholinergic stimulation were characterized. 1. Measurement of membrane potentials by means of the fluorescent dye diSC3-(5) revealed a value of approximately -59 mV, which remained unaffected on stimulation. 2. Stimulation caused a rapid net loss of 42K+ that was strongly inhibited by the "maxi" K+-channel inhibitor "charybdotoxin" present in scorpion venom. 3. It was calculated that the number of open "maxi" K+-channels per cell was approximately 40 in the unstimulated state and approximately 3000 in the stimulated state. 4. Stimulation caused a transient decrease in the acinar ATP content. 5. Intracellular pH (pHi) measured by means of the fluorescent dye, BCECF, was dependent upon the presence of extracellular HCO3- as well as Na+. Under physiological conditions pHi was 7.27 and stimulation caused a transient decrease of 0.1 pH units due to HCO3- efflux. The decrease was followed by pHi recovery mediated by a Na+/H+ exchange mechanism.

Model of anion and monovalent cation transport as neutral ion pairs through lipophilic water channels of the Na,K ATPase complex

A model of anion and monovalent cation transport through a lipophilic water channel of the Na,K ATPase complex is presented. Literature data for the Na,K ATPase cation binding sites are combined with data for the anion binding sites of Band 3 to obtain adjacent cation and anion combining sites at the inner and outer channel mouths. Cations and anions form neutral ion pairs or undissociated acids at these sites and then partition much more favorably into lipophilic channel water, passing through the channel in diffusive fashion. Cation movements in an "uphill" direction occur without an enzyme translocating moiety and its specific energetic requirement. The pertinent factors are the exclusion of unpaired cations by the tight channel and the site selectivity or pickup ratios for Na/K at each side which dominate over bulk and transmembrane concentration ratios. ATP hydrolysis provides phosphate for ion pairing.

Chloride transport properties of human leukemic cell lines K562 and HL60

The Cl- transport characteristics of the human leukemic cell lines K562 and HL60, with erythroid and granulocytic phenotypic features, respectively, were investigated. Cl- effluxes were measured with 36Cl- under equilibrium conditions in both cell lines and were found to be three orders of magnitude slower than the unidirectional efflux of Cl- in normal erythrocytes. Induction of differentiation of the K562 cell line with hemin does not affect the rate of Cl- transport, while induction of the HL60 cell line with dimethyl sulfoxide results in a small decrease in the rate of Cl- transport. Cl- transport in both cell lines could be divided into two components. One component is inhibited by treatment with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), displays counter-transport characteristics, and has a high energy of activation--all properties characteristic of the human erythrocyte-facilitated anion exchange system. The second component is insensitive to DIDS, is partially inhibited by furosemide, and has a low energy of activation.

Volume-responsive sodium movements in dog red blood cells

As dog red blood cells are shrunken in vitro, their sodium permeability increases progressively. Some new features of this volume-responsive transport process are described. Retardation of sodium movements in shrunken cells occurs when chloride is replaced by the more conductive anions: nitrate or thiocyanate. Micromolar concentrations of amiloride or quinidine inhibit the increment of sodium flux associated with a reduction in cell volume. In the presence of a large outwardly directed sodium gradient, dog red blood cells can progressively alkalinize the medium in which they are suspended. This pH change is stimulated by cell shrinkage, reversed by cell swelling, retarded when chloride is replaced by nitrate or thiocyanate, and inhibited by micromolar concentrations of amiloride or quinidine. The similarities between the shrinkage-associated sodium flux and the alkalinization phenomenon suggest that the mechanism responsible for increased sodium permeability in shrunken cells can be made to operate as a sodium-hydrogen exchanger.