Structural aspects of the gastric H,K ATPase

Functional modifications associated with gastrointestinal tract organogenesis during metamorphosis in Atlantic halibut (Hippoglossus hippoglossus)

BACKGROUND

Flatfish metamorphosis is a hormone regulated post-embryonic developmental event that transforms a symmetric larva into an asymmetric juvenile. In altricial-gastric teleost fish, differentiation of the stomach takes place after the onset of first feeding, and during metamorphosis dramatic molecular and morphological modifications of the gastrointestinal (GI-) tract occur. Here we present the functional ontogeny of the developing GI-tract from an integrative perspective in the pleuronectiforme Atlantic halibut, and test the hypothesis that the multiple functions of the teleost stomach develop synchronously during metamorphosis.

RESULTS

Onset of gastric function was determined with several approaches (anatomical, biochemical, molecular and in vivo observations). In vivo pH analysis in the GI-tract lumen combined with quantitative PCR (qPCR) of α and β subunits of the gastric proton pump (H+/K+-ATPase) and pepsinogen A2 indicated that gastric proteolytic capacity is established during the climax of metamorphosis. Transcript abundance of ghrelin, a putative orexigenic signalling molecule produced in the developing stomach, correlated (p < 0.05) with the emergence of gastric proteolytic activity, suggesting that the stomach's role in appetite regulation occurs simultaneously with the establishment of proteolytic function. A 3D models series of the GI-tract development indicated a functional pyloric sphincter prior to first feeding. Observations of fed larvae in vivo confirmed that stomach reservoir function was established before metamorphosis, and was thus independent of this event. Mechanical breakdown of food and transportation of chyme through the GI-tract was observed in vivo and resulted from phasic and propagating contractions established well before metamorphosis. The number of contractions in the midgut decreased at metamorphic climax synchronously with establishment of the stomach's proteolytic capacity and its increased peristaltic activity. Putative osmoregulatory competence of the GI-tract, inferred by abundance of Na+/K+-ATPase α transcripts, was already established at the onset of exogenous feeding and was unmodified by metamorphosis.

CONCLUSIONS

The functional specialization of the GI-tract was not exclusive to metamorphosis, and its osmoregulatory capacity and reservoir function were established before first feeding. Nonetheless, acid production and the proteolytic capacity of the stomach coincided with metamorphic climax, and also marked the onset of the stomach's involvement in appetite regulation via ghrelin.

Current trends in the management of gastroesophageal reflux disease: a review

Gastroesophageal reflux disease (GERD) is a chronic disorder of the upper gastrointestinal tract with global distribution. The incidence is on the increase in different parts of the world. In the last 30 to 40 years, research findings have given rise to a more robust understanding of its pathophysiology, clinical presentation, and management. The current definition of GERD (The Montreal definition, 2006) is not only symptom-based and patient-driven, but also encompasses esophageal and extraesophageal manifestations of the disease. The implication is that the disease can be confidently diagnosed based on symptoms alone. Nonerosive reflux disease (NERD) remains the predominant form of GERD. Current thinking is that NERD and erosive reflux disease (ERD) are distinct phenotypes of GERD rather than the old concept which regarded them as components of a disease spectrum. Non erosive reflux disease is a very heterogeneous group with significant overlap with other functional gastrointestinal disorders. There is no gold standard for the diagnosis of GERD. Esophageal pH monitoring and intraluminal impedance monitoring have thrown some light on the heterogeneity of NERD. A substantial proportion of GERD patients continue to have symptoms despite optimal PPI therapy, and this has necessitated research into the development of new drugs. Several safety concerns have been raised about chronic use of proton pump inhibitors but these are yet to be substantiated in controlled studies. The debate about efficacy of long-term medical treatment compared to surgery continues, however, recent data indicate that modern surgical techniques and long-term PPI therapy have comparable efficacy. These and other issues are subjects of further research.

Intravenous proton pump inhibitors for peptic ulcer bleeding: Clinical benefits and limits

Peptic ulcer bleeding is a common disease and recurrent bleeding is an independent risk factor of mortality. Infusion with proton pump inhibitors (PPIs) prevents recurrent bleeding after successful endoscopic therapy. A gastric acidic environment of less than pH 5.4 alters coagulation function and activates pepsin to disaggregate platelet plugs. Gastric acid is secreted by H⁺, K⁺-ATPase, naming the proton pump. This update review focuses on the mechanism and the role of PPIs in the clinical management of patients with peptic ulcer bleeding. An intravenous omeprazole bolus followed by high-dose continuous infusion for 72 h after successful endoscopic therapy can prevent the recurrent bleeding. In the Asian, however, the infusion dosage can possibly be diminished whilst preserving favorable control of the intragastric pH and thereby still decreasing rates of recurrent bleeding. Irrespective of the infusion dosage of PPIs, rates of recurrent bleeding remain high in patients with co-morbidities. Because recurrent peptic ulcer bleeding may be prolonged in those with co-morbidities, a low-dose infusion of IV PPIs for up to 7-day may result in better control of recurrent bleeding of peptic ulcers. Due to the inter-patient variability in CYP2C19 genotypes, the infusion form of new generation PPIs, such as esomeprazole, should be promising for the prevention of recurrent bleeding. This article offers a comprehensive review of clinical practice, highlighting the indication, the optimal dosage, the duration, and the potential limitation of PPIs infusion for peptic ulcer bleeding.

Esomeprazole: potent acid suppression in the treatment of acid-related disorders

Esomeprazole (S-omeprazole), an enantiomer of the racemate omeprazole, is the first proton pump inhibitor to be developed as an isomer. This confers improved pharmacokinetics and pharmacodynamics compared with the racemate R/S-omeprazole. The difference in the pharmacokinetics of esomeprazole compared with omeprazole and the R-isomer is due to reductions in total body clearance and first-pass metabolism in the liver. Pharmacodynamic studies showed that esomeprazole 40 mg provides greater intragastric acid control than respective doses of all the other proton pump inhibitors on the market. Several well-designed clinical trials, employing both endoscopic and symptomatic response criteria, have compared the efficacy of esomeprazole with that of other proton pump inhibitors in the management of gastroesophageal reflux disease patients, and in the eradication of Helicobacter pylori. In addition, the efficacy of esomeprazole for the healing and prevention of nonsteroidal anti-inflammatory drug-associated dyspeptic symptoms and ulcers has been established. The aim of this review is to provide an overview of the pharmacokinetics, pharmacodynamics and consequent clinical importance of esomeprazole in the treatment of acid-related disorders.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

1-Isopropyl-4-nitro-6-meth-oxy-1H-benzimidazole

There are two independent mol­ecules in the asymmetric unit of the title compound, C₁₁H₁₃N₃O₃. The inter­planar angles for the two rings of the benzimidazole ring system is 2.21 (12)° in one mol­ecule and 0.72 (12)° in the other. The nitro group is twisted in the same direction relative to the least-squares plane through its attached benzene ring in both mol­ecules, with inter­planar angles of 15.22 (9) and 18.02 (8)°. In the crystal structure, mol­ecules are stacked along the a axis through π–π inter­actions (centroid–centroid distance 4.1954 Å). C—H⋯O hydrogen bonds are also present.

Direct measurement of gastric H+/K+-ATPase activities in patients with or without Helicobacter pylori-associated chronic gastritis

AIM: The role of Helicobacter pylori (H pylori ) infection in gastric acid secretion of patients with chronic gastritis remains controversial. This study was designed to elucidate the effect of H pylori on H⁺/K⁺-ATPase activities in gastric biopsy specimens.

METHODS: Eighty-two patients with chronic gastritis who had undergone upper endoscopy were included in this study. H pylori infection was confirmed by rapid urease test and histology. Gastric H⁺/K⁺-ATPase activities and serum gastrin concentrations were measured by an enzymatic method and radioimmunoassay, respectively. For those patients who received triple therapy for eradicating H pylori, changes in the activity of gastric H⁺/K⁺-ATPase and serum gastrin levels were also measured.

RESULTS: The mean gastric H⁺/K⁺-ATPase activity in H pylori-positive group (42 patients) was slightly higher than that in H pylori-negative group (29 patients) (169.65±52.9 and 161.38±43.85 nmol P_i/(mg·h), respectively, P = 0.301). After eradication of H pylori, the gastric H⁺/K⁺-ATPase activities slightly decreased compared to prior therapy (165.03±59.50 and 158.42±38.93 nmol P_i/(mg·h), respectively, P = 0.805). The mean basal gastrin concentration was slightly higher in H pylori-positive patients than in H pylori-negative patients (87.92±39.65 pg/mL vs 75.04± 42.57 pg/mL, P = 0.228). The gastrin levels fell significantly after the eradication of H pylori. (Before treatment 87.00±30.78 pg/mL, after treatment 64.73±18.96 pg/mL, P = 0.015).

CONCLUSION: Gastric H⁺/K⁺-ATPase activities are not associated with H pylori status in patients with chronic gastritis.

Identification of the β-subunit for nongastric H-K-ATPase in rat anterior prostate

The structural organization of nongastric H-K-ATPase, unlike that of closely related Na-K-ATPase and gastric H-K-ATPase, is not well characterized. Recently, we demonstrated that nongastric H-K-ATPase α-subunit (αng) is expressed in apical membranes of rodent prostate. Its highest level, as well as relative abundance, with respect to α1-isoform of Na-K-ATPase, was observed in anterior lobe. Here, we aimed to determine the subunit composition of nongastric H-K-ATPase through the detailed analysis of the expression of all known X-K-ATPase β-subunits in rat anterior prostate (AP). RT-PCR detects transcripts of β-subunits of Na-K-ATPase only. Measurement of absolute protein content of these three β-subunit isoforms, with the use of quantitative Western blotting of AP membrane proteins, indicates that the abundance order is β1> β3≫ β2. Immunohistochemical experiments demonstrate that β1is present predominantly in apical membranes, coinciding with αng, whereas β3is localized in the basolateral compartment, coinciding with α1. This is the first direct demonstration of the αng-β1colocalization in situ indicating that, in rat AP, αngassociates only with β1. The existence of αng-β1complex has been confirmed by immunoprecipitation experiments. These results indicate that β1-isoform functions as the authentic subunit of Na-K-ATPase and nongastric H-K-ATPase. Putatively, the intracellular polarization of X-K-ATPase isoforms depends on interaction with other proteins.

The use of proton pump inhibitors in children: a comprehensive review

Proton pump inhibitors (PPIs) belong to a group of chemically related compounds whose primary function is the inhibition of acid production in the final common metabolic pathway of gastric parietal cells. PPIs are highly selective and effective in their action and have few short- or long-term adverse effects. These pharmacologic features have made the development of PPIs the most significant advancement in the management of acid peptic related disorders in the last two decades. There are numerous published adult studies that describe the pharmacology, efficacy and safety of these anti-secretory agents; however, in the pediatric population, there are very few comparable studies, particularly multicenter studies with significant patient enrollment. In preparing this article, our aim was to perform a comprehensive review of the literature on the clinical pharmacology and use of PPIs in the pediatric population, and to briefly review some recent articles. Relevant literature was identified by performing MEDLINE/Pubmed searches from January 1990 to December 2001. Combinations of the following search terms were use to analyze these databases: proton pump inhibitor, children, pediatrics, gastroesophageal reflux disease (GERD), esophagitis, intestinal metaplasia, Helicobacter pylori, omeprazole, lansoprazole, pantoprazole, rabeprazole, esomeprazole, and safety. Abstracts from the 14th annual conference of the North American Society for Pediatric Gastroenterology, Hepatology and Nutrition (NASPGHAN) 2001, and the Disease and Digestive Week 2001, were also included in the review. All pediatric studies reviewed were limited to either omeprazole or lansoprazole. The dosage range used for the management of GERD and related disorders with lansoprazole was 0.73-1.66 mg/kg/day (maximum 30 mg/day). The dosage range for GERD management using omeprazole was 0.3-3.5 mg/kg (maximum 80 mg/day). The dosage range for omeprazole used for H. pylori was 0.5-1.5 mg/kg/day, with a maximum dosage of 40 mg/day, and lansoprazole-containing regimens for H. pylori eradication used dosages ranging from 0.6-1.2 mg/kg/day, with a maximum dosage of 30 mg/day. Few severe adverse events were reported with the use of either drug. Eradication rates for H. pylori were 56-87% for lansoprazole-based triple therapy, and 75-94% for omeprazole-based eradication regimens. To date, there are no published controlled trials of sufficient power comparing the efficacy of the five commercially available PPIs in children, for a variety of acid peptic diseases. Studies suggest that PPIs are highly effective for the management of GERD and related disorders, and are a critically needed component of triple therapy to eradicate H. pylori. PPIs have a very good tolerability profile in adults and children, but long-term tolerability studies are needed, particularly in the pediatric population. Multicenter studies are critically needed to evaluate the second-generation PPIs, to compare PPI efficacy to each other, and to assess the importance of developmental and genetic pharmacology of these drugs in children with acid-peptic disease.

Dose-dependent control of intragastric pH by pantoprazole, 10, 20 or 40 mg, in healthy volunteers

BACKGROUND

Proton pump inhibitors have emerged as the most effective class of drugs for the treatment of gastro-oesophageal reflux. Pantoprazole is a proton pump inhibitor that has demonstrated high clinical efficacy.

AIM

To evaluate the effect of once-daily doses of pantoprazole, 10, 20 and 40 mg, on gastric acidity in healthy volunteers.

METHODS

Thirty-six subjects received pantoprazole in a three-way crossover design study. Ambulatory 24-h intragastric pH and distal oesophageal pH were monitored at baseline and on the last day of each treatment period. The measured endpoints were the median intragastric and oesophageal pH, the percentage of time the intragastric pH < 4 and oesophageal pH < 4 and the area under the curve for gastric acidity over 24 h. Safety was evaluated by incidence and severity of adverse events.

RESULTS

Pantoprazole demonstrated a linear dose- dependent suppression of gastric acidity over the dose range 10-40 mg. The dose of 40 mg demonstrated a significantly greater response than the lower doses, particularly at night. All pantoprazole doses were well tolerated.

CONCLUSIONS

Pantoprazole demonstrates a dose-related effect in the range 10-40 mg once daily. The once-daily dose of 40 mg provides the highest and most consistent control of gastric pH, especially at night.

Selective Fe2+-catalyzed oxidative cleavage of gastric H+,K+-ATPase: implications for the energy transduction mechanism of P-type cation pumps

In the presence of ascorbate/H(2)O(2), Fe(2+) ions or the ATP-Fe(2+) complex catalyze selective cleavage of the alpha subunit of gastric H(+),K(+)-ATPase. The electrophoretic mobilities of the fragments and dependence of the cleavage patterns on E(1) and E(2) conformational states are essentially identical to those described previously for renal Na(+),K(+)-ATPase. The cleavage pattern of H(+),K(+)-ATPase by Fe(2+) ions is consistent with the existence of two Fe(2+) sites: site 1 within highly conserved sequences in the P and A domains, and site 2 at the cytoplasmic entrance to trans-membrane segments M3 and M1. The change in the pattern of cleavage catalyzed by Fe(2+) or the ATP-Fe(2+) complex induced by different ligands provides evidence for large conformational movements of the N, P, and A cytoplasmic domains of the enzyme. The results are consistent with the Ca(2+)-ATPase crystal structure (Protein Data Bank identification code; Toyoshima, C., Nakasako, M., Nomura, H., and Ogawa, H. (2000) Nature 405, 647-655), an E(1)Ca(2+) conformation, and a theoretical model of Ca(2+)-ATPase in an E(2) conformation (Protein Data Bank identification code ). Thus, it can be presumed that the movements of N, P, and A cytoplasmic domains, associated with the E(1) <--> E(2) transitions, are similar in all P-type ATPases. Fe(2+)-catalyzed cleavage patterns also reveal sequences involved in phosphate, Mg(2+), and ATP binding, which have not yet been shown in crystal structures, as well as changes which occur in E(1) <--> E(2) transitions, and subconformations induced by H(+),K(+)-ATPase-specific ligands such as SCH28080.

Clinical pharmacology and safety profile of esomeprazole, the first enantiomerically pure proton pump inhibitor

Awareness of important differences in the pharmacological profile of individual optical isomers of chiral drugs led to the development of esomeprazole, the S-isomer of omeprazole, a new pharmacological entity designed to improve the clinical outcome of available proton pump inhibitors in the management of acid-related disorders. The superior acid control achieved by esomeprazole is mainly due to an advantageous metabolism compared with racemate omeprazole, leading to improved bioavailability and to enhanced delivery of the drug to the gastric proton pump.

Monitoring of secondary and tertiary structure changes in the gastric H+/K+-ATPase by infrared spectroscopy

Conformational changes occurring in the catalytic cycle of the H+/K+-ATPase were monitored by Fourier transform infrared spectroscopy (FTIR). Caged compounds were used to release ATP, in the presence of Ca2+, to induce the transition between the E1 and E1-P conformation of the H+/K+-ATPase. In addition to bands associated with the photolysis of the caged compounds, some peaks of the difference infrared spectra were associated with changes in secondary structure and modifications of the ionization in the side chains of amino-acid residues (Glu or Asp). These changes were specific to the reaction between the ligand and the enzyme. We estimated that 39 amino acids changed their secondary structure during the reaction and four amino-acid residues were deprotonated. Similar spectral changes appeared when ADP was released from its precursor. The release of Pi from the same caged molecule did not induce similar changes. Changes in tertiary structure occurring during the binding of adenosine and phosphorylation of the enzyme were demonstrated by recording hydrogen/deuterium exchange kinetics by attenuated total reflectance FTIR spectroscopy (ATR-FTIR). At least 129 amide protons were involved in a tertiary structure change induced by ATP. This suggested that secondary structure change transduced a much larger tertiary structure modification.

Biochemical properties of a newly synthesized H(+)/K(+) ATPase inhibitor, 1-(2-methyl-4-methoxyphenyl)-4-

A new compound, 1-(2-methyl-4-methoxyphenyl)-4-[(3-hydroxypropyl)amino]-6-methyl-2,3-dihydropyrrolo[3,2-c]quinoline (DBM-819), inhibited gastric H(+)/K(+) ATPase in the rabbit (EC 3.6.1.3) with an IC(50) value of 5 microM. However, DBM-819 was a weak inhibitor of kidney Na(+)/K(+) ATPase in the dog, indicating that it has selectivity for the gastric H(+)/K(+) ATPase. The inhibition was reversible and non-competitive with respect to the activating cation K(+). The presence of dithiothreitol did not protect the H(+)/K(+) ATPase from inactivation. The inhibition by DBM-819 was potentiated by acid pretreatment of the compound, suggesting that DBM-819 is converted into a more active intermediate under acidic conditions. The results suggest that DBM-819 is a potent, selective and reversible inhibitor of gastric H(+)/K(+) ATPase, and that the essential cysteine residue may not be involved in the DBM-819-mediated inactivation of gastric H(+)/K(+) ATPase.

Intravenous pantoprazole rapidly controls gastric acid hypersecretion in patients with Zollinger-Ellison syndrome

BACKGROUND & AIMS

Parenteral control of gastric acid hypersecretion in conditions such as Zollinger-Ellison syndrome (ZES) or idiopathic gastric acid hypersecretion is necessary perioperatively or when oral medications cannot be taken for other reasons (e.g., during chemotherapy, acute upper gastrointestinal bleeding, or in intensive care unit settings).

METHODS

We evaluated the efficacy and safety of 15-minute infusions of the proton pump inhibitor pantoprazole (80-120 mg every 8-12 hours) in controlling acid output for up to 7 days. Effective control was defined as acid output >10 milliequivalents per hour (mEq/h) (<5 mEq/h in patients with prior acid-reducing surgery) for 24 hours.

RESULTS

The 21 patients enrolled had a mean age of 51.9 years (range, 29-75) and a mean disease duration of 8.1 years (range, <0.5-21); 13 were male, 7 had multiple endocrine neoplasia syndrome type I, 4 had undergone acid-reducing surgery, 2 had received chemotherapy, and 13 had undergone gastrinoma resections without cure. Basal acid output (mean +/- SD) was 40.2 +/- 27.9 mEq/h (range, 11.2-117.9). In all patients, acid output was controlled within the first hour (mean onset of effective control, 41 minutes) after an initial 80-mg intravenous pantoprazole dose. Pantoprazole, 80 mg every 12 hours, was effective in 17 of 21 patients (81%) for up to 7 days. Four patients required upward dose titration, 2 required 120 mg pantoprazole every 12 hours, and 2 required 80 mg every 8 hours. At study end, acid output remained controlled for 6 hours beyond the next expected dose in 71% of patients (n = 15); mean acid output increased to 4.0 mEq/h (range, 0-9.7). No serious or unexpected adverse events were observed.

CONCLUSIONS

Intravenous pantoprazole, 160-240 mg/day administered in divided doses by 15-minute infusion, rapidly and effectively controlled acid output within 1 hour and maintained control for up to 7 days in all ZES patients.

Comparison of covalent with reversible inhibitor binding sites of the gastric H,K-ATPase by site-directed mutagenesis

The gastric H,K-ATPase is covalently inhibited by substituted pyridyl-methylsulfinyl-benzimidazoles, such as omeprazole, that convert to thiophilic probes of luminally accessible cysteines in the acid space. The K(+) competitive inhibitor, SCH28080, prevented inhibition of acid transport by omeprazole. In stably expressing HEK293 cells, the benzimidazole-reactive cysteines, Cys-321 (transmembrane helix (TM) 3), Cys-813 and Cys-822 (TM5/6), and Cys-892 (TM7/8) were mutated to the amino acids found in the SCH28080-resistant Na,K-ATPase and kinetic parameters of H,K-ATPase activity analyzed. Mutations of Cys-822 and Cys-892 had insignificant effects on the K(i(app)), K(m(app)) or V(max), but mutations of Cys-813 to threonine and Cys-321 to alanine decreased the affinity for SCH28080. Mutation of Cys-321 to alanine produced mixed kinetics of inhibition, still with higher affinity for the cation-free form of phosphoenzyme. Since the phenylmethoxy ring of the imidazo-pyridine inhibitors binds to TM1/2, as shown by earlier photoaffinity studies, and the mutations in TM6 (Cys-813 --> Thr) as well as the end of TM3 (Cys-321 --> Ala) decrease the affinity for SCH28080, the TM1/2, TM3, and TM6 helices lie within approximately 16 A of each other based on the size of the active, extended conformation of SCH28080.

Stabilization of the H,K-ATPase M5M6 membrane hairpin by K+ ions. Mechanistic significance for p2-type atpases

The integral membrane protein, the gastric H,K-ATPase, is an alpha-beta heterodimer, with 10 putative transmembrane segments in the alpha-subunit and one such segment in the beta-subunit. All transmembrane segments remain within the membrane domain following trypsinization of the intact gastric H,K-ATPase in the presence of K+ ions, identified as M1M2, M3M4, M5M6, and M7, M8, M9, and M10. Removal of K+ ions from this digested preparation results in the selective loss of the M5M6 hairpin from the membrane. The release of the M5M6 fragment is directed to the extracellular phase as evidenced by the accumulation of the released M5M6 hairpin inside the sealed inside out vesicles. The stabilization of the M5M6 hairpin in the membrane phase by the transported cation as well as loss to the aqueous phase in the absence of the transported cation has been previously observed for another P2-type ATPase, the Na, K-ATPase (Lutsenko, S., Anderko, R., and Kaplan, J. H. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7936-7940). Thus, the effects of the counter-transported cation on retention of the M5M6 segment in the membrane as compared with the other membrane pairs may be a general feature of P2-ATPase ion pumps, reflecting a flexibility of this region that relates to the mechanism of transport.

Evidence for a salt bridge between transmembrane segments 5 and 6 of the yeast plasma-membrane H+-ATPase

The plasma-membrane H+-ATPase of Saccharomyces cerevisiae, which belongs to the P2 subgroup of cation-transporting ATPases, is encoded by the PMA1 gene and functions physiologically to pump protons out of the cell. This study has focused on hydrophobic transmembrane segments M5 and M6 of the H+-ATPase. In particular, a conserved aspartate residue near the middle of M6 has been found to play a critical role in the structure and biogenesis of the ATPase. Site-directed mutants in which Asp-730 was replaced by an uncharged residue (Asn or Val) were abnormally sensitive to trypsin, consistent with the idea that the proteins were poorly folded, and immunofluorescence confocal microscopy showed them to be arrested in the endoplasmic reticulum. Similar defects are known to occur when either Arg-695 or His-701 in M5 is replaced by a neutral residue (Dutra, M. B., Ambesi, A., and Slayman, C. W. (1998) J. Biol. Chem. 273, 17411-17417). To search for possible charge-charge interactions between Asp-730 and Arg-695 or His-701, double mutants were constructed in which positively and negatively charged residues were swapped or eliminated. Strikingly, two of the double mutants (R695D/D730R and R695A/D730A) regained the capacity for normal biogenesis and displayed near-normal rates of ATP hydrolysis and ATP-dependent H+ pumping. These results demonstrate that neither Arg-695 nor Asp-730 is required for enzymatic activity or proton transport, but suggest that there is a salt bridge between the two residues, linking M5 and M6 of the 100-kDa polypeptide.

Proton pump inhibitors. Pharmacology and rationale for use in gastrointestinal disorders

Proton pump inhibitors (PPIs) are drugs which irreversibly inhibit proton pump (H+/K+ ATPase) function and are the most potent gastric acid-suppressing agents in clinical use. There is now a substantial body of evidence showing improved efficacy of PPIs over the histamine H2 receptor antagonists and other drugs in acid-related disorders. Omeprazole 20 mg/day, lansoprazole 30 mg/day, pantoprazole 40 mg/day or rabeprazole 20 mg/day for 2 to 4 weeks are more effective than standard doses of H2-receptor antagonists in healing duodenal and gastric ulcers. Patients with gastric ulcers should receive standard doses of PPIs as for duodenal ulcers but for a longer time period (4 to 8 weeks). There is no conclusive evidence to support the use of a particular PPI over another for either duodenal or gastric ulcer healing. For Helicobacter pylori-positive duodenal ulceration, a combination of a PPI and 2 antibacterials will eradicate H. pylori in over 90% of cases and significantly reduce ulcer recurrence. Patients with H. pylori-positive gastric ulcers should be managed similarly. PPIs also have efficacy advantages over ranitidine and misoprostol and are better tolerated than misoprostol in patients taking nonsteroidal anti-inflammatory drugs (NSAIDs). In endoscopically proven gastro-oesophageal reflux disease, standard daily doses of the PPIs are more effective than H2-receptor antagonists for healing, and patients should receive a 4 to 8 week course of treatment. For severe reflux, with ulceration and/or stricture formation, a higher dose regimen (omeprazole 40 mg, lansoprazole 60 mg, pantoprazole 80 mg or rabeprazole 40 mg daily) appears to yield better healing rates. There is little evidence that PPIs lead to resolution of Barrett's oesophagus or a reduction of subsequent adenocarcinoma development, but PPIs are indicated in healing of any associated ulceration. In Zollinger-Ellison syndrome, PPIs have become the treatment of choice for the management of gastric acid hypersecretion.

Gastric H+/K(+)-ATPase: targeting signals in the regulation of physiologic function

The H+/K(+)-ATPase of gastric parietal cells catalyzes acid secretion in the stomach. Regulation of this pump's function involves its targeting to an intracellular storage compartment, which fuses with the apical plasma membrane in response to secretagogue stimulation. The H+/K(+)-ATPase must also be returned to this intracellular storage compartment to inactivate acid secretion. Recent research suggests that a number of targeting signals and protein-protein interactions participate in governing this process.