Sodium acts as a potassium analog on gastric H,K-ATPase

Control of gastric H,K-ATPase activity by cations, voltage and intracellular pH analyzed by voltage clamp fluorometry in Xenopus oocytes

Whereas electrogenic partial reactions of the Na,K-ATPase have been studied in depth, much less is known about the influence of the membrane potential on the electroneutrally operating gastric H,K-ATPase. In this work, we investigated site-specifically fluorescence-labeled H,K-ATPase expressed in Xenopus oocytes by voltage clamp fluorometry to monitor the voltage-dependent distribution between E₁P and E₂P states and measured Rb⁺ uptake under various ionic and pH conditions. The steady-state E₁P/E₂P distribution, as indicated by the voltage-dependent fluorescence amplitudes and the Rb⁺ uptake activity were highly sensitive to small changes in intracellular pH, whereas even large extracellular pH changes affected neither the E₁P/E₂P distribution nor transport activity. Notably, intracellular acidification by approximately 0.5 pH units shifted V_0.5, the voltage, at which the E₁P/E₂P ratio is 50∶50, by −100 mV. This was paralleled by an approximately two-fold acceleration of the forward rate constant of the E₁P→E₂P transition and a similar increase in the rate of steady-state cation transport. The temperature dependence of Rb⁺ uptake yielded an activation energy of ∼90 kJ/mol, suggesting that ion transport is rate-limited by a major conformational transition. The pronounced sensitivity towards intracellular pH suggests that proton uptake from the cytoplasmic side controls the level of phosphoenzyme entering the E₁P→E₂P conformational transition, thus limiting ion transport of the gastric H,K-ATPase. These findings highlight the significance of cellular mechanisms contributing to increased proton availability in the cytoplasm of gastric parietal cells. Furthermore, we show that extracellular Na⁺ profoundly alters the voltage-dependent E₁P/E₂P distribution indicating that Na⁺ ions can act as surrogates for protons regarding the E₂P→E₁P transition. The complexity of the intra- and extracellular cation effects can be rationalized by a kinetic model suggesting that cations reach the binding sites through a rather high-field intra- and a rather low-field extracellular access channel, with fractional electrical distances of ∼0.5 and ∼0.2, respectively.

Mineralocorticoids stimulate the activity and expression of renal H+,K+-ATPases

In the renal collecting duct, mineralocorticoids drive Na(+) reabsorption, K(+) secretion, and H(+) secretion through coordinated actions on apical and basolateral transporters. Whether mineralocorticoids act through H(+),K(+)-ATPases to maintain K(+) and acid-base homeostasis is unknown. Here, treatment of mice with the mineralocorticoid desoxycorticosterone pivalate (DOCP) resulted in weight gain, a decrease in blood [K(+)] and [Cl(-)], and an increase in blood [Na(+)] and [HCO(3)(-)]. DOCP treatment increased the rate of H(+),K(+)-ATPase-mediated H(+) secretion in intercalated cells of the inner cortical collecting duct. mRNA expression of the catalytic subunit HKα(1) did not significantly change, whereas HKα(2) mRNA expression dramatically increased in the outer and inner medulla of DOCP-treated mice. A high-K(+) diet abrogated this increase in renal HKα(2) expression, showing that DOCP-mediated stimulation of HKα(2) expression depends on dietary K(+) intake. DOCP treatment of mice lacking HKα(1) (HKα(1)(-/-)) resulted in greater urinary Na(+) retention than observed in either wild-type mice or mice lacking both HKα(1) and HKα(2) (HKα(1,2)(-/-)). DOCP-treated HKα(1,2)(-/-) mice exhibited a lower blood [HCO(3)(-)] and less Na(+) and K(+) retention than either wild-type or HKα(1)(-/-) mice. Taken together, these results indicate that H(+),K(+)-ATPases-especially the HKα(2)-containing H(+),K(+)-ATPases-play an important role in the effects of mineralocorticoids on K(+), acid-base, and Na(+) balance.

Pharmacological profiles of the murine gastric and colonic H,K-ATPases

BACKGROUND

The H,K-ATPase, consisting of α and ß subunits, belongs to the P-type ATPase family. There are two isoforms of the α subunit, HKα₁ and HKα₂ encoded by different genes. The ouabain-resistant gastric HKα₁-H,K-ATPase is Sch28080-sensitive. However, the colonic HKα₂-H,K-ATPase from different species shows poor primary structure conservation of the HKα₂ subunit between species and diverse pharmacological sensitivity to ouabain and Sch28080. This study sought to determine the contribution of each gene to functional activity and its pharmacological profile using mouse models with targeted disruption of HKα₁, HKα₂, or HKbeta genes.

METHODS

Membrane vesicles from gastric mucosa and distal colon in wild-type (WT), HKα₁, HKα₂, or HKß knockout (KO) mice were extracted. K-ATPase activity and pharmacological profiles were examined.

RESULTS

The colonic H,K-ATPase demonstrated slightly greater affinity for K(+) than the gastric H,K-ATPase. This K-ATPase activity was not detected in the colon of HKα₂ KO but was observed in HKß KO with properties indistinguishable from WT. Neither ouabain nor Sch28080 had a significant effect on the WT colonic K-ATPase activity, but orthovanadate abolished this activity. Amiloride and its analogs benzamil and 5-N-ethyl-N-isopropylamiloride inhibited K-ATPase activity of HKα₁-containing H,K-ATPase; the dose dependence of inhibition was similar for all three inhibitors. In contrast, the colonic HKα₂-H,K-ATPase was not inhibited by these compounds.

CONCLUSIONS

These data demonstrate that the mouse colonic H,K-ATPase exhibits a ouabain- and Sch28080-insensitive, orthovanadate-sensitive K-ATPase activity. Interestingly, pharmacological studies suggested that the mouse gastric H,K-ATPase is sensitive to amiloride.

GENERAL SIGNIFICANCE

Characterization of the pharmacological profiles of the H,K-ATPases is important for understanding the relevant knockout animals and for considering the specificity of the inhibitors.

The renal H+-K+-ATPases: physiology, regulation, and structure

The H(+)-K(+)-ATPases are ion pumps that use the energy of ATP hydrolysis to transport protons (H(+)) in exchange for potassium ions (K(+)). These enzymes consist of a catalytic alpha-subunit and a regulatory beta-subunit. There are two catalytic subunits present in the kidney, the gastric or HKalpha(1) isoform and the colonic or HKalpha(2) isoform. In this review we discuss new information on the physiological function, regulation, and structure of the renal H(+)-K(+)-ATPases. Evaluation of enzymatic functions along the nephron and collecting duct and studies in HKalpha(1) and HKalpha(2) knockout mice suggest that the H(+)-K(+)-ATPases may function to transport ions other than protons and potassium. These reports and recent studies in mice lacking both HKalpha(1) and HKalpha(2) suggest important roles for the renal H(+)-K(+)-ATPases in acid/base balance as well as potassium and sodium homeostasis. Molecular modeling studies based on the crystal structure of a related enzyme have made it possible to evaluate the structures of HKalpha(1) and HKalpha(2) and provide a means to study the specific cation transport properties of H(+)-K(+)-ATPases. Studies to characterize the cation specificity of these enzymes under different physiological conditions are necessary to fully understand the role of the H(+)-K(+) ATPases in renal physiology.

The human non-gastric H,K-ATPase has a different cation specificity than the rat enzyme

The primary sequence of non-gastric H,K-ATPase differs much more between species than that of Na,K-ATPase or gastric H,K-ATPase. To investigate whether this causes species-dependent differences in enzymatic properties, we co-expressed the catalytic subunit of human non-gastric H,K-ATPase in Sf9 cells with the beta(1) subunit of rat Na,K-ATPase and compared its properties with those of the rat enzyme (Swarts et al., J. Biol. Chem. 280, 33115-33122, 2005). Maximal ATPase activity was obtained with NH(4)(+) as activating cation. The enzyme was also stimulated by Na(+), but in contrast to the rat enzyme, hardly by K(+). SCH 28080 inhibited the NH(4)(+)-stimulated activity of the human enzyme much more potently than that of the rat enzyme. The steady-state phosphorylation level of the human enzyme decreased with increasing pH, [K(+)], and [Na(+)] and nearly doubled in the presence of oligomycin. Oligomycin increased the sensitivity of the phosphorylated intermediate to ADP, demonstrating that it inhibited the conversion of E(1)P to E(2)P. All three cations stimulated the dephosphorylation rate dose-dependently. Our studies support a role of the human enzyme in H(+)/Na(+) and/or H(+)/NH(4)(+) transport but not in Na(+)/K(+) transport.

Conversion of the Low Affinity Ouabain-binding Site of Non-gastric H,K-ATPase into a High Affinity Binding Site by Substitution of Only Five Amino Acids

P-type ATPases of the IIC subfamily exhibit large differences in sensitivity toward ouabain. This allows a strategy in which ouabain-insensitive members of this subfamily are used as template for mutational elucidation of the ouabain-binding site. With this strategy, we recently identified seven amino acids in Na,K-ATPase that conferred high affinity ouabain binding to gastric H,K-ATPase (Qiu, L. Y., Krieger, E., Schaftenaar, G., Swarts, H. G. P., Willems, P. H. G. M., De Pont, J. J. H. H. M., and Koenderink, J. B. (2005) J. Biol. Chem. 280, 32349-32355). Because important, but identical, amino acids were not recognized in that study, here we used the non-gastric H,K-ATPase, which is rather ouabain-insensitive, as template. The catalytic subunit of this enzyme, in which several amino acids from Na,K-ATPase were incorporated, was expressed with the Na,K-ATPase beta1 subunit in Xenopus laevis oocytes. A chimera containing 14 amino acids, located in M4, M5, and M6, which are unique to Na,K-ATPase, displayed high affinity ouabain binding. Four of these residues, all located in M5, appeared dispensable for high affinity binding. Individual mutation of the remaining 10 residues to their non-gastric H,K-ATPase counterparts yielded five amino acids (Glu312,Gly319, Pro778, Leu795, and Cys802) whose mutation resulted in a loss of ouabain binding. In a final gain-of-function experiment, we introduced these five amino acids in different combinations in non-gastric H,K-ATPase and demonstrated that all five were essential for high affinity ouabain binding. The non-gastric H,K-ATPase with these five mutations had a similar apparent affinity for ouabain as the wild type Na,K-ATPase and showed a 2000 times increased affinity for ouabain in the NH4+-stimulated ATPase activity in membranes of transfected Sf9 cells.

The coxsackievirus 2B protein increases efflux of ions from the endoplasmic reticulum and Golgi, thereby inhibiting protein trafficking through the Golgi

Coxsackievirus infection leads to a rapid reduction of the filling state of the endoplasmic reticulum (ER) and Golgi Ca2+ stores. The coxsackievirus 2B protein, a small membrane protein that localizes to the Golgi and to a lesser extent to the ER, has been proposed to play an important role in this effect by forming membrane-integral pores, thereby increasing the efflux of Ca2+ from the stores. Here, evidence is presented that supports this idea and that excludes the possibility that 2B reduces the uptake of Ca2+ into the stores. Measurement of intra-organelle-free Ca2+ in permeabilized cells revealed that the ability of 2B to reduce the Ca2+ filling state of the stores was preserved at steady ATP. Biochemical analysis in a cell-free system further showed that 2B had no adverse effect on the activity of the sarco/endoplasmic reticulum calcium ATPase, the Ca2+-ATPase that transports Ca2+ from the cytosol into the stores. To investigate whether 2B specifically affects Ca2+ homeostasis or other ion gradients, we measured the lumenal Golgi pH. Expression of 2B resulted in an increased Golgi pH, indicative for the efflux of H+ from the Golgi lumen. Together, these data support a model that 2B increases the efflux of ions from the ER and Golgi by forming membrane-integral pores. We have demonstrated that a major consequence of this activity is the inhibition of protein trafficking through the Golgi complex.

The non-gastric H,K-ATPase is oligomycin-sensitive and can function as an H+,NH4(+)-ATPase

We used the baculovirus/Sf9 expression system to gain new information on the mechanistic properties of the rat non-gastric H,K-ATPase, an enzyme that is implicated in potassium homeostasis. The alpha2-subunit of this enzyme (HKalpha2) required a beta-subunit for ATPase activity thereby showing a clear preference for NaKbeta1 over NaKbeta3 and gastric HKbeta. NH4(+), K+, and Na+ maximally increased the activity of HKalpha2-NaKbeta1 to 24.0, 14.2, and 5.0 micromol P(i) x mg(-1) protein x h(-1), respectively. The enzyme was inhibited by relatively high concentrations of ouabain and SCH 28080, whereas it was potently inhibited by oligomycin. From the phosphorylation level in the presence of oligomycin and the maximal NH4(+)-stimulated ATPase activity, a turnover number of 20,000 min(-1) was determined. All three cations decreased the steady-state phosphorylation level and enhanced the dephosphorylation rate, disfavoring the hypothesis that Na+ can replace H+ as the activating cation. The potency with which vanadate inhibited the cation-activated enzyme decreased in the order K+ > NH4(+) > Na+, indicating that K+ is a stronger E2 promoter than NH4(+), whereas in the presence of Na+ the enzyme is in the E1 form. For K+ and NH4(+), the E2 to E1 conformational equilibrium correlated with their efficacy in the ATPase reaction, indicating that here the transition from E2 to E1 is rate-limiting. Conversely, the low maximal ATPase activity with Na+ is explained by a poor stimulatory effect on the dephosphorylation rate. These data show that NH4(+) can replace K+ with similar affinity but higher efficacy as an extracellular activating cation in rat nongastric H,K-ATPase.

Asn792 participates in the hydrogen bond network around the K+-binding pocket of gastric H,K-ATPase

Asn792 present in M5 of gastric H,K-ATPase is highly conserved within the P-type ATPase family. A direct role in K+ binding was postulated for Na,K-ATPase but was not found in a recent model for gastric H,K-ATPase (Koenderink, J. B., Swarts, H. G. P., Willems, P. H. G. M., Krieger, E., and De Pont, J. J. H. H. M. (2004) J. Biol. Chem. 279, 16417-16424). Therefore, its role in K+ binding and E1/E2 conformational equilibrium in gastric H,K-ATPase was studied by site-directed mutagenesis and expression in Sf9 cells. N792Q and N792A, but not N792D and N792E, had a markedly reduced K+ affinity in both the ATPase and dephosphorylation reactions. In addition, N792A shifted the conformational equilibrium to the E1 form. In double mutants, the effect of N792A on K+ sensitivity was overruled by either E820Q (K(+)-independent activity) or E343D (no dephosphorylation activity). Models were made for the mutants based on the E2 structure of Ca(2+)-ATPase. In the wild-type model the acid amide group of Asn792 has hydrogen bridges to Lys791, Ala339, and Val341. Comparison of the effects of the various mutants suggests that the hydrogen bridge between the carbonyl oxygen of Asn792 and the amino group of Lys791 is essential for the K+ sensitivity and the E2 preference of wild-type enzyme. Moreover, there was a high positive correlation (r = 0.98) between the in silico calculated energy difference of the E2 form (mutants versus wild type) and the experimentally measured IC50 values for vanadate, which reflects the direction of the E2<-->E1 conformational equilibrium. These data strongly support the validity of the model in which Asn792 participates in the hydrogen bond network around the K(+)-binding pocket.

Novel p-type ATPases mediate high-affinity potassium or sodium uptake in fungi

Fungi have an absolute requirement for K+, but K+ may be partially replaced by Na+. Na+ uptake in Ustilago maydis and Pichia sorbitophila was found to exhibit a fast rate, low Km, and apparent independence of the membrane potential. Searches of sequences with similarity to P-type ATPases in databases allowed us to identify three genes in these species, Umacu1, Umacu2, and PsACU1, that could encode P-type ATPases of a novel type. Deletion of the acu1 and acu2 genes proved that they encoded the transporters that mediated the high-affinity Na+ uptake of U. maydis. Heterologous expressions of the Umacu2 gene in K+ transport mutants of Saccharomyces cerevisiae and transport studies in the single and double Deltaacu1 and Deltaacu2 mutants of U. maydis revealed that the acu1 and acu2 genes encode transporters that mediated high-affinity K+ uptake in addition to Na+ uptake. Other fungi also have genes or pseudogenes whose translated sequences show high similarity to the ACU proteins of U. maydis and P. sorbitophila. In the phylogenetic tree of P-type ATPases all the identified ACU ATPases define a new cluster, which shows the lowest divergence with type IIC, animal Na+,K(+)-ATPases. The fungal high-affinity Na+ uptake mediated by ACU ATPases is functionally identical to the uptake that is mediated by some plant HKT transporters.

A conformation-specific interhelical salt bridge in the K+ binding site of gastric H,K-ATPase

Homology modeling of gastric H,K-ATPase based on the E2 model of sarcoplasmic reticulum Ca2+-ATPase (Toyoshima, C., and Nomura, H. (2002) Nature 392, 835-839) revealed the presence of a single high-affinity binding site for K+ and an E2 form-specific salt bridge between Glu820 (M6) and Lys791 (M5). In the E820Q mutant this salt bridge is no longer possible, and the head group of Lys791, together with a water molecule, fills the position of the K+ ion and apparently mimics the K+-filled cation binding pocket. This gives an explanation for the K+-independent ATPase activity and dephosphorylation step of the E820Q mutant (Swarts, H. G. P., Hermsen, H. P. H., Koenderink, J. B., Schuurmans Stekhoven, F. M. A. H., and De Pont, J. J. H. H. M. (1998) EMBO J. 17, 3029-3035) and, indirectly, for its E1 preference. The model is strongly supported by a series of reported mutagenesis studies on charged and polar amino acid residues in the membrane domain. To further test this model, Lys791 was mutated alone and in combination with other crucial residues. In the K791A mutant, the K+ affinity was markedly reduced without altering the E2 preference of the enzyme. The K791A mutation prevented, in contrast to the K791R mutation, the spontaneous dephosphorylation of the E820Q mutant as well as its conformational equilibrium change toward E1. This indicates that the salt bridge is essential for high-affinity K+ binding and the E2 preference of H,K-ATPase. Moreover, its breakage (E820Q) can generate a K+-insensitive activity and an E1 preference. In addition, the study gives a molecular explanation for the electroneutrality of H,K-ATPases.

Osteocyte control of bone formation via sclerostin, a novel BMP antagonist

There is an unmet medical need for anabolic treatments to restore lost bone. Human genetic bone disorders provide insight into bone regulatory processes. Sclerosteosis is a disease typified by high bone mass due to the loss of SOST expression. Sclerostin, the SOST gene protein product, competed with the type I and type II bone morphogenetic protein (BMP) receptors for binding to BMPs, decreased BMP signaling and suppressed mineralization of osteoblastic cells. SOST expression was detected in cultured osteoblasts and in mineralizing areas of the skeleton, but not in osteoclasts. Strong expression in osteocytes suggested that sclerostin expressed by these central regulatory cells mediates bone homeostasis. Transgenic mice overexpressing SOST exhibited low bone mass and decreased bone strength as the result of a significant reduction in osteoblast activity and subsequently, bone formation. Modulation of this osteocyte-derived negative signal is therapeutically relevant for disorders associated with bone loss.

K(+)-independent gastric H(+),K(+)-atpase activity. Dissociation of K(+)-independent dephosphorylation and preference for the E1 conformation by combined mutagenesis of transmembrane glutamate residues

Several mutations of residues Glu(795) and Glu(820) present in M5 and M6 of the catalytic subunit of gastric H(+),K(+)-ATPase have resulted in a K(+)-independent, SCH 28080-sensitive ATPase activity, caused by a high spontaneous dephosphorylation rate. The mutants with this property also have a preference for the E(1) conformation. This paper investigates the question of whether these two phenomena are coupled. This possibility was studied by combining mutations in residue Glu(343), present in M4, with those in residues 795 and 820. When in combined mutants Glu and/or Gln residues were present at positions 343, 795, and 820, the residue at position 820 dominated the behavior: a Glu giving K(+)-activated ATPase activity and an E(2) preference and a Gln giving K(+)-independent ATPase activity and an E(1) preference. With an Asp at position 343, the enzyme could be phosphorylated, but the dephosphorylation was blocked, independent of the presence of either a Glu or a Gln at positions 795 and 820. However, in these mutants, the direction of the E(2) <--> E(1) equilibrium was still dominated by the 820 residue: a Glu giving E(2) and a Gln giving E(1). This indicates that the preference for the E(1) conformation of the E820Q mutation is independent of an active dephosphorylation process.

Structural modifications in the membrane-bound regions of the gastric H+/K+-ATPase upon ligand binding

Extensive trypsin proteolysis was used to examine the accessibility of membrane bound segments of the gastric H+/K+-ATPase under different experimental conditions known to induce either the E1 or the E2 conformation. Membrane-anchored peptides were isolated after trypsinolysis and identified by sequencing. We show that several membrane bound segments are involved in the conformational change. In the N-terminal region, a M1-M2 peptide (12 kDa) was found to be associated with the membrane fraction after digestion in the presence of K+ or in the presence of vanadate (12 kDa and 15 kDa). In the M3 and M4 region, no difference was observed for the peptide obtained in E1 or E2-K conformations, but the peptide generated in the presence of vanadate begins 12 amino-acid residues earlier in the sequence. Cytoplasmic loop region: we show here that a peptide beginning at Asp574 and predicted to end at Arg693 is associated with the membrane for a vanadate-induced conformation. In the M5-M6 region, the membrane-anchored peptide obtained on E1 is 39 amino acids shorter than the E2 peptide. In the M7-M8 region, the same peptide encompassing the M7 and M8 transmembrane segments was produced for E1 and E2 conformations.

Chimeras of X+, K+-ATPases. The M1-M6 region of Na+, K+-ATPase is required for Na+-activated ATPase activity, whereas the M7-M10 region of H+, K+-ATPase is involved in K+ de-occlusion

In this study we reveal regions of Na(+),K(+)-ATPase and H(+),K(+)-ATPase that are involved in cation selectivity. A chimeric enzyme in which transmembrane hairpin M5-M6 of H(+),K(+)-ATPase was replaced by that of Na(+),K(+)-ATPase was phosphorylated in the absence of Na(+) and showed no K(+)-dependent reactions. Next, the part originating from Na(+),K(+)-ATPase was gradually increased in the N-terminal direction. We demonstrate that chimera HN16, containing the transmembrane segments one to six and intermediate loops of Na(+),K(+)-ATPase, harbors the amino acids responsible for Na(+) specificity. Compared with Na(+),K(+)-ATPase, this chimera displayed a similar apparent Na(+) affinity, a lower apparent K(+) affinity, a higher apparent ATP affinity, and a lower apparent vanadate affinity in the ATPase reaction. This indicates that the E(2)K form of this chimera is less stable than that of Na(+),K(+)-ATPase, suggesting that it, like H(+),K(+)-ATPase, de-occludes K(+) ions very rapidly. Comparison of the structures of these chimeras with those of the parent enzymes suggests that the C-terminal 187 amino acids and the beta-subunit are involved in K(+) occlusion. Accordingly, chimera HN16 is not only a chimeric enzyme in structure, but also in function. On one hand it possesses the Na(+)-stimulated ATPase reaction of Na(+),K(+)-ATPase, while on the other hand it has the K(+) occlusion properties of H(+),K(+)-ATPase.

Significance of lysine/glycine cluster structure in gastric H+,K+-ATPase

Gastric H+,K+-ATPase consists of alpha- and beta-subunits. The catalytic alpha-subunit contains a very unique structure consisting of lysine and glycine clusters, KKK(or KKKK)AG(G/R)GGGK-(K/R)K, in the amino-terminal cytoplasmic region. This structure is well conserved in all gastric H+,K+-ATPases from different animal species, and was postulated to be the site controlling the access of cations (or proton) to its binding site. In this report, we studied the role of this unique structure by expressing several H+,K+-ATPase mutants of the alpha-subunit together with the wild-type beta-subunit in HEK-293 cells. Even after replacing all the positively-charged amino acid residues (six lysines and one arginine) in the cluster with alanine or removing all the glycine residues in the cluster, the mutants preserved the H+,K+-ATPase activity, and showed similar affinity for ATP and K+ as well as similar pH profiles as those of wild-type H+,K+-ATPase, indicating that the cluster is not indispensable for H+,K+-ATPase activity and not directly involved in determination of the affinity for cation (proton).

H+-K+-ATPases: regulation and role in pathophysiological states

Molecular cloning experiments have identified the existence of two H+-K+-ATPases (HKAs), colonic and gastric. Recent functional and molecular studies indicate the presence of both transporters in the kidney, which are presumed to mediate the exchange of intracellular H+ for extracellular K+. On the basis of these studies, a picture is evolving that indicates differential regulation of HKAs at the molecular level in acid-base and electrolyte disorders. Of the two transporters, gastric HKA is expressed constitutively along the length of the collecting duct and is responsible for H+ secretion and K+ reabsorption under normal conditions and may be stimulated with acid-base perturbations and/or K+ depletion. This regulation may be species specific. To date there are no data to indicate that the colonic HKA (HKAc) plays a role in H+ secretion or K+ reabsorption under normal conditions. However, HKAc shows adaptive regulation in pathophysiological conditions such as K+ depletion, NaCl deficiency, and proximal renal tubular acidosis, suggesting an important role for this exchanger in potassium, HCO-3, and sodium (or chloride) reabsorption in disease states. The purpose of this review is to summarize recent functional and molecular studies on the regulation of HKAs in physiological and pathophysiological states. Possible signals responsible for regulation of HKAs in these conditions will be discussed. Furthermore, the role of these transporters in acid-base and electrolyte homeostasis will be evaluated in the context of genetically altered animals deficient in HKAc.

The beta-subunits of Na+,K+-ATPase and gastric H+,K+-ATPase have a high preference for their own alpha-subunit and affect the K+ affinity of these enzymes

The alpha- and beta-subunits of Na+,K+-ATPase and H+,K+-ATPase were expressed in Sf9 cells in different combinations. Immunoprecipitation of the alpha-subunits resulted in coprecipitation of the accompanying beta-subunit independent of the type of beta-subunit. This indicates cross-assembly of the subunits of the different ATPases. The hybrid ATPase with the catalytic subunit of Na+,K+-ATPase and the beta-subunit of H+,K+-ATPase (NaKalphaHKbeta) showed an ATPase activity, which was only 12 +/- 4% of the activity of the Na+,K+-ATPase with its own beta-subunit. Likewise, the complementary hybrid ATPase with the catalytic subunit of H+,K+-ATPase and the beta-subunit of Na+,K+-ATPase (HKalphaNaKbeta) showed an ATPase activity which was 9 +/- 2% of that of the recombinant H+,K+-ATPase. In addition, the apparent K+ affinity of hybrid NaKalphaHKbeta was decreased, while the apparent K+ affinity of the opposite hybrid HKalphaNaKbeta was increased. The hybrid NaKalphaHKbeta could be phosphorylated by ATP to a level of 21 +/- 7% of that of Na+,K+-ATPase. These values, together with the ATPase activity gave turnover numbers for NaKalphabeta and NaKalphaHKbeta of 8800 +/- 310 min-1 and 4800 +/- 160 min-1, respectively. Measurements of phosphorylation of the HKalphaNaKbeta and HKalphabeta enzymes are consistent with a higher turnover of the former. These findings suggest a role of the beta-subunit in the catalytic turnover. In conclusion, although both Na+,K+-ATPase and H+,K+-ATPase have a high preference for their own beta-subunit, they can function with the beta-subunit of the other enzyme, in which case the K+ affinity and turnover number are modified.

Mutagenesis of glutamate 820 of the gastric H+,K+-ATPase alpha-subunit to aspartate decreases the apparent ATP affinity

Mutagenesis of Glu820, present in the catalytic subunit of gastric H+,K+-ATPase, into an Asp hardly affects K+-stimulated ATPase and K+-stimulated dephosphorylation of the enzyme. The ATP phosphorylation rate of the E820D mutant, however, is rather low and the apparent affinity for ATP in the phosphorylation process of this mutant is 2-3 times lower than that of the wild type enzyme. The reduction in the ATP phosphorylation rate of the E820D mutant has only an effect on the ATPase activity at low temperature. These findings suggest that Glu820 might play a role in H+ stimulation of the phosphorylation process.

Constitutive activation of gastric H+,K+-ATPase by a single mutation

In the reaction cycle of P-type ATPases, an acid-stable phosphorylated intermediate is formed which is present in an intracellularly located domain of the membrane-bound enzymes. In some of these ATPases, such as Na+,K+-ATPase and gastric H+, K+-ATPase, extracellular K+ ions stimulate the rate of dephosphorylation of this phosphorylated intermediate and so stimulate the ATPase activity. The mechanism by which extracellular K+ ions stimulate the dephosphorylation process is unresolved. Here we show that three mutants of gastric H+,K+-ATPase lacking a negative charge on residue 820, located in transmembrane segment six of the alpha-subunit, have a high SCH 28080-sensitive, but K+-insensitive ATPase activity. This high activity is caused by an increased 'spontaneous' rate of dephosphorylation of the phosphorylated intermediate. A mutant with an aspartic acid instead of a glutamic acid residue in position 820 showed hardly any ATPase activity in the absence of K+, but K+ ions stimulated ATPase activity and the dephosphorylation process. These findings indicate that the negative charge normally present on residue 820 inhibits the dephosphorylation process. K+ ions do not stimulate dephosphorylation of the phosphorylated intermediate directly, but act by neutralizing the inhibitory effect of a negative charge in the membrane.

Does the colonic H,K-ATPase also act as an Na,K-ATPase?

We previously have demonstrated that the colonic P-ATPase alpha subunit cDNA encodes an H,K-ATPase when expressed in Xenopus laevis oocytes. Besides its high level of amino acid homology (75%) with the Na,K-ATPase, the colonic H,K-ATPase also shares a common pharmacological profile with Na,K-ATPase, because both are ouabain-sensitive and Sch 28080-insensitive. These features raise the possibility that an unrecognized property of the colonic H, K-ATPase would be Na+ translocation. To test this hypothesis, ion-selective microelectrodes were used to measure the intracellular Na+ activity of X. laevis oocytes expressing various combinations of P-ATPase subunits. The results show that expression in oocytes of the colonic H,K-ATPase affects intracellular Na+ homeostasis in a way similar to the expression of the Bufo marinus Na,K-ATPase; intracellular Na+ activity is lower in oocytes expressing the colonic H,K-ATPase or the B. marinus Na,K-ATPase than in oocytes expressing the gastric H,K-ATPase or a beta subunit alone. In oocytes expressing the colonic H,K-ATPase, the decrease in intracellular Na+ activity persists when diffusive Na+ influx is enhanced by functional expression of the amiloride-sensitive epithelial Na+ channel, suggesting that the decrease is related to increased active Na+ efflux. The Na+ decrease depends on the presence of K+ in the external medium and is inhibited by 2 mM ouabain, a concentration that inhibits the colonic H,K-ATPase. These data are consistent with the hypothesis that the colonic H,K-ATPase may transport Na+, acting as an (Na,H),K-ATPase. Despite its molecular and functional characterization, the physiological role of the colonic (Na,H),K-ATPase in colonic and renal ion homeostasis remains to be elucidated.

The negative charge of glutamic acid-820 in the gastric H+,K+-ATPase alpha-subunit is essential for K+ activation of the enzyme activity

To investigate the role of Glu820, located in transmembrane domain M6 of the alpha-subunit of gastric H+,K+-ATPase, a number of mutants was prepared and expressed in Sf9 cells using a baculovirus encoding for both H+,K+-ATPase subunits. The wild-type enzyme and the E820D (Glu820-->Asp) mutant showed a similar biphasic activation by K+ on the ATPase activity (maximum at 1 mM). The mutant E820A had a markedly decreased K+ affinity (maximum at 40-100 mM). The other mutants, E820Q, E820N, E820L and E820K, showed no K+-activated ATPase activity at all, whereas all mutants formed a phosphorylated intermediate. After preincubation with K+ before phosphorylation mutant E820D showed a similar K+-sensitivity as the wild-type enzyme. The mutants E820N and E820Q had a 10-20 times lower sensitivity, whereas the other three mutants were hardly sensitive towards K+. Upon preincubation with 3-(cyanomethyl)-2-methyl-8-(phenylmethoxy) imidazo [1,2a]-pyridine (SCH28080), all mutants showed similar sensitivity for this drug as the wild-type enzyme, except mutant E820Q, which could only partly be inhibited, and mutant E820K, which was completely insensitive towards SCH28080. These experiments suggest that, with a relatively large residue at position 820, the binding of SCH28080 is obstructed. The various mutants showed a behaviour in K+-stimulated-dephosphorylation experiments similar to that for K+-activated-ATPase-activity measurements. These results indicate that K+ binding, and indirectly the transition to the E2 form, is only fully possible when a negatively charged residue is present at position 820 in the alpha-subunit.

Role of negatively charged residues in the fifth and sixth transmembrane domains of the catalytic subunit of gastric H+,K+-ATPase

The role of six negatively charged residues located in or around the fifth and sixth transmembrane domain of the catalytic subunit of gastric H+,K+-ATPase, which are conserved in P-type ATPases, was investigated by site-directed mutagenesis of each of these residues. The acid residues were converted into their corresponding acid amides. Sf9 cells were used as the expression system using a baculovirus with coding sequences for the alpha- and beta-subunits of H+,K+-ATPase behind two different promoters. Both subunits of all mutants were expressed like the wild type enzyme in intracellular membranes of Sf9 cells as indicated by Western blotting experiments, an enzyme-linked immunosorbent assay, and confocal laser scan microscopy studies. The mutants D824N, E834Q, E837Q, and D839N showed no 3-(cyanomethyl)-2-methyl-8(phenylmethoxy)-imidazo[1, 2a]pyridine (SCH 28080)-sensitive ATP dependent phosphorylation capacity. Mutants E795Q and E820Q formed a phosphorylated intermediate, which, like the wild type enzyme, was hydroxylamine-sensitive, indicating that an acylphosphate was formed. Formation of the phosphorylated intermediate from the E795Q mutant was similarly inhibited by K+ (I50 = 0.4 mM) and SCH 28080 (I50 = 10 nM) as the wild type enzyme, when the membranes were preincubated with these ligands before phosphorylation. The dephosphorylation reaction was K+-sensitive, whereas ADP had hardly any effect. Formation of the phosphorylated intermediate of mutant E820Q was much less sensitive toward K+ (I50 = 4.5 mM) and SCH 28080 (I50 = 1.7 microM) than the wild type enzyme. The dephosphorylation reaction of this intermediate was not stimulated by either K+ or ADP. In contrast to the wild type enzyme and mutant E795Q, mutant E820Q did not show any K+-stimulated ATPase activity. These findings indicate that residue Glu820 might be involved in K+ binding and transition to the E2 form of gastric H+,K+-ATPase.