Inhibition of phosphate transport across the human erythrocyte membrane by chemical modification of sulfhydryl groups

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.

Role of sulfhydryl groups in band 3 in the inhibition of phosphate transport across erythrocyte membrane in visceral leishmaniasis

Membrane destabilization in erythrocytes plays an important role in the premature hemolysis and development of anemia during visceral leishmaniasis (VL). Marked degradation of the anion channel protein band 3 is likely to allow modulation of anion flux across the red cell membrane in infected animals. The present study describes the effect of structural modification of band 3 on phosphate transport in VL using (31)P NMR. The result showed progressive decrease in the rate and extent of phosphate transport during the post-infection period. Interdependence between the intracellular ionic levels seems to be a determining factor in the regulation of anion transport across the erythrocyte membrane in control and infected conditions. Infection-induced alteration in band 3 made the active sites of transport more susceptible to binding with amino reactive agents. Inhibition of transport by oxidation of band 3 and subsequent reversal by reduction using dithiothreitol suggests the contribution of sulfhydryl group in the regulation of anion exchange across the membrane. Quantitation of sulfhydryl groups in the anion channel protein showed the inhibition to be closely related to the decrease of sulfhydryl groups in the infected hamsters. Downregulation of phosphate transport during leishmanial infection may be ascribed to the sulfhydryl modification of band 3 resulting in the impaired functioning of this protein under the diseased condition.

Towards an understanding of organic anion transporters: Structure-function relationships

Organic anion transporters (OAT) play essential roles in the body disposition of clinically important anionic drugs, including anti-viral drugs, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories. The activities of OATs are directly linked to drug toxicity and drug-drug interactions. So far, four members of the OAT family have been identified: OAT1, OAT2, OAT3, and OAT4. These transporters share several common structural features including 12 transmembrane domains, multiple glycosylation sites localized in the first extracellular loop between transmembrane domains 1 and 2, and multiple phosphorylation sites present in the intracellular loop between transmembrane domains 6 and 7, and in the carboxyl terminus. The impact of these structural features on the function of these transporters has just begun to be explored. In the present review, the author will summarize recent progress made from her laboratory as well as from others, on the molecular characterization of the structure-function relationships of OATs, including particular amino acid residues/regions of the transporter protein ("molecular domains") that potentially determine transport characteristics.

Responses of thiols to an oxidant challenge: differences between blood and tissues in the rat

Treatment of rats with diamide (100 mg/kg i.p.) altered the thiol components of the blood to a very different extent than in tissues (liver, kidney, lung, spleen, heart and testis). A total consumption (10 min) and regeneration (120 min) of blood glutathione (GSH), matched by a parallel increase and decrease in glutathione-protein mixed disulfides (GS-SP) was observed. In contrast, no modification of non-protein SH groups (NPSH) and protein SH groups (PSH), GS-SP and malondialdehyde (MDA) was observed in liver, kidney, lung, testis spleen and heart within same time range. In particular, only glutathione disulfide (GSSG) levels and some activities of antioxidant enzymes were modified to a small extent and in an opposite direction in some organs. For example, GSSG, and glucose-6-phosphate dehydrogenase (G-6-PDH) and catalase (CAT) activities appeared up-regulated in one tissue and down-regulated in another. The least modified organ was the liver, whereas lung and spleen were the most affected (lung, GSSG, significantly increased whereas G-6-PDH, glutaredoxin (GRX), GPX, superoxide dimutase (SOD) levels were significantly lowered; spleen, GSSG and the activity of glutathione reductase (GR), G-6-PDH and glutathione transferase (GST) were significantly decreased). The different responses of erythrocytes and organs to diamide were explained by the high affinity of hemoglobin and by the relatively high potential of thiol regeneration in organs. The rapid reversibility of the process of protein S-thiolation in blood and the small effects in organs leads us to propose the existence of an inter-organ cooperation in the rat that regulates protein S-thiolation in blood. Plasma thiols may well play a role in this process.

Decreased plasma membrane thiol concentration is associated with increased osmotic fragility of erythrocytes in zinc-deficient rats

Zinc deficiency leads to pathological signs that are related to impaired function of plasma membrane proteins. The purpose of this study was to assess the effect of dietary zinc status on the sulfhydryl (SH) content of erythrocyte plasma membranes and erythrocyte function. Three experiments were performed. In the first, immature male rats were fed for 21 d either a low-zinc (<1.0 mg/kg) diet free choice (-ZnAL), an adequate-zinc (100 mg/kg) diet free choice (+ZnAL), or the adequate-zinc diet limited to the intake of -ZnAL pair-mates (+ZnPF). Tail blood was sampled to measure osmotic fragility and SH concentration of erythrocyte membrane proteins. The zinc-deficient rats were then repleted for 2 d and erythrocytes assayed for fragility and SH content. In the second experiment blood was sampled at 3-d intervals to determine the time course of change in fragility and SH concentration. In the third experiment the SH concentration of erythrocyte band 3 protein and the binding of zinc to isolated plasma membranes were measured. SH concentration decreased from approximately 75 nmol/mg protein to 68 nmol/mg protein during 21 d of depletion and returned to control level within 2 d of repletion. There was an inverse relationship between osmotic fragility and SH concentration of erythrocyte membrane proteins. Maximal decrease in SH occurred within 6 d of consuming the low-zinc diet. The SH content of band 3 protein isolated from deficient rats was also significantly lower than that of pair-fed controls (45 vs. 51 nmol/mg protein). The zinc-binding affinity of plasma membrane proteins tended to be decreased by zinc deficiency. In summary, low-zinc status lowers the plasma membrane SH concentration, and the decreased reducing potential is inversely related to osmotic fragility, and presumably, with impaired volume recovery of erythrocytes.

Effect of band 3 subunit equilibrium on the kinetics and affinity of ankyrin binding to erythrocyte membrane vesicles

The membrane-spanning protein, band 3, anchors the spectrin-based membrane skeleton to the lipid bilayer via the bridging protein, ankyrin. To understand how band 3 subunit stoichiometry influences this membrane-skeletal junction, we have induced changes in the band 3 association equilibrium and assayed the kinetics and equilibrium properties of ankyrin binding. We observe that band 3 oligomers convert slowly to dimers and ultimately monomers following removal of ankyrin. Addition of excess ankyrin back to these membranes enriched in dissociated band 3 then shifts band 3 almost entirely to tetramers, confirming that the tetrameric form of band 3 constitutes the preferred oligomeric state of ankyrin binding. 4, 4'-Diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) labeling of band 3, which is shown to shift most of the band 3 population to dimers, eliminates the majority of ankyrin-binding sites on the membrane and greatly reduces retention of band 3 in detergent-extracted membrane skeletons. Furthermore, DIDS- modified membranes lack all low affinity ankyrin-binding sites and roughly half of all high affinity sites. Since labeled membranes lack the rapid kinetic phase of ankyrin binding and exhibit only half of the normal amplitude of the slow kinetic phase, it can be concluded that the rapid phase of ankyrin association involves low affinity sites and the slow phase involves high affinity sites. A model accounting for these data and most previous data on ankyrin-band 3 interactions is provided.

Lysosomal sulphate transport is dependent upon sulphydryl groups

Using thiol blocking agents, we examined the role of sulphydryl groups for function of the lysosomal sulphate transport system. Monothiol binding reagents, p-hydroxymercuribenzoic acid (p-HMB) and p-chloromercuribenzene sulphonic acid (p-CMBS), dithiol binding reagents such as CuCl2, the alkylating agent, N-ethylmaleimide (NEM), and NADH all inhibited lysosomal sulphate transport. The inhibitory effects of NEM and Cu2+ were not additive, suggesting that they both act upon the same critical sulphydryl group(s). Unlike the case for NEM, the inhibitory effects of Cu2+ were reversed by the reducing agent, dithiothreitol. Exposure to NEM resulted in a seven-fold increase in Km to 867 microM versus a control value of 126 microM and a modest decrease in Vmax to 99 pmolperunit beta-hexosaminidase per 30 s versus a control value of 129 pmolperunit beta-hexosaminidase per 30 s. Similar although somewhat less dramatic results were obtained using Cu2+ with an increase of Km to 448 microM and a Vmax of 77 pmolperunit beta-hexosaminidase per 30 s. The sulphate transport activity of detergent solubilized lysosomal membranes could be bound to a p-chloromercuribenzoic acid (p-CMB)-Sepharose sulphydryl affinity resin and eluted with mercaptoethanol. Sulphydryl groups thus appear to play a role in sulphate transport through effects on substrate affinity. Sulphydryl-binding appears to be a strategy that may be useful for purification of the transporter.

Se-mediated domain-domain communication in band 3 of human erythrocytes

Na2SeO3 could affect the anion flux of Band 3 of inside-out erythrocyte membrane vesicles (IOVs). Such effect was believed to be based on the interaction of SH groups of Band 3 with Na2SeO3. This effect could be eliminated when the cytoplasmic domain of Band 3 was proteolytically removed by trypsin. This suggested that SH groups in the cytoplasmic domain were involved in such interaction. Measurement of the pH dependence of intrinsic fluorescence intensity provided evidence that conformational changes of Band 3 occurred as a consequence of interaction with selenite. KI quenching of intrinsic fluorescence of Band 3 could also show that there was a conformational change in the cytoplasmic domain of Band 3 after reaction with Na2SeO3. Such conformational change in turn could be transmitted to the membrane domain of Band 3 monitored by quenching of intrinsic fluorescence of Band 3 using hypocrellin B (HB) (a photosensitive pigment obtained from a parasitic fungus growing in Yunnan, China). It is suggested that the cytoplasmic domain of Band 3 is not necessary for its anion flux, but is essential for the regulation (e.g., by Se) of its active site located at the membrane domain, and hence, it may provide evidence of communication between the cytoplasmic domain and the membrane domain of Band 3.

Effects of intracellular pH on high pressure-induced hemolysis of anion transport inhibitor-treated erythrocytes

Effects of anion transport inhibitors such as 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate on hemolysis of human erythrocytes at 200 MPa were examined by changing intracellular pH (7.2-7.9). These inhibitors suppressed the hemolysis at neutral pH but enhanced it at alkaline pH. However, such an enhancement was suppressed by cross-linking of membrane proteins using diamide. From the near-UV CD spectra of band 3 and the relation between hemolysis and anion transport in intact or trypsin-treated erythrocytes, it was found that such hemolytic properties were characterized by the binding of inhibitors to band 3. In addition, spectrin detachment from the erythrocyte membrane by high pressure was considerably suppressed by DIDS treatment at neutral pH, but not by DIDS labeling at alkaline pH. These results suggest that the interaction of the cytoplasmic domain of band 3 with the cytoskeleton, which is induced by the binding of ligands to the exofacial domain of band 3, is dependent on the intracellular pH, i.e., the linking is tightened at neutral pH but relaxed at alkaline pH.

Unlike its human counterpart, band 3 anion exchange protein from goat erythrocyte membrane shows a lack of reactivity against various -SH oxidants and protease treatments

Studies involving a number of -SH oxidants and proteases were made to analyse the organization of band 3 in goat erythrocyte membrane. -SH oxidizing agents such as diamide, Cu2+.o-phenanthroline and phenylene dimaleimide, known to cause cross-linking of human erythrocyte band 3, failed to show any cross-linking in the case of goat band 3 protein. When resolved to their individual components using -SH reducing agent beta-mercaptoethanol, high molecular weight protein adducts formed as a result of diamide treatment did not show any band 3 on two-dimensional electrophoresis. Also no proteolysis of band 3 was detected when intact goat erythrocytes were exposed to pronase, though marked proteolysis was noticed in the case of human band 3 proteins under similar conditions. These studies involving -SH oxidant and protease treatments suggest a different organization for goat erythrocyte band 3 protein as compared to that of human in erythrocyte membrane.

Band 3, the anion exchanger of the erythrocyte membrane, is also a flippase

The transbilayer reorientation (flip-flop) of the long-chain amphiphilic anion DENSA (5-(N-decyl)aminonaphthalene-2-sulfonic acid) in the erythrocyte membrane was studied by fluorescence spectroscopy. DENSA intercalates into the membrane at a high membrane/water partition coefficient (3.2.10(5)) and rapidly reorients from the outer to the inner layer in a first order process (k = 0.11 min-1, 37 degrees C, pH 7.4) leading to a steady-state distribution inner:outer layer of about 30:70. The activation energy of the fully reversible and symmetric flip process is about 110 kJ/mol. DIDS and various other established covalent and non-covalent inhibitors of anion transport via the erythrocyte anion exchanger, band 3 (AE 1), suppress the flip to a minimum of about 30-35% of the control. The flip is also inhibited by Cl- with a half maximal inhibitory concentration equal to that required for the inhibition of the exchange flux of ordinary anions via band 3. These findings indicate the involvement of a band 3 mediated (DIDS-sensitive) component of the flip and a DIDS-insensitive one, possibly involving, at least to some extent, simple transbilayer 'diffusion'. This latter component is stimulated by diamide, an SH oxidant known to increase the permeability of the membrane lipid domain of the erythrocyte. Alcohols (butanol, hexanol) accelerate both flip components. Papain treatment, known to inhibit 'ordinary' anion exchange, accelerates both flip and flop. The results suggest that band 3 protein, besides being a conventional transporter of anions, can act as a flippase translocating anionic, membrane-intercalated amphiphiles approaching the transporter from the lipid domain. The flippase mode of operation of band 3 must, however, differ in its mechanism from the conventional exchange mode.

A role for band 4.2 in human erythrocyte band 3 mediated anion transport

Human erythrocyte band 3 was purified essentially free of peripheral proteins, in particular band 4.2, using affinity chromatography. Band 3 protein was then reconstituted into liposomes of lipid type and ratio approximating that of erythrocyte membranes. Stilbenedisulfonate inhibition of band 3 mediated efflux of radiolabeled sulfate from preloaded liposomes was used to test the functionality and correct orientation of the protein. When sulfate efflux, mediated by purified band 3, was compared with partially purified band 3, which contained detectable amounts of bands 4.1 and 4.2, a clear difference in efflux was measured. Sulfate efflux was approximately 30% faster from liposomes containing purified band 3 compared with those containing partially purified protein. In order to investigate further any specific effect of band 4.2 protein on band 3 mediated anion transport, band 4.2 was purified. Increasing amounts of band 4.2 were complexed with purified band 3 and then reconstituted into liposomes. Increasing amounts of band 4.2 complexed with band 3 caused a decrease in band 3 mediated anion transport. The effect of band 4.2 on band 3 mediated anion transport appears to be specific since increasing concentrations of band 4.2 added exogenously to band 3 in reconstituted vesicles (rather than complexed with band 3 before reconstitution) produced no significant changes in sulfate efflux. Further, when increasing amounts of band 4.2 were added to the functionally active transmembrane domain of band 3 and then reconstituted into vesicles, there was also no significant change in sulfate efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Near-UV circular dichroism of band 3. Evidence for intradomain conformational changes and interdomain interactions

Near-UV circular dichroism (CD) was used to identify differences in the tertiary structure of human erythrocyte band 3, the chloride/bicarbonate exchange protein, consequent to covalent binding of anion transport inhibitors to the intramonomeric stilbenedisulfonate (ISD) site. Isolated intact band 3 and its membrane domain (B3MD) were compared. Spectral differences were observed which involved intradomain effects, in that they were seen both with intact band 3 and with B3MD, or interdomain effects, in that they were observed only for B3MD, but were inhibited when the cytoplasmic domain was attached. The intradomain effect involved a significant loss in optical activity in the Phe/Tyr region of the spectrum below 280 nm. It was seen only when the ISD site had stilbenedisulfonates bound covalently at pH 7.4. Raising the pH to 9.6 after adduct formation "normalized" this spectral change irreversibly. The interdomain effect was identified in the Trp spectral region at 292 nm. There was a significant increase in optical activity at 292 nm when bulky covalent ligands such as DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate) were bound to B3MD, but not when the same ligands were bound to intact band 3. These latter results offer evidence that certain aspects of the conformational response of the integral domain are inhibited by the presence of an attached cytoplasmic domain. The potential significance of interdomain interactions to band 3 function is discussed briefly.