Methods and analysis of erythrocyte anion fluxes

Altered Structure and Anion Transport Properties of Band 3 (AE1, SLC4A1) in Human Red Cells Lacking Glycophorin A

We have studied the properties of band 3 in different glycophorin A (GPA)-deficient red cells. These red cells lack either both GPA and glycophorin B (GPB) (M(k)M(k) cells) or GPA (En(a-) cells) or contain a hybrid of GPA and GPB (MiV cells). Sulfate transport was reduced in all three red cell types to approximately 60% of that in normal control red cells as a result of an increased apparent K(m) for sulfate. Transport of the monovalent anions iodide and chloride was also reduced. The reduced iodide transport resulted from a reduction in the V(max) for iodide transport. The anion transport site was investigated by measuring iodide fluorescence quenching of eosin-5-maleimide (EMA)-labeled band 3. The GPA-deficient cells had a normal K(d) for iodide binding, in agreement with the unchanged K(m) found in transport studies. However, the apparent diffusion quenching constant (K(q)) was increased, and the fluorescence polarization of band 3-bound EMA decreased in the variant cells, suggesting increased flexibility of the protein in the region of the EMA-binding site. This increased flexibility is probably associated with the decrease in V(max) observed for iodide transport. Our results suggest that band 3 in the red cell can take up two different structures: one with high anion transport activity when GPA is present and one with lower anion transport activity when GPA is absent.

Substrate-dependent reversal of anion transport site orientation in the human red blood cell anion-exchange protein, AE1

The tightly coupled, one-for-one exchange of anions mediated by the human red blood cell AE1 anion-exchange protein involves a ping-pong mechanism, in which AE1 alternates between a state with the anion-binding site facing inward toward the cytoplasm (Ei) and a state with the site facing outward toward the external medium (Eo). The conformational shift (Ei <--> Eo) is only permitted when a suitable substrate such as Cl(-) or HCO(3)(-) (B(-)) is bound. With no anions bound, or with Cl(-) bound, far more AE1 molecules are in the inward-facing than the outward-facing forms (Ei Eo, ECli EClo). We have constructed a model for CI(-)-B(-) exchange based on Cl(-)-Cl(-) and B(-)-B(-) exchange data, and have used it to predict the heteroexchange flux under extremely asymmetric conditions, with either all Cl(-) inside and all B(-) outside (Cli-Bo) or vice versa (Bi-Clo). The experimental values of the ratio of the exchange rate for Bi-Clo to that for Cli-Bo are only compatible with the model if the asymmetry of bicarbonate-loaded sites (A(B) = EBo/EBi) > 10, the opposite of the asymmetry for unloaded or Cl-loaded sites. Furthermore, the Eo form has a higher affinity for HCO(3)(-) than for Cl(-), whereas the Ei form has a higher affinity for Cl(-). The fact that this "passive" system exhibits changes in substrate selectivity with site orientation ("sidedness"), a characteristic usually associated with energy-coupled "active" pumps, suggests that changes in affinity with changes in sidedness are a more general property of transport proteins than previously thought.

Persistence of external chloride and DIDS binding after chemical modification of Glu-681 in human band 3

Although its primary function is monovalent anion exchange, the band 3 protein also cotransports divalent anions together with protons at low pH. The putative proton binding site, Glu-681 in human erythrocyte band 3, is conserved throughout the anion exchanger family (AE family). To determine whether or not the monovalent anion binding site is located near Glu-681, we modified this residue with Woodward's reagent K (N-ethyl-5-phenylisoxazolium-3'-sulfonate; WRK). Measurements of Cl(-) binding by (35)Cl-NMR show that external Cl(-) binds to band 3 even when Cl(-) transport is inhibited approximately 95% by WRK modification of Glu-681. This indicates that the external Cl(-) binding site is not located near Glu-681 and thus presumably is distant from the proton binding site. DIDS inhibits Cl(-) binding even when WRK is bound to Glu-681, indicating that the DIDS binding site is also distant from Glu-681. Our data suggest that the DIDS site and probably also the externally facing Cl(-) transport site are located nearer to the external surface of the membrane than Glu-681.

Inducible expression of erythrocyte band 3 protein

A permanent cell line with inducible expression of the human anion exchanger protein 1 (hAE1) was constructed in a derivative of human embryonic kidney cells (HEK-293). In the absence of the inducer, muristerone A, the new cell line had no detectable hAE1 protein by Western analysis or additional 36Cl flux. Increasing dose and incubation time with muristerone A increased the amount of protein (both unglycosylated and glycosylated). The 4,4'-dinitrostilbene-2, 2'-disulfonate (DNDS)-inhibitable rapid Cl exchange flux was increased up to 40-fold in induced cells compared with noninduced cells. There was no DNDS-inhibitable rapid flux component in noninduced cells. This result demonstrates inducible expression of a new rapid Cl transport pathway that is DNDS sensitive. The additional transport of 36Cl and 35SO4 had the characteristics of hAE1-mediated transport in erythrocytes: 1) inhibition by 250 microM DNDS, 2) activation of 36Cl efflux by external Cl with a concentration producing half-maximal effect of 4.8 mM, 3) activation of 36Cl efflux by external anions that was selective in the order NO3 = Cl > Br > I, and 4) activation of 35SO4 influx by external protons. Under the assumption that the turnover numbers of hAE1 were the same as in erythrocytes, there was good agreement (+/-3-fold) between the number of copies of glycosylated hAE1 and the induced tracer fluxes. This is the first expression of hAE1 in a mammalian system to track the kinetic characteristics of the native protein.

Effects of external pH on binding of external sulfate, 4.4-dinitro-stilbene-2,2'-disulfonate (DNDS), and chloride to the band 3 anion exchange protein

A model in which two positively-charged titratable sites enhance the affinity for anionic substrates can explain the increase in external iodide dissociation constant (K(O)(I)) with increasing pH(O) (Liu, S. J., F.-Y. Law, and P.A. Knauf. 1996.f Gen.Physiol. 107:271-291). If sulfate binds to the same external site as I-, this model predicts that the SO(4)= dissociation constant (K(O)(S)) should also increase. The data at pH 0 8.5 to 10 fit this prediction, and the pK for the titration is not significantly different from that (pKc) for the low-pK group that affects K(O)(1). The dissociation constant for the apparently competitive inhibitor, DNDS (4,4-dinitrostilbene-2,2'-disulfonate), also increases greatly as pH(O) increases. Particularly at high pH(O), a noncompetitive inhibition by DNDS is also evident. Increasing pH(O) from 7.2 to 11.2 increases the competitive dissociation constant by 700-fold, but the noncompetitive is only increased 20-fold. The pK values for these effects are similar to pKc for K(O)(1), as expected if DNDS binds near the external transport site, but it seems likely that additional titratable groups also affect DNDS binding. The apparent affinity for external Cl- is also affected by pH(O), in a manner similar to that observed for I-. Pretreatment with the amino-selective reagent, bis-sulfosuccinimidyl suberate (BSSS), decreases the apparent Cl- affinity at pH 8.5, but two titrations are still evident, the first (lower) of which decreases the apparent C- affinity, and the second of which surprisingly increases it. Thus, the BSSS-reactive amino groups (probably Lys-539 and Lys-851) do not seem to be involved in the titrations that affect Cl- affinity. In general, the data support the concept that a positively charged amino group (or groups), together with a guanidino group, plays an important role in the binding of substrates and inhibitors at or near the external transport site.

Effects of external pH on substrate binding and on the inward chloride translocation rate constant of band 3

To test the hypothesis that amino acid residues in band 3 with titratable positive charges play a role in the binding of anions to the outside-facing transport site, we measured the effects of changing external pH (pH(O)) on the dissociation constant for binding of external iodide to the transport site, K(O)(I). K(O)(I) increased with increasing pH(O), and a significant increase was seen even at pH(O) values as low as 9.9. The dependence of K(O)(I) on pH(O) can be explained by a model with one titratable site with pK 9.5 +/- 0.2 (probably lysine), which increases anion affinity for the external transport site when it is in the positively charged form. A more complex model, analogous to one recently proposed by Bjerrum (1992), with two titratable sites, one with pK 9.3 +/- 0.3 (probably lysine) and another with pK > 11 (probably arginine), gives a slightly better fit to the data. Thus, titratable positively charged residues seem to be functionally important for the binding of substrate anions to the outward-facing anion transport site. In addition, analysis of Dixon plot slopes for L inhibition of Cl- exchange at different pH 0 values, coupled with the assumption that pH(O) has parallel effects on external I- and Cl- binding, indicates that k', the rate-constant for inward translocation of the complex of Cl- with the extracellular transport site, decreases with increasing pH(O). The data are compatible with a model in which titration of the pK 9.3 residue decreases k to 14 +/- 10% of its value at neutral pH(O). This result, however, together with Bjerrum's (1992) observation that the maximum flux J(M)) increases 1.6-fold when this residue is deprotonated, makes quantitative predictions that raise significant questions about the adequacy of the two titratable site ping-pong model or the assumptions used in analyzing the data.

Ionophore properties of monensin derivatives studied on human erythrocytes by 23Na NMR and K+ and H+ potentiometry: relationship with antimicrobial and antimalarial activities

Eight derivatives of monensin with a modified C25-C26 moiety were synthesized. Their ionophore properties were studied on human erythrocytes by measuring Na+ influx with 23Na NMR and concomitant K+ and H+ efflux by potentiometry. Modification of OH-26 led to inversion of selectivity of transport in favor of K+/Na+ in comparison with monensin. This selectivity disappeared by suppression of the C26-OH moiety. Finally the ionophore ability was lost if the head-to-tail chelation of the monensin skeleton was prevented by blocking the terminal OH-25 and -26 functions. All the compounds were inactive on Gram-negative bacteria and fungi. MIC measured on Bacillus cereus showed that derivatives with increased K+/Na+ selectivity were clearly the most active against Bacillus growth. Most of the compounds showed potential antimalarial properties in the nanomolar range when tested in vitro against Plasmodium falciparum. The IC50S measured were correlated with the whole Na+ and K+ transport efficiency rather than with the ionic selectivity. In both cases determination of initial fluxes of transport for both cations (Na+ and K+) was necessary to investigate the relationship between biological and ionophore properties.

35Cl nuclear magnetic resonance line broadening shows that eosin-5-maleimide does not block the external anion access channel of band 3

It has been suggested that Lys-430 of band 3, with which eosin-5-maleimide (EM) reacts, is located in the external channel through which anions gain access to the external transport site, and that EM inhibits anion exchange by blocking this channel. To test this, we have used 35Cl nuclear magnetic resonance (NMR) to measure Cl- binding to the external transport site in control and EM-treated human red blood cells. Intact cells were used rather than ghosts, because in this case all line broadening (LB) results from binding to external sites. In an NMR spectrometer with a 9.4-T magnetic field, red blood cells at 50% concentration (v/v) in 150 mM Cl- medium at 3 degrees C caused 19.0 +/- 1.2 Hz LB. Of this, 7.9 +/- 0.7 Hz was due to Cl- binding to the high affinity band 3 transport sites, because it was prevented by an apparently competitive inhibitor of anion exchange, 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS). The LB was not due to hemoglobin released from the cells, as little LB remained in the supernatant after cells were removed by centrifugation. Saturable Cl- binding remained in EM-treated cells, although the binding was no longer DNDS-sensitive, because EM prevents binding of DNDS. The lower limit for the rate at which Cl- goes from the binding site to the external medium is 2.15 x 10(5) s-1 for control cells and 1.10 x 10(5) s-1 for EM-treated cells, far higher than the Cl- translocation rate at 3 degrees C (about 400 s-1). Thus, EM does not inhibit Cl- exchange by blocking the external access channel. EM may therefore be useful for fixing band 3 in one conformation for studies of Cl- binding to the external transport site.

WW-781, a potent reversible inhibitor of red cell Cl- flux, binds to band 3 by a two-step mechanism

WW-781 ([3-methyl-1-p-sulfophenyl-5-pyrazolone-(4)]-[1,3-dibutylbarbit uric acid]-pentamethine oxonol), a fluorescent dye that has been used for measuring membrane potentials by optical methods, inhibits human red blood cell Cl- exchange, which is mediated by the membrane protein known as band 3 or capnophorin. The inhibition is slowly reversible upon removal of WW-781 from the medium, with a half time of approximately 4.7 min in 150 mM Cl- medium at 0 degrees C. The mechanism of inhibition by WW-781 involves a two-step binding reaction. WW-781 binds rapidly to band 3 to form an initial complex, which can also rapidly dissociate. Formation of this initial complex is followed by the much slower formation of a second complex (with a rate constant of approximately 1.1 min-1), probably involving a protein conformational change, through which WW-781 is more tightly bound to band 3. At low concentrations, WW-781 inhibits Cl- exchange with a stoichiometry of 1 WW-781 molecule per band 3 monomer, suggesting that under these conditions the binding of WW-781 is highly selective for the band 3 protein.

Synthesis of 5-nitro-2-[N-3-(4-azidophenyl)-propylamino]-benzoic acid: photoaffinity labeling of human red blood cell ghosts with a 5-nitro-2-(3-phenylpropylamino)-benzoic acid analog

A photoaffinity analog of the potent epithelial chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid has been synthesized and characterized. In the dark, this reagent, 5-nitro-2-[N-3-(4-azidophenyl)-propylamino]-benzoic acid, and the parent compound reversibly inhibited chloride efflux in human red blood cell ghosts. Irradiation of ghost membranes with 350 microM arylazide analog reduced the rate of chloride efflux to 33% of the control value. The photoinactivation process was not reversed by exhaustive washing of ghost membranes. Covalent incorporation of the photoaffinity reagent was supported by difference ultraviolet spectroscopy, which indicated the attachment of the substituted 2-amino-5-nitrobenzoic acid chromophore to ghost membranes. The novel photolabeling agent described here should be a useful structural probe for chloride channels in erythrocyte membranes and epithelial cells.