Vonoprazan-amoxicillin dual regimen with Saccharomyces boulardii as a rescue therapy for Helicobacter pylori: Current perspectives and implications.

Yu et al's study in the World Journal of Gastroenterology (2023) introduced a novel regimen of Vonoprazan-amoxicillin dual therapy combined with Saccharomyces boulardii (S. boulardii) for the rescue therapy against Helicobacter pylori (H. pylori), a pathogen responsible for peptic ulcers and gastric cancer. Vonoprazan is a potassium-competitive acid blocker renowned for its rapid and long-lasting acid suppression, which is minimally affected by mealtime. Compared to proton pump inhibitors, which bind irreversibly to cysteine residues in the H+/K+-ATPase pump, Vonoprazan competes with the K+ ions, prevents the ions from binding to the pump and blocks acid secretion. Concerns with increasing antibiotic resistance, effects on the gut microbiota, patient compliance, and side effects have led to the advent of a dual regimen for H. pylori. Previous studies suggested that S. boulardii plays a role in stabilizing the gut barrier which improves H. pylori eradication rate. With an acceptable safety profile, the dual-adjunct regimen was effective regardless of prior treatment failure and antibiotic resistance profile, thereby strengthening the applicability in clinical settings. Nonetheless, S. boulardii comes in various formulations and dosages, warranting further exploration into the optimal dosage for supplementation in rescue therapy. Additionally, larger, randomized, double-blinded controlled trials are warranted to confirm these promising results.

Bioinformatic Study of Possible Acute Regulation of Acid Secretion in the Stomach.

The gastric H+,K+-ATPase is an integral membrane protein which derives energy from the hydrolysis of ATP to transport H+ ions from the parietal cells of the gastric mucosa into the stomach in exchange for K+ ions. It is responsible for the acidic environment of the stomach, which is essential for digestion. Acid secretion is regulated by the recruitment of the H+,K+-ATPase from intracellular stores into the plasma membrane on the ingestion of food. The similar amino acid sequences of the lysine-rich N-termini α-subunits of the H+,K+- and Na+,K+-ATPases, suggests similar acute regulation mechanisms, specifically, an electrostatic switch mechanism involving an interaction of the N-terminal tail with the surface of the surrounding membrane and a modulation of the interaction via regulatory phosphorylation by protein kinases. From a consideration of sequence alignment of the H+,K+-ATPase and an analysis of its coevolution with protein kinase C and kinases of the Src family, the evidence points towards a phosphorylation of tyrosine-7 of the N-terminus by either Lck or Yes in all vertebrates except cartilaginous fish. The results obtained will guide and focus future experimental research.

[Intracellular Mechanism of Gastric Acid Secretion: What is the True Switch?]

In 1985, I was accepted as postdoc by Professor Forte of UC Berkeley. He discovered H+,K+-ATPase and established the membrane recycling theory as the activation mechanism for acid secretion using whole animals. H+,K+-ATPase is an enzyme that exchanges H+ with K+. In resting state, it locates on the tubulovesicles and the pump does not work because the membrane lacks K+ permeability. Upon stimulation, the tubulovesicles fuse to the apical membrane and acquire K+ permeability, turning the pump on. The main route was known to be protein kinase A (PKA), but its specific targets remained unknown. Right after I joined Forte's lab, I was fortunate enough to reproduce the above mechanism in vitro, and I discovered proteins of molecular weight 120 kDa and 80 kDa that were specifically phosphorylated in stimulated parietal cells. After I returned to Japan, the former was cloned and named as parchorin, which is one of the chloride intracellular channels. Although no progress was made on ezrin, I found out the importance of PIP2 and Arf6 using permeabilized gland models. After I left parietal cell research, the link between ezrin and Arf6 was revealed. PKA phosphorylates S66 of ezrin and also MST4. The former changes the N-terminal structure of ezrin to bind syntaxin3, and the latter phosphorylates ACAP4, an Arf6-GAP, to accelerate binding to ezrin. Subsequently, H+,K+-ATPase, SNAREs, ezrin, and Arf6-GAP are aligned on the apical membrane. However, there are still many unsolved questions and the intracellular mechanism of parietal cells remains an attractive research area.

Specific protonation of acidic residues confers K+ selectivity to the gastric proton pump.

The gastric proton pump (H+,K+-ATPase) transports a proton into the stomach lumen for every K+ ion exchanged in the opposite direction. In the lumen-facing state of the pump (E2), the pump selectively binds K+ despite the presence of a 10-fold higher concentration of Na+. The molecular basis for the ion selectivity of the pump is unknown. Using molecular dynamics simulations, free energy calculations, and Na+ and K+-dependent ATPase activity assays, we demonstrate that the K+ selectivity of the pump depends upon the simultaneous protonation of the acidic residues E343 and E795 in the ion-binding site. We also show that when E936 is protonated, the pump becomes Na+ sensitive. The protonation-mimetic mutant E936Q exhibits weak Na+-activated ATPase activity. A 2.5-Å resolution cryo-EM structure of the E936Q mutant in the K+-occluded E2-Pi form shows, however, no significant structural difference compared with wildtype except less-than-ideal coordination of K+ in the mutant. The selectivity toward a specific ion correlates with a more rigid and less fluctuating ion-binding site. Despite being exposed to a pH of 1, the fundamental principle driving the K+ ion selectivity of H+,K+-ATPase is similar to that of Na+,K+-ATPase: the ionization states of the acidic residues in the ion-binding sites determine ion selectivity. Unlike the Na+,K+-ATPase, however, protonation of an ion-binding glutamate residue (E936) confers Na+ sensitivity.

Trimethoprim inhibits renal H+-K+-ATPase in states of K+ depletion.

There is growing consensus that under physiological conditions, collecting duct H+ secretion is independent of epithelial Na+ channel (ENaC) activity. We have recently shown that the direct ENaC inhibitor benzamil acutely impairs H+ excretion by blocking renal H+-K+-ATPase. However, the question remains whether inhibition of ENaC per se causes alterations in renal H+ excretion. To revisit this question, we studied the effect of the antibiotic trimethoprim (TMP), which is well known to cause K+ retention by direct ENaC inhibition. The acute effect of TMP (5 µg/g body wt) was assessed in bladder-catheterized mice, allowing real-time measurement of urinary pH, electrolyte, and acid excretion. Dietary K+ depletion was used to increase renal H+-K+-ATPase activity. In addition, the effect of TMP was investigated in vitro using pig gastric H+-K+-ATPase-enriched membrane vesicles. TMP acutely increased natriuresis and decreased kaliuresis, confirming its ENaC-inhibiting property. Under control diet conditions, TMP had no effect on urinary pH or acid excretion. Interestingly, K+ depletion unmasked an acute urine alkalizing effect of TMP. This finding was corroborated by in vitro experiments showing that TMP inhibits H+-K+-ATPase activity, albeit at much higher concentrations than benzamil. In conclusion, under control diet conditions, TMP inhibited ENaC function without changing urinary H+ excretion. This finding further supports the hypothesis that the inhibition of ENaC per se does not impair H+ excretion in the collecting duct. Moreover, TMP-induced urinary alkalization in animals fed a low-K+ diet highlights the importance of renal H+-K+-ATPase-mediated H+ secretion in states of K+ depletion.NEW & NOTEWORTHY The antibiotic trimethoprim (TMP) often mediates K+ retention and metabolic acidosis. We suggest a revision of the underlying mechanism that causes metabolic acidosis. Our results indicate that TMP-induced metabolic acidosis is secondary to epithelial Na+ channel-dependent K+ retention. Under control dietary conditions, TMP does not per se inhibit collecting duct H+ secretion. These findings add further argument against a physiologically relevant voltage-dependent mechanism of collecting duct H+ excretion.

ATP12A Proton Pump as an Emerging Therapeutic Target in Cystic Fibrosis and Other Respiratory Diseases.

ATP12A encodes the catalytic subunit of the non-gastric proton pump, which is expressed in many epithelial tissues and mediates the secretion of protons in exchange for potassium ions. In the airways, ATP12A-dependent proton secretion contributes to complex mechanisms regulating the composition and properties of the fluid and mucus lining the respiratory epithelia, which are essential to maintain the airway host defense and the respiratory health. Increased expression and activity of ATP12A in combination with the loss of other balancing activities, such as the bicarbonate secretion mediated by CFTR, leads to excessive acidification of the airway surface liquid and mucus dysfunction, processes that play relevant roles in the pathogenesis of cystic fibrosis and other chronic inflammatory respiratory disorders. In this review, we summarize the findings dealing with ATP12A expression, function, and modulation in the airways, which led to the consideration of ATP12A as a potential therapeutic target for the treatment of cystic fibrosis and other airway diseases; we also highlight the current advances and gaps regarding the development of therapeutic strategies aimed at ATP12A inhibition.

Lupeol: A Triterpenoid Isolated from the Stem Bark of Hymenocardia Acida (tul.) Exhibits a van der Waal Antagonism on the Alpha Subunit of Gastric H+K+Atpase - A Promising Antiulcer Principle.

BACKGROUND: Hymenocardia acida (HA) is one of the numerous medicinal plants in Nigeria with ethnomedicinal history of usage in the treatment of ulcer. The study aimed at isolating antiulcer principle(s) from the stem bark of HA as well as the mechanism of action determination. METHODS: Antiulcer screenings of the crude extract, aqueous fraction, and bulked VLC fractions were performed using in vivo and in vitro models. Docking was carried out by using PyRx. RESULTS: Crude extract (HA; 1 mg/mL) and the aqueous fraction of H. acida (HAA; 1 mg/mL) showed an acid neutralizing capacity (MEq) of 0.3948 and 0.4035, respectively which is significantly different from 0.431 MEq showed by negative control (distilled water) at p<0.05. BVLC 3 (1 mg/mL) showed a significant value of 0.4049 MEq. However, HA showed a dose-dependent decrease in activity across doses examined, with 100 mg/kg showing an ulcer index of 10.00±2.89 (61.50%) and cimetidine (positive control; 100 mg/kg), also showed the highest ulcer index of 3.67±0.88 (85.9%), which is significantly different from ulcer index of 26.00±6.35 (0.00%) p<0.05 observed in the negative control (5% dimethylsulphoxide). The highest ulcer index of 8.00±1.32 (65.10%) was noted in BVLC 3. Bioactive BVLC 3, resulted in an isolated compound (BF3B2A). The compound was suggested to be lupeol, with a docking score of -7.7. It showed a van der Waal interaction with some key amino acid residues in the vonoprazan binding site. CONCLUSION: The experimental studies justify the ethnomedicinal claim of usage among locals.

Differential expression of KCNQ1 and ATP4A genes according to the sex and age in the stomach.

We compared the expression of six genes in stomach tissue samples between healthy men and women in different age groups to study sexually dimorphic gene expression. Real-Time RT-PCR was used to compare gene expression between men and women. Our results showed that the expression of KCNQ1 (p = 0.01) was significantly higher in non-menopausal women compared to post-menopausal women. In addition, the expression level of the ATP4A gene in men under 35 years was significantly higher than in men above 50 (p = 0.026). Sexually and age dimorphic gene expression in some genes throughout life may affect gastric function.

The revolution of personalized pharmacotherapies for cystic fibrosis: what does the future hold?

INTRODUCTION: Cystic fibrosis (CF), a potentially fatal genetic disease, is caused by loss-of-function mutations in the gene encoding for the CFTR chloride/bicarbonate channel. Modulator drugs rescuing mutant CFTR traffic and function are now in the clinic, providing unprecedented breakthrough therapies for people with CF (PwCF) carrying specific genotypes. However, several CFTR variants are unresponsive to these therapies. AREA COVERED: We discussed several therapeutic approaches that are under development to tackle the fundamental cause of CF, including strategies targeting defective CFTR mRNA and/or protein expression and function. Alternatively, defective chloride secretion and dehydration in CF epithelia could be restored by exploiting pharmacological modulation of alternative targets, i.e., ion channels/transporters that concur with CFTR to maintain the airway surface liquid homeostasis (e.g., ENaC, TMEM16A, SLC26A4, SLC26A9, and ATP12A). Finally, we assessed progress and challenges in the development of gene-based therapies to replace or correct the mutant CFTR gene. EXPERT OPINION: CFTR modulators are benefiting many PwCF responsive to these drugs, yielding substantial improvements in various clinical outcomes. Meanwhile, the CF therapy development pipeline continues to expand with the development of novel CFTR modulators and alternative therapeutic strategies with the ultimate goal of providing effective therapies for all PwCF in the foreseeable future.

Discovery of novel benzimidazole derivatives as potent potassium-competitive acid blockers for the treatment of acid-related diseases.

H+, K+-ATPase, as the most critical enzyme in gastric acid secretion, has long been an attractive target for the treatment of acid-related diseases. In this study, a series of benzimidazole derivatives were designed and synthesized through conformational restriction and skeleton hopping strategies by using vonoprazan as the lead compound. Among them, compounds A12 (IC50 = 9.32 µM) and A18 (IC50 = 5.83 µM) showed better inhibition at the enzyme level. In addition, gastric acid secretion inhibition was assessed in vivo, and the results showed that A12 and A18 significantly inhibited basal gastric acid secretion, 2-deoxy-d-glucose (2DG) stimulated gastric acid secretion and histamine-stimulated gastric acid secretion. In further in vitro metabolic experiments, A12 and A18 demonstrated excellent stability and low toxicity. Pharmacokinetic studies showed that the p.o. and i.v. half-lives of A18 were 3.21 h and 8.67 ± 1.15 h, respectively. In summary, A18 might be a novel and effective potassium-competitive acid blocker, and this study provides strong support for it use in the treatment of acid-related diseases.

Parkinson's disease-associated ATP13A2/PARK9 functions as a lysosomal H+,K+-ATPase.

Mutations in the human ATP13A2 (PARK9), a lysosomal ATPase, cause Kufor-Rakeb Syndrome, an early-onset form of Parkinson's disease (PD). Here, we demonstrate that ATP13A2 functions as a lysosomal H+,K+-ATPase. The K+-dependent ATPase activity and the lysosomal K+-transport activity of ATP13A2 are inhibited by an inhibitor of sarco/endoplasmic reticulum Ca2+-ATPase, thapsigargin, and K+-competitive inhibitors of gastric H+,K+-ATPase, such as vonoprazan and SCH28080. Interestingly, these H+,K+-ATPase inhibitors cause lysosomal alkalinization and α-synuclein accumulation, which are pathological hallmarks of PD. Furthermore, PD-associated mutants of ATP13A2 show abnormal expression and function. Our results suggest that the H+/K+-transporting function of ATP13A2 contributes to acidification and α-synuclein degradation in lysosomes.

Targeting ATP12A, a Nongastric Proton Pump α Subunit, for Idiopathic Pulmonary Fibrosis Treatment.

Idiopathic pulmonary fibrosis (IPF) is a pathological condition of unknown etiology that results from injury to the lung and an ensuing fibrotic response that leads to the thickening of the alveolar walls and obliteration of the alveolar space. The pathogenesis is not clear, and there are currently no effective therapies for IPF. Small airway disease and mucus accumulation are prominent features in IPF lungs, similar to cystic fibrosis lung disease. The ATP12A gene encodes the α-subunit of the nongastric H+, K+-ATPase, which functions to acidify the airway surface fluid and impairs mucociliary transport function in patients with cystic fibrosis. It is hypothesized that the ATP12A protein may play a role in the pathogenesis of IPF. The authors' studies demonstrate that ATP12A protein is overexpressed in distal small airways from the lungs of patients with IPF compared with normal human lungs. In addition, overexpression of the ATP12A protein in mouse lungs worsened bleomycin induced experimental pulmonary fibrosis. This was prevented by a potassium competitive proton pump blocker, vonoprazan. These data support the concept that the ATP12A protein plays an important role in the pathogenesis of lung fibrosis. Inhibition of the ATP12A protein has potential as a novel therapeutic strategy in IPF treatment.

Randomized controlled trial to evaluate the efficacy and safety of fexuprazan compared with esomeprazole in erosive esophagitis.

BACKGROUND: Fexuprazan, a novel potassium-competitive acid blocker, reversibly suppresses the K+/H+-ATPase enzyme in proton pumps within gastric parietal cells. Fexuprazan's suppression of gastric acid was maintained in healthy individuals for 24 h in a dose-dependent manner. AIM: To compare fexuprazan to esomeprazole and establish its efficacy and safety in patients with erosive esophagitis (EE). METHODS: Korean adult patients with endoscopically confirmed EE were randomized 1:1 to receive fexuprazan 40 mg or esomeprazole 40 mg once daily for eight weeks. The primary endpoint was the proportion of patients with healed EE confirmed by endoscopy at week 8. The secondary endpoints included the healing rate of EE at week 4, symptom response, and quality of life assessment. Safety profiles and serum gastrin levels were compared between the groups. RESULTS: Of the 263 randomized, 218 completed the study per protocol (fexuprazan 40 mg, n = 107; esomeprazole 40 mg, n = 111). Fexuprazan was non-inferior to esomeprazole regarding the healing rate at week 8 [99.1% (106/107) vs 99.1% (110/111)]. There were no between-group differences in the EE healing rate at week 4 [90.3% (93/103) vs 88.5% (92/104)], symptom responses, and quality of life assessments. Additionally, serum gastrin levels at weeks 4 and 8 and drug-related side effects did not significantly differ between the groups. CONCLUSION: Fexuprazan 40 mg is non-inferior to esomeprazole 40 mg in EE healing at week 8. We suggest that fexuprazan is an alternative promising treatment option to PPIs for patients with EE.

Airway surface hyperviscosity and defective mucociliary transport by IL-17/TNF-α are corrected by ß-adrenergic stimulus.

The fluid covering the surface of airway epithelia represents a first barrier against pathogens. The chemical and physical properties of the airway surface fluid are controlled by the activity of ion channels and transporters. In cystic fibrosis (CF), loss of CFTR chloride channel function causes airway surface dehydration, bacterial infection, and inflammation. We investigated the effects of IL-17A plus TNF-α, 2 cytokines with relevant roles in CF and other chronic lung diseases. Transcriptome analysis revealed a profound change with upregulation of several genes involved in ion transport, antibacterial defense, and neutrophil recruitment. At the functional level, bronchial epithelia treated in vitro with the cytokine combination showed upregulation of ENaC channel, ATP12A proton pump, ADRB2 ß-adrenergic receptor, and SLC26A4 anion exchanger. The overall result of IL-17A/TNF-α treatment was hyperviscosity of the airway surface, as demonstrated by fluorescence recovery after photobleaching (FRAP) experiments. Importantly, stimulation with a ß-adrenergic agonist switched airway surface to a low-viscosity state in non-CF but not in CF epithelia. Our study suggests that CF lung disease is sustained by a vicious cycle in which epithelia cannot exit from the hyperviscous state, thus perpetuating the proinflammatory airway surface condition.

The chemical constituents and gastroprotective effects of Calanthe fimbriata Franch.

Calanthe fimbriata Franch. is a Tujia ethnic herb, which has traditionally been used to treat gastric ulcers, chronic hepatitis, etc. We explored the chemical constitutes, gastroprotective effects, and the active fraction of C. fimbriata, as well as elucidating the underlying mechanisms. Firstly, four in vitro antioxidant tests were applied to determine the oxidation resistance of C. fimbriata methanol extract and its fractions. The gastroprotective effects were evaluated in ethanol-induced gastric ulcer rats, gastric histopathology was visualized by H&E staining, and the acidity of gastric juice was measured by titrating with NaOH solution. The contents of malondialdehyde, catalase, superoxide dismutase, gastrin, and the activity of H+K+-ATPase were estimated using commercial kits. EtOAc fraction of C. fimbriata methanol extract (CfEF) exhibited significant gastroprotective effects by ameliorating stomach pathological changes and elevating the pH value of gastric juice. It also manifested remarkable antioxidant activities in vitro and in vivo. Using various chromatographic methods and spectroscopic techniques, 22 compounds were isolated and characterized from CfEF, in which alkaloids were the predominant components. All of these substances were derived from C. fimbriata for the first time. The results indicated that CfEF is a promising source of gastroprotective agents. The antioxidant activity of this herb, as well as prevention of gastrin secretion and inhibition of H+K+ -ATPase, was found to be the underlying mechanism of action.

Gastric Th17 Cells Specific for H+/K+-ATPase and Serum IL-17 Signature in Gastric Autoimmunity.

Human gastric autoimmunity [autoimmune gastritis (AIG)] is characterized by inflammation of the gastric mucosa and parietal cell loss. The gastric parietal cell proton pump H+/K+-adenosine triphosphatase (H+/K+-ATPase) is the major autoantigen in AIG. Our work aimed to investigate the gastric H+/K+-ATPase-specific T helper 17 (Th17) responses in AIG and serum interleukin (IL)-17 cytokine subfamily in AIG patients, in healthy subjects [healthy controls (HCs)], and in patients with iron deficiency anemia (IDA) without AIG. We analyzed the activation of gastric lamina propria mononuclear cells (LPMCs) by H+/K+-ATPase and the IL-17A and IL-17F cytokine production in eight patients with AIG and four HCs. Furthermore, we compared serum levels of IL-17A, IL-17F, IL-21, IL-17E, IL-22, and IL-23 in 43 AIG patients, in 47 HCs, and in 20 IDA patients without AIG. Gastric LPMCs from all AIG patients, but not those from HCs, were activated by H+/K+-ATPase and were able to proliferate and produce high levels of IL-17A and IL-17F. AIG patients have significantly higher serum IL-17A, IL-17F, IL-21, and IL-17E (393.3 ± 410.02 pg/ml, 394.0 ± 378.03 pg/ml, 300.46 ± 303.45 pg/ml, 34.92 ± 32.56 pg/ml, respectively) than those in HCs (222.99 ± 361.24 pg/ml, 217.49 ± 312.1 pg/ml, 147.43 ± 259.17 pg/ml, 8.69 ± 8.98 pg/ml, respectively) and those in IDA patients without AIG (58.06 ± 107.49 pg/ml, 74.26 ± 178.50 pg/ml, 96.86 ± 177.46 pg/ml, 10.64 ± 17.70 pg/ml, respectively). Altogether, our results indicate that IL-17A and IL-17F are produced in vivo in the stomach of AIG patients following activation with H+/K+-ATPase and that serum IL-17A, IL-17F, IL-21, and IL-17E levels are significantly elevated in AIG patients but not in patients without AIG. These data suggest a Th17 signature in AIG and that IL-17A, IL-17F, IL-21, and IL-17E may represent a relevant tool for AIG management.

Novel Isoxazole Derivative Attenuates Ethanol-Induced Gastric Mucosal Injury through Inhibition of H+/K+-ATPase Pump, Oxidative Stress and Inflammatory Pathways.

Isoxazole derivatives are significant enough due to their wide range of pharmacological and therapeutic activities. The purpose of the current study is to use computational, in vitro, in vivo, and extensive molecular approaches to examine the possible anti-ulcer activity of 4-benzylidene-3 methyl-1,2-isoxazol-5(4H)-one (MBO). Biovia Discovery Studio visualizer (DSV) was utilized for virtual screening. A tissue antioxidant investigation, H+/K+-ATPase test, and anti-H. pylori activities were carried out. ELISA, immunohistochemistry, and PCR methods were employed for the proteome analysis. An ethanol-induced stomach ulcer model was used to examine the anti-ulcer potential in rats. The binding affinities for MBO ranged from -5.4 to -8.2 Kcal/mol. In vitro findings revealed inhibitory activity against H. pylori and the H+/K+-ATPase pump. It also enhanced levels of glutathione, catalase, and glutathione-S-transferase and reduced lipid peroxidation levels in gastric tissues of rats. In vivo results showed the gastro-protective effect of MBO (30 mg/kg) in ulcerative rat stomachs. The proteomic study revealed decreased expression of inflammatory markers (cyclooxygenase-2, p-NFkB, and TNF-α). In RT-PCR analysis, the expression levels of H+/K+-ATPase were reduced. Furthermore, ADMET (absorption, distribution, metabolism, excretion and toxicity) studies revealed that MBO has high GIT solubility and has a safer profile for cardiac toxicity. This study suggests that MBO displayed anti-ulcer potential, which may have been mediated through the inhibition of the H+/K+-ATPase pump, as well as antioxidant and anti-inflammatory pathways. It has the potential to be a lead molecule in the treatment of peptic ulcers with fewer adverse effects.

The transported cations impose differences in the thermostability of the gastric H,K-ATPase. A kinetic analysis.

This work analyses the thermostability of a membrane protein, the gastric H,K-ATPase, by means of a detailed kinetic characterization of its inactivation process, which showed to exhibit first-order kinetics. We observed parallel time courses for the decrease of ATPase activity, the decrease of the autophosphorylation capacity and the loss of tertiary structure at 49 °C. Higher temperatures were required to induce a significant change in secondary structure. The correspondence between the kinetics of Trp fluorescence measured at 49 °C and the decrease of the residual activity after heating at that temperature, proves the irreversibility of the inactivation process. Inactivation proceeds at different rates in E1 or E2 conformations. The K+-induced E2 state exhibits a lower inactivation rate; the specific effect is exerted with a K0.5 similar to that found at 25 °C, providing a further inkling that K+ occlusion by the H,K-ATPase is not really favoured. Increasing [H+] from pH 8 to pH 7, which possibly shifts the protein to E1, produces a subtle destabilizing effect on the H,K-ATPase. We performed a prediction of potential intramolecular interactions and found that the differential stability between E1 and E2 may be mainly explained by the higher number of hydrophobic interactions in the α- and ß-subunits of E2 conformation.