Ionic dependence of bumetanide binding to the rabbit parotid Na/K/Cl cotransporter

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Cryo-EM structure of the human NKCC1 transporter reveals mechanisms of ion coupling and specificity

The sodium-potassium-chloride transporter NKCC1 of the SLC12 family performs Na⁺ -dependent Cl^- - and K⁺ -ion uptake across plasma membranes. NKCC1 is important for regulating cell volume, hearing, blood pressure, and regulation of hyperpolarizing GABAergic and glycinergic signaling in the central nervous system. Here, we present a 2.6 Å resolution cryo-electron microscopy structure of human NKCC1 in the substrate-loaded (Na⁺ , K⁺ , and 2 Cl^- ) and occluded, inward-facing state that has also been observed for the SLC6-type transporters MhsT and LeuT. Cl^- binding at the Cl1 site together with the nearby K⁺ ion provides a crucial bridge between the LeuT-fold scaffold and bundle domains. Cl^- -ion binding at the Cl2 site seems to undertake a structural role similar to conserved glutamate of SLC6 transporters and may allow for Cl^- -sensitive regulation of transport. Supported by functional studies in mammalian cells and computational simulations, we describe a putative Na⁺ release pathway along transmembrane helix 5 coupled to the Cl2 site. The results provide insight into the structure-function relationship of NKCC1 with broader implications for other SLC12 family members.

Gramicidin-perforated patch recording revealed the oscillatory nature of secretory Cl- movements in salivary acinar cells

Elevations of cytoplasmic free calcium concentrations ([Ca²⁺]_i) evoked by cholinergic agonists stimulate isotonic fluid secretion in salivary acinar cells. This process is driven by the apical exit of Cl⁻ through Ca²⁺-activated Cl⁻ channels, while Cl⁻ enters the cytoplasm against its electrochemical gradient via a loop diuretic-sensitive Na⁺-K⁺-2Cl⁻ cotransporter (NKCC) and/or parallel operations of Cl⁻-HCO₃⁻ and Na⁺-H⁺ exchangers, located in the basolateral membrane. To characterize the contributions of those activities to net Cl⁻ secretion, we analyzed carbachol (CCh)-activated Cl⁻ currents in submandibular acinar cells using the “gramicidin-perforated patch recording configuration.” Since the linear polypeptide antibiotic gramicidin creates monovalent cation-selective pores, CCh-activated Cl⁻ currents in the gramicidin-perforated patch recording were carried by Cl⁻ efflux via Cl⁻ channels, dependent upon Cl⁻ entry through Cl⁻ transporters expressed in the acinar cells. CCh-evoked oscillatory Cl⁻ currents were associated with oscillations of membrane potential. Bumetanide, a loop diuretic, decreased the CCh-activated Cl⁻ currents and hyperpolarized the membrane potential. In contrast, neither methazolamide, a carbonic anhydrase inhibitor, nor elimination of external HCO₃⁻ had significant effects, suggesting that the cotransporter rather than parallel operations of Cl⁻-HCO₃⁻ and Na⁺-H⁺ exchangers is the primary Cl⁻ uptake pathway. Pharmacological manipulation of the activities of the Ca²⁺-activated Cl⁻ channel and the NKCC revealed that the NKCC plays a substantial role in determining the amplitude of oscillatory Cl⁻ currents, while adjusting to the rate imposed by the Ca²⁺-activated Cl⁻ channel, in the gramicidin-perforated patch configuration. By concerting with and being controlled by the cation steps, the oscillatory form of secretory Cl⁻ movements may effectively provide a driving force for fluid secretion in intact acinar cells.

Ion transport and ligand binding by the Na-K-Cl cotransporter, structure-function studies

The cation-Cl cotransporters (CCCs) mediate the coupled movement of Na and/or K to that of Cl across the plasmalemma of animal cells. Eight CCCs have been identified to date: two Na-K-Cl cotransporters (NKCC), four K-Cl cotransporters (KCCs), one Na-Cl cotransporter (NCC) and one CCC interacting protein (CIP). All of the NKCCs and KCCs are inhibited by loop diuretics; mercury and other modifying agents are also known to block NKCC-mediated transport. In this work, we have utilized a mutational approach to study the interaction between different substrates and the NKCCs. We relied on the strategy of exchanging domains between functionally distinct carriers (the shark NKCCl and the human NKCCl) to identify residues or group of residues that are involved in the interaction with ions, loop diuretics and Hg. Our results show that the N- and C-termini have no role in determining the species differences in ion transport and bumetanide binding. On the other hand, the interaction between Hg and the NKCCs is found to partially involve the C-terminus through residues that contain available sulfhydryl groups. Within the transmembrane segments, variant residues in the 2nd, 4th and 7th predicted alpha-helices are shown to encode the differences in ion transport between the shark and the human cotransporters. For loop diuretic binding, several regions throughout the central domain appear to be involved. Interestingly, these regions are not the same as those involved in cation or anion transport, and in Hg binding.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Characterization of a phosphorylation event resulting in upregulation of the salivary Na(+)-K(+)-2Cl(-) cotransporter

Previous studies from our laboratory have shown a close correlation between increased Na(+)-K(+)-2Cl(-) cotransporter activity and increased cotransporter phosphorylation after beta-adrenergic stimulation of rat parotid acinar cells. We demonstrate here that these effects are paralleled by an increase in the number of high-affinity binding sites for the cotransporter inhibitor bumetanide in membranes prepared from stimulated acini. We also show that the sensitivity of cotransporter fluxes to inhibition by bumetanide is the same in both resting and isoproterenol-stimulated cells, consistent with the hypothesis that beta-adrenergic stimulation and the accompanying phosphorylation result in the activation of previously quiescent transporters rather than in a change in the properties of already active proteins. In addition, we demonstrate that the increased phosphorylation on the cotransporter resulting from beta-adrenergic stimulation is localized to a 30-kDa phosphopeptide obtained by cyanogen bromide digestion. Immunoprecipitation and Western blotting experiments demonstrate that this peptide is derived from the NH(2)-terminal cytosolic tail of the cotransporter, which surprisingly does not contain the sole protein kinase A consensus site on the molecule.

Mutagenic mapping of the Na-K-Cl cotransporter for domains involved in ion transport and bumetanide binding

The human and shark Na-K-Cl cotransporters (NKCCs) are 74% identical in amino acid sequence yet they display marked differences in apparent affinities for the ions and bumetanide. In this study, we have used chimeras and point mutations to determine which transmembrane domains (tm's) are responsible for the differences in ion transport and in inhibitor binding kinetics. When expressed in HEK-293 cells, all the mutants carry out bumetanide-sensitive 86Rb influx. The kinetic behavior of these constructs demonstrates that the first seven tm's contain all of the residues conferring affinity differences. In conjunction with our previous finding that tm 2 plays an important role in cation transport, the present observations implicate the fourth and seventh tm helices in anion transport. Thus, it appears that tm's 2, 4, and 7 contain the essential affinity-modifying residues accounting for the human-shark differences with regard to cation and anion transport. Point mutations have narrowed the list of candidates to 13 residues within the three tm's. The affinity for bumetanide was found to be affected by residues in the same tm 2-7 region, and also by residues in tm's 11 and 12. Unlike for the ions, changes in bumetanide affinity were nonlinear and difficult to interpret: the Ki(bumetanide) of a number of the constructs was outside the range of sNKCC1 and hNKCC1 Kis.

The role of transmembrane domain 2 in cation transport by the Na-K-Cl cotransporter

The human and shark Na-K-Cl cotransporters (NKCC), although 74% identical in amino acid sequence, exhibit marked differences in ion transport and bumetanide binding. We have utilized shark-human chimeras of NKCC1 to search for regions that confer the kinetic differences. Two chimeras (hs3.1 and its reverse sh3.1) with a junction point located at the beginning of the third transmembrane domain were examined after stable transfection in HEK-293 cells. Each carried out bumetanide-sensitive 86Rb influx with cation affinities intermediate between shark and human cotransporters. In conjunction with the previous finding that the N and C termini are not responsible for differences in ion transport, the current observations identify the second transmembrane domain as playing an important role. Site-specific mutagenesis of two pairs of residues in this domain revealed that one pair is indeed involved in the difference in Na affinity, and a second pair is involved in the difference in Rb affinity. Substitution of the same residues with corresponding residues from NKCC2 or the Na-Cl cotransporter resulted in cation affinity changes, consistent with the hypothesis that alternative splicing of transmembrane domain 2 endows different versions of NKCC2 with unique kinetic behaviors. None of the changes in transmembrane domain 2 was found to substantially affect Km(Cl), demonstrating that the affinity difference for Cl is specified by the region beyond predicted transmembrane domain 3. Finally, unlike Cl, bumetanide binding was strongly affected by shark-human replacement of transmembrane domain 2, indicating that the bumetanide-binding site is not the same as the Cl-binding site.

Molecular and clinical implications of loop diuretic ototoxicity

Recent advances in molecular biology have been applied to inner ear research. Loop diuretic ototoxicity has been suggested, but not proven, to share a common mechanism with diuretic effects on renal tubules. The discovery of the molecular nature of the Na-K-2Cl cotransporter in the cochlea provided a better understanding of loop diuretic ototoxicity. In this review, we describe clinical reports of loop diuretic ototoxicity and other information obtained by physiological, biochemical and morphological investigations related to the mechanism sensitive to loop diuretics. Based on recent evidence for the molecular nature of the Na-K-2Cl cotransporter expressed in the mammalian cochlea, the underlying mechanisms of ototoxicity induced by loop diuretics are described.

Ultrastructural localization of the Na-K-Cl cotransporter in the lateral wall of the rabbit cochlear duct

Localization of the immunoreactivity in the lateral wall of the rabbit cochlear duct was examined using a post-embedding immunogold method with a polyclonal antiserum raised against the rabbit parotid Na-K-Cl cotransporter. In the stria vascularis, the labeling was significant on the basolateral membrane infolding of marginal cells, whereas no labeling was seen on the luminal membrane of these cells. Immunoreactivity was also detected on the cell membranes of various other cells. These include fibrocytes of the spiral ligament and the spiral prominence, and vascular endothelial cells in the stria vascularis and the spiral ligament. In contrast, virtually no gold particles were seen on the membrane of intermediate cells, basal cells of the stria vascularis, the epithelial cells of the spiral prominence, or Reissner's membrane. Our result on the localization of the Na-K-Cl cotransporter in marginal cells is consistent with electrophysiological studies (Wangemann et al. (1995) Hear. Res. 84, 19-29). Our result on fibrocytes is discussed in relation to K+ circulation into endolymph from perilymph (Schulte and Steel (1994) Hear. Res. 78, 65-76).

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

Involvement of direct phosphorylation in the regulation of the rat parotid Na(+)-K(+)-2Cl- cotransporter

We identify a 175-kDa membrane phosphoprotein (pp175) in rat parotid acini whose properties correlate well with the Na(+)-K(+)-2Cl- cotransporter previously characterized functionally and biochemically in this tissue. pp175 was the only phosphoprotein immunoprecipitated by an anti-Na(+)-K(+)-2Cl- cotransporter antibody and the only membrane protein whose phosphorylation state was conspicuously altered after a brief (45-s) exposure of acini to the beta-adrenergic agonist isoproterenol. Phosphopeptide mapping provided evidence for three phosphorylation sites on pp175, only one of which was labeled in response to isoproterenol treatment. The half-maximal effect of isoproterenol on phosphorylation of pp175 (approximately 20 nM) was in excellent agreement with its previously demonstrated up-regulatory effect on cotransport activity. Increased phosphorylation of pp175 was also seen following acinar treatment with a permeant cAMP analogue and with forskolin, conditions that have likewise been shown to up-regulate the cotransporter. Combined with earlier results from our laboratory, these data provide strong evidence that the up-regulation of the cotransporter by these agents is due to direct phosphorylation mediated by protein kinase A. AlF(-)4 treatment, which results in an up-regulation of cotransport activity comparable with that observed with isoproterenol (approximately 6-fold), caused a similar increase in phosphorylation of pp175. However, hypertonic shrinkage and treatment with the protein phosphatase inhibitor calyculin A, which also up-regulate the cotransporter (approximately 3-fold and approximately 6-fold, respectively) caused no change in the phosphorylation level. Furthermore, although acinar treatment with the muscarinic agonist carbachol results in a dramatic up-regulation of cotransport activity and a concomitant phosphorylation of pp175, no phosphorylation of pp175 was seen with the Ca(2+)-mobilizing agent thapsigargin, which is able to fully mimic the up-regulatory effect of carbachol on transport activity. Taken together, these results indicate that direct phosphorylation is only one of the mechanisms involved in secretagogue-induced regulation of the rat parotid Na(+)-K(+)-2Cl- cotransporter.

Anion dependence of bumetanide binding and ion transport by the rabbit parotid Na(+)-K(+)-2Cl- co-transporter: evidence for an intracellular anion modifier site

The anion dependence of [3H]bumetanide binding and 22Na+ transport by the rabbit parotid Na(+)-K(+)-2Cl- co-transporter was studied in acinar basolateral membrane vesicles (BLMVs). Cl-, Br- and NO3- have a biphasic effect on binding consistent with the presence of two anion sites associated with the bumetanide binding event, a high-affinity stimulatory site and a lower-affinity inhibitory site. We show that formate shares only the stimulatory site and SO4(2-) only the inhibitory site. The initial rate of [3H]bumetanide binding was stimulated by formate or low [Cl-] and inhibited by SO4(2-) or high [Cl-], but the rate of [3H]bumetanide dissociation was not affected by the presence of these anions in the dissociation medium. However, when [3H]bumetanide was bound to BLMVs in the presence of formate its rate of dissociation was more than four times faster than when binding took place in the presence of Cl-. These observations indicate that the binding of bumetanide and the stimulatory anion are ordered such that the anion must necessarily bind first and subsequently cannot dissociate until after bumetanide dissociates. In zero-trans-flux experiments, extravesicular SO4(2-) and formate had no effect on 22Na+ transport via the co-transporter [Turner and George (1988) J. Membr. Biol. 102, 71-77]. Thus neither of the anion sites associated with bumetanide binding is a Cl- transport site. However, we show here that SO4(2-) inhibits transport when present in the intravesicular space. Since the BLMV preparation is predominantly oriented cytosolic-side-in, this observation indicates the existence of an inhibitory cytosolic anion modifier site. Our data suggest that this site is identical to the inhibitory anion site associated with bumetanide binding.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. Na-K-Cl cotransporters are present in a wide variety of cells and tissues, including reabsorptive and secretory epithelia, nerve and muscle cells, endothelial cells, fibroblasts, and blood cells. Na-K-Cl cotransport plays a vital role in renal salt reabsorption and in salt secretion by intestinal, airway, salivary gland, and other secretory epithelia. Cotransport function also appears to be important in the maintenance and regulation of cell volume and of ion gradients by both epithelial and nonepithelial cells. Na-K-Cl cotransport activity is inhibited by "loop" diuretics, including the clinically efficacious agents bumetanide and furosemide. The regulation of Na-K-Cl cotransport is mediated, at least in some cases, through direct phosphorylation of the cotransport protein. Cotransporter regulation is highly tissue specific, perhaps in part related to the presence of different Na-K-Cl cotransporter isoforms. In epithelia, both absorptive (kidney-specific) and secretory isoforms have been identified by cDNA cloning and sequencing and Northern blot analysis; alternatively spliced variants of the kidney-specific isoform have also been identified. The absorptive and secretory isoforms exhibit approximately 60% identity at the amino acid sequence level; these sequences in turn show approximately 45% overall homology with those of thiazide-sensitive, bumetanide-insensitive, Na-Cl cotransport proteins of winter flounder urinary bladder and mammalian kidney. This review focuses on recent developments in the identification of Na-K-Cl cotransport proteins in epithelial and on the regulation of epithelial Na-K-Cl cotransporter function at cellular and molecular levels.

Identification, characterization and purification of a 160 kD bumetanide-binding glycoprotein from the rabbit parotid

We demonstrate the presence of a 160 kD protein in rabbit parotid basolateral membranes that can be labeled with the irreversible sulfhydryl reagent [14C]-N-ethylmaleimide in a bumetanide-protectable fashion. The specificity of this labeling, and our previous evidence for the existence of an essential sulfhydryl group closely associated with the bumetanide-binding site on the parotid Na(+)-K(+)-Cl-cotransporter (J. Membrane Biol. 112:51-58, 1989), provide strong evidence that this protein is a part or all of the parotid bumetanide-binding site. When this protein is treated with endoglycosidase F/N-glycosidase F to remove N-linked oligosaccharides, its apparent molecular weight decreases to 135 kD. The pI of this deglycosylated protein is approximately 6.4. The bumetanide-binding protein was purified using two preparative electrophoresis steps. First, a Triton X-100 extract enriched in this protein was run on preparative electrophoresis to obtain fractions containing proteins in the 160 kD range. These were then deglycosylated with endoglycosidase F/N-glycosidase F and selected fractions were pooled and rerun on preparative electrophoresis to obtain a final 135 kD fraction. The enrichment of the bumetanide-binding protein in this final 135 kD fraction estimated from [14C]-N-ethylmaleimide labeling was approximately 48 times relative to the starting membrane extract. Since the bumetanide-binding site represents approximately 2% of the total protein in this starting extract, this enrichment indicates a high degree of purity of this protein in the 135 kD fraction.

Regulation of the cytosolic pH set point for activation of the Na+/H+ antiport in human platelets: the roles of the Na+/Ca2+ exchange, the Na(+)-K(+)-2Cl- cotransport and cellular volume

To explore further the mechanisms that regulate the Na+/H+ antiport in human platelets, we examined the effect of Na+ pump inhibition by ouabain and K+ removal from the extracellular medium on parameters of this transport system. Treatment with ouabain resulted in increased cytosolic free Ca2+ and Na+, coupled with an alkaline shift in the cytosolic pH set point for the Na+/H+ antiport. Inhibition of the Na+ pump by the removal of K+ from the medium increased the cytosolic Na+ but not the cytosolic Ca2+; yet this treatment also produced a substantial alkaline shift in the cytosolic pH set point for the Na+/H+ antiport. This effect appeared to relate to a decline in cellular volume and it was attenuated by the Na(+)-K(+)-2 Cl- cotransport inhibitor, bumetanide. These findings indicate: (a) a link between the Na+ pump and the Na+/H+ antiport, mediated by the Na+/Ca2+ exchange and the cytosolic free Ca2+, and (b) a link between the Na+/H+ antiport and the Na(+)-K(+)-2Cl- cotransport through cellular volume.

[3H]bumetanide binding to mouse kidney membranes: identification of corresponding membrane proteins

Crude plasma membranes from whole mouse kidneys have two classes of [3H]bumetanide binding sites. High-affinity sites (K1/2 approximately equal to 0.04 microM; Bmax = 1-2 pmol/mg protein) are similar to those identified on dog kidney membranes (B. Forbush and H.C. Palfrey. J. Biol. Chem. 258: 11787-11792, 1983) both with respect to affinity and in that Na, K, and Cl are required for [3H]bumetanide binding. Low-affinity sites (K1/2 approximately equal to 1 microM; Bmax = 7-14 pmol/mg) are unaffected by removal of these ions; such sites are not seen with dog kidney. When mouse kidney membranes are photolabeled with 4-[3H]benzoyl-5-sulfamoyl-3-(3-thenyloxy)benzoic acid [( 3H]BSTBA), a photoreactive bumetanide analogue, specific incorporation of the label is seen in two regions. As with dog kidney [M. Haas and B. Forbush. Am. J. Physiol. 253 (Cell Physiol. 22): C243-C252, 1987], an approximately 150-kDa protein is labeled with high affinity (K1/2 approximately equal to 0.05 microM). This labeling also requires Na, K, and Cl and appears to correspond to the high-affinity [3H]bumetanide binding sites and to the Na-K-Cl cotransport system. A second peak of [3H]BSTBA photolabeling, centered at approximately 75 kDa, incorporates the label with lower affinity (K1/2 = 2-3 microM). The photolabeling at approximately 75 kDa is unaffected by Na, K, and Cl concentrations and thus may correspond, at least in part, to the low-affinity [3H]bumetanide binding sites. Western blot analysis of [3H]BSTBA-labeled mouse kidney membranes was performed using an antiserum raised to proteins of approximately 82 and approximately 39 kDa isolated from mouse Ehrlich ascites tumor cells using a bumetanide affinity gel (P. B. Dunham, F. Jessen, and E. K. Hoffmann. Proc. Natl. Acad. Sci. USA 87: 6828-6832, 1990). This antiserum cross-reacts with a approximately 150-kDa mouse kidney protein, the staining profile of which on Western blot corresponds very closely to the peak of specific [3H]BSTBA incorporation in this region. The antiserum also reacts with proteins in the range of 65-85 kDa, overlapping the low-affinity peak of [3H]BSTBA incorporation.

Role of phospholipids in the binding of bumetanide to the rabbit parotid Na/K/Cl cotransporter

It was recently reported (Turner, R.J., George, J.N., 1990, J. Membrane Biol. 113:203-210) that the high affinity bumetanide binding site of the rabbit parotid Na/K/Cl cotransporter could be extracted from a basolateral membrane preparation from this gland using relatively low concentrations of the non-ionic detergent Triton X-100. At the detergent:protein ratios required for complete membrane solubilization bumetanide binding activity in this extract was lost but could be recovered by the addition of crude soybean lipids. In the present paper the ability of various purified lipids to restore high affinity bumetanide binding activity in detergent solubilized rabbit parotid basolateral membranes is studied. We show that the effect of exogenous lipid on the detergent-inactivated bumetanide binding site is to increase the affinity of binding without affecting the number of binding sites. Of the 11 lipid species tested, several relatively minor, negatively charged membrane phospholipids are the most effective in restoring binding activity (phosphatidylserine approximately phosphatidylglycerol greater than phosphatidylinositol greater than cardiolipin), while the major mammalian plasma membrane lipid components phosphatidylcholine, phosphatidylethanolamine, sphingomyelin and cholesterol are without effect. In addition, we show that in the presence of these minor lipids the affinity of bumetanide binding is considerably increased over that observed in the native membrane (e.g., Kd approximately 0.06 microM in membranes extracted with 0.3% Triton and treated with 0.15% wt/vol phosphatidylserine, vs. Kd approximately 3 microM in native basolateral membranes). This dramatic dependence of bumetanide binding affinity on the presence of certain lipid species suggests that the properties of the bumetanide binding protein in situ may be quite dependent on the minor lipid content of the plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Solubilization and partial purification of the rabbit parotid Na/K/Cl-dependent bumetanide binding site

We demonstrate that the high affinity bumetanide binding site of the rabbit parotid acinar cell can be extracted from a basolateral membrane fraction using relatively low concentrations (0.07%, wt/vol; 1 mg membrane protein/ml) of the nonionic detergent Triton X-100. This extracted site cannot be sedimented by ultracentrifugation at 100,000 x g x 1 hr. Bumetanide binding to this site retains the ionic characteristics of bumetanide binding to native membranes but shows a fivefold increase in binding affinity (Kd = 0.57 +/- 0.15 microM vs. Kd = 3.3 +/- 0.7 microM for native membranes). Inactivation of the extracted bumetanide binding site observed at detergent/protein ratios greater than 1 can be prevented or (partially) reversed by the addition of exogenous lipid (0.2% soybean phosphatidylcholine). When the 0.07% Triton extract is fractionated by sucrose density gradient centrifugation in 0.24% Triton X-100, 0.2% exogenous lipid and 200 mM salt, the high affinity bumetanide binding site sediments as a single band with S20,w = 8.8 +/- 0.8 S. This corresponds to a molecular weight approximately 200 kDa for the bumetanide binding protein-detergent-lipid complex and represents a sevenfold purification of this site relative to the starting membrane fraction. In contrast to previous attempts to purify Na/K/Cl cotransport proteins and their associated bumetanide binding sites, the present method avoids harsh detergent treatment as well as direct covalent modification (inactivation) of the transporter itself. As a consequence, one can follow the still active protein through a series of extraction and purification steps by directly monitoring its bumetanide binding properties.

Inactivation of the rabbit parotid Na/K/Cl cotransporter by N-ethylmaleimide

The inactivation of the rabbit parotid Na/K/Cl cotransporter by the irreversible sulfhydryl reagent N-ethylmaleimide (NEM) is studied by monitoring its effect on high affinity bumetanide binding to the carrier. NEM reduces the number of bumetanide binding sites with no significant change in the affinity of those remaining. NEM also reduces KCl-dependent 22Na flux via the cotransporter by the same factor as the reduction in bumetanide binding sites. Both bumetanide and its analogue furosemide can protect against the effect of NEM. The concentration range over which this protection occurs is in good agreement with affinities of these two compounds for the high affinity bumetanide binding site (2.6 and 8.5 microM, respectively), indicating an association of this site with the site of action of NEM. Also consistent with this hypothesis are the observations that (i) sodium and potassium, both of which are required for high affinity bumetanide binding, increase the rate of inactivation of binding by NEM and (ii) chloride, at concentrations previously shown to competitively inhibit bumetanide binding, protects the cotransporter against NEM. The effects of NEM on bumetanide binding are mimicked by another highly specific sulfhydryl reagent, methyl methanethiolsulfonate. The apparent rate constant for inactivation of high affinity bumetanide binding by NEM is a hyperbolic function of NEM concentration consistent with a model in which the inactivation reaction is first order in [NEM] and proceeds through an intermediate adsorptive complex. The data indicate that the presence of a reduced sulfhydryl group at or closely related to the bumetanide binding site is essential for the operation of the parotid Na/K/Cl cotransporter.