Characterization of gastric mucosal membranes. IX. Fractionation and purification of K+-ATPase-containing vesicles by zonal centrifugation and free-flow electrophoresis technique

Structural Study of H,K-ATPase by Electron Microscopy and Image Reconstruction

H,K-ATPase is an integral protein of the plasma membrane of parietal cells. It is believed to constitute the pump responsible for secretion of acid into stomach. Its catalytic subunit (Mr 110,000) shows striking sequence homology to those of other transport ATPases. Recent studies suggest that there is also a glycoprotein (ca 300 amino acids) associated with the H,K pump, which is very homologous to the β subunit of the Na,K-ATPase.

The enzyme is isolated in protein-rich membrane vesicles from hog stomachs. Formation of two-dimensional crystals is induced in suspensions of the enzyme by methods that had proved successful for crystallization of the Na,K-ATPase.

Benzimidazole covalent probes and the gastric H(+)/K(+)-ATPase as a model system for protein labeling in a copper-free setting

Affinity probes are useful tools for determining molecular targets and elucidating mechanism of action for novel, bioactive compounds. In the case of covalent inhibitors, activity based probes are particularly valuable for ensuring acceptable selectivity margins. However, there is a variety of bioorthogonal chemistry reactions available for modifying compounds of interest with clickable tags. Here, we describe a direct comparison of tetrazine ligation and strain promoted azide-alkyne cycloaddition using benzimidazole based probes to bind their known target, the gastric proton pump, ATP4A. This study validates the use of chemical probes for target identification and illustrates the superior efficiency of tetrazine ligation for copper-free click systems. In addition, we have identified several novel binding partners of benzimidazole probes: Isoform 2 of deleted in malignant brain tumors 1 protein (DMBT1) and three uncharacterized proteins.

Computational insights into the interaction mechanism of triazolyl substituted tetrahydrobenzofuran derivatives with H(+),K(+)-ATPase at different pH

The interaction mechanism of triazolyl substituted tetrahydrobenzofuran derivatives (compound 1 (N, N-Dipropyl-1-(2-phenyl-4,5,6,7-tetrahydrobenzofuran-4-yl)-1H-1,2,3-triazole-4-methanamine) and 2 (1-(2-Phenyl-4,5,6,7-tetrahydrobenzofuran-4-yl)-4-(morpholin-4-ylmethyl)-1H-1,2,3-triazole)) with H(+),K(+)-ATPase at different pH were studied by induced-fit docking, QM/MM optimization and MM/GBSA binding free energy calculations of two forms (neutral and protonated form) of compounds. The inhibition activity of compound 1 is measured and almost unchanged at different pH, while the activity of compound 2 increases significantly with pH value decreased. This phenomenon could be explained by their protonated form percentages and the calculated binding free energies of protonated and neutral mixture of compounds at different pH. The binding free energy of protonated form is higher than that of neutral form of compound, and the protonated form could be a powerful inhibitor of H(+),K(+)-ATPase. By the decomposed energy comparisons of residues in binding sites, Asp137 should be the key binding site to protonated form of compound because of the hydrogen bond and electrostatic interactions. These calculation results could help for further rational design of novel H(+),K(+)-ATPase inhibitors.

A novel use of methylene blue as a pharmacological tool

INTRODUCTION

A new use of methylene blue as an ulcerogenic agent and the mechanisms involved were identified with an objective to exploit methylene blue as a pharmacological tool to study investigational antiulcer agents.

METHODS

Ulcerogenic potential was assessed using electron microscopy and measurement of an ulcer index after administering methylene blue (5-125 mg kg(-1), p.o.) or absolute ethanol (99%v/v, 2 ml, p.o.) to fasted rats. Estimation of thiobarbituric acid reactive substances, superoxide dismutase, reduced glutathione and catalase was used to assess oxidative stress. H(+)/K(+) ATPase activity, gastric mucosal blood flow and gastric acid secretion were measured to study the mechanism of methylene induced gastric ulcer.

RESULTS

Methylene blue (100 mg kg(-1), p.o.) produced marked ulceration of the gastric mucosa due to increased levels of thiobarbituric acid reactive substances, activities of the H(+)/K(+) ATPase and superoxide dismutase, and decreased blood flow to the gastric mucosa, activity of catalase combined with reduced glutathione levels.

DISCUSSION

It may be concluded that methylene blue activates the H(+)/K(+) ATPase to increase gastric acid secretion and reduces blood supply to gastric mucosa to produce oxidative stress that subsequently causes ulceration of gastric mucosa. Methylene blue can be used as an ulcerogenic agent to study mechanisms of investigational antiulcer agents.

Effect of butanol fraction of Panax ginseng head on gastric lesion and ulcer

From our previous result that Panax ginseng head extract had inhibition of gastric damages, the extract was fractionated. Among the hexane, chloroform, butanol and water fractions, butanol fraction showed the most potent inhibition of HCl.ethanol-induced gastric lesion, aspirin-induced gastric ulcer, acetic acid-induced ulcer and Shay ulcer. Butanol fraction showed significant increase in mucin secretion, and inhibited malondialdehyde (MDA) and H+/ K+ATPase activity in the stomach. This results indicate that the effectiveness of the fraction on gastric damages might be related to inhibition of acid secretion, increment of mucin secretion and antioxidant property.

Cibenzoline, an ATP-sensitive K(+) channel blocker, binds to the K(+)-binding site from the cytoplasmic side of gastric H(+),K(+)-ATPase

1. Cibenzoline, (+/-)-2-(2,2-diphenylcyclopropyl-2-imidazoline succinate, has been clinically used as one of the Class I type antiarrhythmic agents and also reported to block ATP-sensitive K(+) channels in excised membranes from heart and pancreatic beta cells. In the present study, we investigated if this drug inhibited gastric H(+),K(+)-ATPase activity in vitro. 2. Cibenzoline inhibited H(+),K(+)-ATPase activity of permeabilized leaky hog gastric vesicles in a concentration-dependent manner (IC(50): 201 microM), whereas no effect was shown on Na(+),K(+)-ATPase activity of dog kidney (IC(50): >1000 microM). Similarly, cibenzoline inhibited H(+),K(+)-ATPase activity of HEK-293 cells (human embryonic kidney cell line) co-transfected with rabbit gastric H(+),K(+)-ATPase alpha- and beta-subunit cDNAs (IC(50): 183 microM). 3. In leaky gastric vesicles, inhibition of H(+),K(+)-ATPase activity by cibenzoline was attenuated by the addition of K(+) (0.5 - 5 mM) in a concentration-dependent manner. The Lineweaver-Burk plot of the H(+),K(+)-ATPase activity shows that cibenzoline increases K(m) value for K(+) without affecting V(max), indicating that this drug inhibits H(+),K(+)-ATPase activity competitively with respect to K(+). 4. The inhibitory effect of H(+),K(+)-ATPase activity by cibenzoline with normal tight gastric vesicles did not significantly differ from that with permeabilized leaky gastric vesicles, indicating that this drug reacted to the ATPase from the cytoplasmic side of the membrane. 5. These findings suggest that cibenzoline is an inhibitor of gastric H(+),K(+)-ATPase with a novel inhibition mechanism, which inhibits gastric H(+),K(+)-ATPase by binding its K(+)-recognition site from the cytoplasmic side.

A model of reversible inhibitors in the gastric H+/K+-ATPase binding site determined by rotational echo double resonance NMR

Several close analogues of the noncovalent H(+)/K(+)-ATPase inhibitor SCH28080 (2-methyl-3-cyanomethyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine) have been screened for activity and examined in the pharmacological site of action by solid-state NMR spectroscopy. TMPIP, the 1,2,3-trimethyl analogue of SCH28080, and variants of TMPIP containing fluorine in the phenylmethoxy ring exhibited IC(50) values for porcine H(+)/K(+)-ATPase inhibition falling in the sub-10 microm range. Deuterium NMR spectra of a (2)H-labeled inhibitor titrated into H(+)/K(+)-ATPase membranes revealed that 80-100% of inhibitor was bound to the protein, and K(+)-competition (2)H NMR experiments confirmed that the inhibitor lay within the active site. The active binding conformation of the pentafluorophenylmethoxy analogue of TMPIP was determined from (13)C-(19)F dipolar coupling measurements using the cross-polarization magic angle spinning NMR method, REDOR. It was found that the inhibitor adopts an energetically favorable extended conformation falling between fully planar and partially bowed extremes. These findings allowed a model to be proposed for the binding of this inhibitor to H(+)/K(+)-ATPase based on the results of independent site-directed mutagenesis studies. In the model, the partially bowed inhibitor interacts with Phe(126) close to the N-terminal membrane spanning helix M1 and residues in the extracellular loop bridging membrane helices M5 and M6 and is flanked by residues in M4.

Metabolism of chiisanoside from Acanthopanax divaricatus var. albeofructus by human intestinal bacteria and its relation to some biological activities

The metabolic pathway of chiisanoside isolated from leaves of Acanthopanax divaricatus var. albeofructus (Araliaceae) by human intestinal bacteria and by the protein fraction of leaves of this plant were investigated, and the cytotoxic and anti-rotaviral activities of chiisanoside and its metabolite, chiisanogenin, were assayed. Chiisanogenin was produced as a main metabolite, when chiisanoside were incubated for 15 h with human intestinal bacteria. This metabolic pathway proceeded more potently with the protein fraction than with human intestinal bacteria. The in vitro cytotoxicity of chiisanogenin was superior to that of chiisanoside. H+/K+ ATPase was more potently inhibited by chiisanogenin than by chiisanoside. However, the anti-rotaviral activity of chiisanoside was more potent than that of chiisanogenin.

Early development of human gastric H,K-adenosine triphosphatase

BACKGROUND

Little is known about the early development of the gastric acid secretion in human neonates. The purpose of this study was to examine the early development of gastric H,K-adenosine triphosphatase (ATPase) by analyzing human gastric biopsy specimens.

METHODS

Eighty-eight neonates from week 25 to week 42 of gestation who were treated in a neonatal intensive care unit underwent gastroscopy with biopsy specimens obtained from the corpus. The expression of gastric H,K-ATPase protein in the gastric biopsy specimens was assessed by Western blot analysis, using an antibody directed against the gastric H,K-ATPase. The amount of H,K-ATPase expressed was compared with age, gender, clinical factors, diseases, and the macroscopic and histologic findings at endoscopy.

RESULTS

The expression of human gastric H,K-ATPase increased significantly with gestational age. There was a significant increase in the expression of gastric H,K-ATPase during the first 82 days after birth. Boys had a significantly higher expression of gastric H,K-ATPase than girls did, when it was adjusted for gestational and postnatal age. Neither the clinical features nor treatments showed significant correlations with the expression of human gastric H,K-ATPase when controlling for gestational and postnatal age.

CONCLUSIONS

This study shows that human gastric H,K-ATPase is expressed from week 25 of gestation, which agrees with earlier findings of gastric pH in preterm infants. The amount of enzyme expressed increases with gestational and postnatal age. The authors speculate that the susceptibility to gastric lesions seen in neonates is not related to the amount of H,K-ATPase. However, studies elucidating the ontogeny of gastric mucosal defense mechanisms are warranted.

Biochemical and pharmacological characteristics of a newly synthesized H+/K+-ATPase inhibitor, YJA20379-8, 3-butyryl-4-[R-1-methylbenzylamino]-8-ethoxy-1,7-naphthyridine, in pigs and rats

We have investigated the effect of the newly synthesized proton-pump inhibitor YJA20379-8, 3-butyryl-4-[R-1-methylbenzylamino]-8-ethoxy-1,7-naphthyridine, on gastric mucosal proton pump (H+/K+-ATPase) activity, gastric acid secretion and gastric lesions in experimental animals. In lyophilized pig gastric microsomes, YJA20379-8 was shown to inhibit H+/K+-ATPase activity; the inhibitory effect was not affected by pH, the IC50 (dose resulting in 50% inhibition) being 28.0 microM and 30.0 microM at pH 6.4 and pH 7.4, respectively. The effect was fully reversed by dilution and subsequent washing of the incubation mixtures of H+/K+-ATPase and YJA20379-8, suggesting the reversible nature of the enzyme inhibition. In pylorus-ligated rats, YJA20379-8 administered by different routes (intraduodenal, subcutaneous, intravenous or oral) resulted in dose-dependent suppression of basal gastric acid secretion. The duration of antisecretory action of 30 mg kg(-1) YJA20379-8 given intraduodenally was very brief (less than 7 h). Pretreatment with YJA20379-8 also dose-dependently prevented gastric lesions induced by absolute ethanol and water-immersion stress in rats. These results suggest that YJA20379-8 might exert its antiulcer activity partly by reversible suppression of acid secretion and partly by protecting the gastric mucosa against ulcerative stimuli.

Biochemical and pharmacological characteristics of a newly synthesized H+-K+ATPase inhibitor, YJA20379-1, 2-amino-4,5-dihydro-8-phenylimidazole [2,1-b]thiazolo[5,4-g]benzothiazole

The biochemical and pharmacological characteristics of a newly synthesized H+-K+ATPase inhibitor, 2-amino-4,5-dihydro-8-phenylimidazole[2,1-b]thiazolo[5,4-g]benzothiazole (YJA20379-1), were investigated. In the pig gastric microsomes, YJA20379-1 inhibited the gastric H+-K+ATPase regardless of pH condition, IC50values being 21 and 24 µM at pH 6.4 and 7.4, respectively. The inhibitory activity of YJA20379-1 was antagonized by dithiothreitol treatment but could not be reversed by dilution and washing of the enzyme preparation. In Sprague-Dawley rats, YJA20379-1, administered i.d., p.o, i.v., or s.c., significantly inhibited basal gastric acid secretion, with ED50values of 4.7, 20.2, 6.3, and 13.4 mg/kg, respectively. The antisecretory action of YJA20379-1 was short lasting (less than 7 h at an oral dosing of 30 mg/kg). Oral administration of YJA20379-1 also prevented the formation of ethanol, indomethacin, and water immersion stress induced gastric lesions and mepirizole-induced duodenal ulcers in rats. Furthermore, YJA20379-1 accelerated the healing of acetic acid induced chronic gastric ulcers in rats. In conclusion, these results suggest that YJA20379-1 has a potent inhibitory activity on the gastric H+-K+ATPase but much shorter duration of antisecretory action than omeprazole, thereby exerting its anti-ulcer effects partly with cytoprotective activity.Key words: proton pump inhibitor, acid secretion, anti-ulcer effects, cytoprotective activity.

Biochemical and pharmacological properties of a new proton pump inhibitor, 2-amino-4,5-dihydropyrido[1,2-a]thiazolo [5,4-g] benzimidazole (YJA20379-5)

This study was designed to determine biochemical and pharmacological properties of a newly synthesized benzimidazole derivative, 2-amino-4,5-dihydropyrido [1,2-a] thiazolo [5,4-g] benzimidazole (YJA20379-5) in vitro and in vivo. In the leaky membrane vesicles of pig gastric mucosa, YJA20379-5 inhibited the K(+)-stimulated H+,K(+)-ATPase activity in a concentration- and time-dependent manner, with IC50 values being 43 microM and 31 microM at pH 6.4 and 7.4, respectively. YJA20379-5, given intraduodenally, had a potent inhibitory effect on the gastric acid secretion in pylorus-ligated rats. The ED50 value for acid secretion was 15.4 mg/kg. YJA20379-5, administered orally, also suppressed gastric damages induced by water-immersion stress, indomethacin and ethanol, and duodenal damage induced by mepirizole in rats; the ED50 values were 17.6, 4.7, 3.0 and 18.7 mg/kg, respectively. Furthermore, repeated oral administration of YJA20379-5 accelerated the spontaneous healing of acetic acid-induced gastric ulcers in rats. It is concluded that the antisecretory activity of YJA20379-5 appears to be associated with inhibition of H+,K(+)-ATPase, while its antigastric and antiduodenal lesion activities are primarily related to the antisecretory effect.

Biochemical and pharmacological characteristics of 3-butyryl-8-methoxy-4-[(2-thiophenyl)amino]quinoline, a new proton-pump inhibitor, in rabbit gastric microsomes and in rats

We have investigated the properties of the newly synthesized proton-pump inhibitor, 3-butyryl-8-methoxy-4-[(2-thiophenyl)amino]quinoline (YJA20379-6), on gastric mucosal proton-pump (H+/K+-ATPase) activity, gastric acid secretion and gastroduodenal lesions in experimental rats. YJA20379-6 markedly inhibited H+/K+-ATPase activity in rabbit isolated gastric mucosal microsomes, confirming its classification as a proton-pump inhibitor. The inhibitory efficacy of YJA20379-6 on the proton pump was approximately 14-times higher than that of omeprazole at pH 7.4. YJA20379-6 given intraduodenally had a potent inhibitory effect on gastric secretion in pylorus-ligated rats (ED50 22.9 mg kg(-1)) but was less active than omeprazole. Pretreatment of rats with YJA20379-6 dose-dependently protected the gastric mucosa from damage induced by water-immersion stress, indomethacin and absolute ethanol, and the duodenal mucosa from damage induced by mepirizole. Repeated administration of YJA20379-6 also dose-dependently accelerated the spontaneous healing of acetic acid-induced gastric ulcers. These results suggest that YJA20379-6 has potent anti-secretory and anti-ulcer effects which are exerted by suppression of H+/K+-ATPase activity in gastric parietal cells. YJA20379-6 might be useful for the clinical treatment of peptic ulcer diseases.

Three-dimensional structure of the porcine gastric H,K-ATPase from negatively stained crystals

A low-resolution three-dimensional model of membrane-bound H,K-ATPase from pig gastric mucosa has been reconstructed by electron microscopy and image processing of two-dimensional crystals in negative stain. The crystal formation is induced by magnesium and vanadate, which stabilize the E2 conformation of the enzyme. The unit cell, with a size of a = b = 123 A, gamma = 90 degrees, has tetragonal p4 symmetry. There are four separate alpha beta protomers within each unit cell. The high-contrast region is limited to the cytoplasmic part of the protein. The total volume of the observed asymmetric protein domain corresponds to a molecular mass of 80-90 kDa. It consists mainly of a large pear-shaped domain measuring 60 x 45 A2, with a height of 50 A as measured perpendicular to the membrane plane. A small stalk segment, 20 A in length, forms a connection to the transmembrane region.

Properties of the reversible K(+)-competitive inhibitor of the gastric (H+/K+)-ATPase, SK&F 97574. I. In vitro activity

SK&F 97574 (3-butyryl-4-(2-methylamino)-8-(2-hydroxyethoxy)quinoline), is a potent inhibitor of the (H+/K+)-ATPase in membrane vesicles isolated from porcine gastric mucosa. It inhibits (H+/K+)-ATPase activity in lyophilised vesicles in a kinetically competitive manner with respect to the activating cation, K+, with an inhibition constant (Ki) of 0.46 +/- 0.003 microM. Inhibition of (H+/K+)-ATPase activity is freely reversible. Binding of SK&F 97574 was shown to be mutually exclusive and the previously reported reversible (H+/K+)-ATPase inhibitors, SCH 28080 and MDPQ. Therefore, despite its structural dissimilarity, SK&F 97574 appears to bind to the same lumenal region of the (H+/K+)-ATPase identified as the binding site for these compounds. SK&F 97574 is a weak base (pKa = 6.86), and would therefore be expected to accumulate in the acidic compartment at the lumenal face of the parietal cell. In intact gastric vesicles (which have the lumenal face of the ATPase on the interior), SK&F 97574 inhibited ATP-dependent H(+)-transport with a similar potency to ATPase activity. SK&F 97574 is therefore relatively membrane permeable, and would be predicted to gain access readily to its site of action in vivo. The effect of pH on inhibition of H+/K(+)-ATPase activity by SK&F 97574 is consistent with its being active only in its protonated form. The selectivity of SK&F 97574 for the gastric (H+/K+)-ATPase was tested by examining its ability to inhibit a closely related p-class pump, the (Na+/K+)-ATPase from dog kidney. SK&F 97574 was found to have a 60-fold greater sensitivity for the former enzyme. The (Na+/K+)-ATPase was not inhibited in a K(+)-competitive manner by SK&F 97574, indicating an entirely different, probably nonspecific, mechanism.

Effects of ranitidine on gastric vesicles containing H+,K(+)-adenosine triphosphatase in rats

BACKGROUND

To ascertain the mechanism for rebound acid hypersecretion after treatment with an H2-receptor blocker, we investigated the effects of ranitidine on gastric H+,K(+)-adenosine triphosphatase (ATPase) in rats.

METHODS

Male Wistar rats received ranitidine (1-50 mg/kg body weight intraperitoneally twice a day for 5 days). The rats were starved for 15 h after the last treatment and then killed, and gastric vesicles containing H+,K(+)-ATPase were prepared.

RESULTS

Treatment with ranitidine dose-dependently increased protein content in the gastric vesicular fraction purified from the gastric mucosa without changing total protein content. Ranitidine also increased the content of a 94,000-dalton protein, the catalytic subunit of H+,K(+)-ATPase. On the other hand, ranitidine did not affect the specific activity of the enzyme (mumol/min/mg of the gastric vesicular protein). Since gastric vesicles in the fasting state mainly consist of the tubulovesicular membrane, these results suggest that ranidine administration increases total tubulovesicular H+,K(+)-ATPase content (mumol/min/rat) by increasing the number of tubulovesicles per parietal cell. The ranitidine-induced increase in total tubulovesicular H+,K(+)-ATPase activity was still evident 1 week after treatment and returned to control level 1 month later.

CONCLUSIONS

All these findings suggest that the increased content and total activity of tubulovesicular H+,K(+)-ATPase after ranitidine treatment may contribute to the mechanism for acid rebound after H2-blocker therapy.

Effect of histamine on soluble and membrane-associated carbonic anhydrase (CA) activity of pig and bovine gastric mucosa

1. As part of their characterization, the effect of histamine (H) on both soluble and membrane-associated carbonic anhydrase (CA) activity of pig and bovine gastric light microsomal membranes was investigated. 2. H did not affect the activity of soluble CA purified from pig oxyntic mucosa, whereas 10(-7) and 10(-4) M H produced a significant enhancement of pig gastric firmly-membrane-associated CA activity at 20 and 30 degrees C (about 2-3-fold), but it failed at 0 degree C. The increase of activity produced by H occurred within 1 min and it was maintained for at least 15 min. 3. H also stimulated bovine gastric firmly-membrane-associated CA activity, the stimulation also being dependent on temperature.

Tertiary amines as antagonists of both the luminal and cytosolic K(+)-site of gastric H,K-ATPase

Tertiary amines like imidazole and triallylamine lower the apparent affinity of K+ in the ATP hydrolysis reaction of pig gastric H,K-ATPase in a pH and amine concentration dependent way. The mechanism and sidedness of this effect was studied by analyzing the partial reactions of the enzyme in both leaky and ion-tight vesicles. In leaky vesicles Tris and Hepes had nearly no effect on the apparent Km for K+ in the ATPase reaction, but imidazole (Ki = 13 mM) and triallylamine (Ki = 1.6 mM) markedly decreased the K+ affinity. The steady-state ATP-phosphorylation level in the absence of K+ was not or only slightly affected by these compounds. The reduction of the ATP-phosphorylation level by K+, however, again depended on both the type and concentration of tertiary amine used. A comparable K(+)-amine antagonism was observed in the dephosphorylation reaction. In tightly sealed vesicles, where no activation of K+ at the luminal side could occur, K+ reduced the affinity for ATP in the phosphorylation reaction. Triallylamine counteracted this effect. The K(+)-activated p-nitrophenylphosphatase activity in these ion-tight vesicles also showed a K(+)-triallylamine antagonism. Inhibition of H,K-ATPase activity in these vesicles by triallylamine was immediate (with nigericin present in order to allow intravesicular K+ activation), suggesting the transmembrane feature of this inhibition. These results indicate that tertiary amines decrease the affinity for K+ at both luminal and cytosolic binding sites by interaction at the cytosolic side of the membrane. This results in shifts in the equilibrium of both the E1.H<==>E1.K transition and in the dephosphorylation reaction, E2-P-->E2.K.

Gastric acid secretion in the chicken: effect of histamine H2 antagonists and H+/K(+)-ATPase inhibitors on gastro-intestinal pH and of sexual maturity calcium carbonate level and particle size on proventricular H+/K+ ATPase activity

1. Cimetidine was more potent 4 hr after a single injection of 25 or 100 mg/kg body wt in increasing gastric pH than other H2 receptor antagonists, ranitidine and famotidine but was less efficient than H+/K(+)-ATPase inhibitors. Omeprazole rose proventricular and gizzard pH at a lower dose than SCH 28080 and Ro 18-5364 (30, 50 and 200 mg/kg body wt, respectively). 2. Proventricular and gizzard pH values were maximal 1 and 4 hr after a single injection of 7.5 mumol/kg body wt omeprazole. Inhibition of acid secretion was maintained for 24 hr after an injection of 100 mumol/kg. 3. H+/K(+)-ATPase activity in vitro was 10 mumol Pi/hr/mg protein in the microsomal fractions of the proventriculus. It was doubled by nigericine and inhibited by SCH 28080. However, western blots by high specific H+/K(+)-ATPase monoclonal antibody 95-A3 and 95-111 recognized a 42 kDa band but hardly exhibited the specific 95 kDa band recognition. 4. Chickens and immature pullets showed a higher H+/K(+)-ATPase activity than laying hens. Calcium level of the diet did not affect the enzyme activity but coarse particles of calcium fed to pullets or laying hens enhanced the H+/K(+)-ATPase activity when compared with ground particles.

pH-dependent association of carbonic anhydrase (CA) with gastric light microsomal membranes isolated from bovine abomasum. Partial characterization of membrane-associated activity

1. The effect of pH on the association of carbonic anhydrase (CA) with bovine gastric light microsomal membranes (LMMs) was investigated (a) by washing LMMs containing CA activity with solutions of different pHs; (b) by studying the adsorption at various pHs of soluble bovine erythrocyte CA to washed gastric LMMs. In both cases, the association of CA with gastric LMMs was dependent on pH, being lower at neutral or alkaline pH. 2. The amount of soluble CA associated with gastric LMMs at pHs 8.0 and 9.0 was reduced when 140 mM K+/10 mM Na+ was added to the incubation medium. 3. Two sources of CA activity in bovine gastric LMMs were assumed: a loosely- and a firmly-membrane-associated activity. Both CA activities were dose-dependently inhibited by acetazolamide (I50: 3.6 x 10(-9) and 8.4 x 10(-9) M, respectively) and by chloride, acetate, iodide, bromide and nitrate at 100 mM. Firmly-membrane-associated activity appeared to be less sensitive to inhibition by acetazolamide, chloride and iodide. 4. Both activities exhibited different behavior and stability following treatment with alkaline Triton X-100. 5. The possible importance of a membrane-associated CA activity in gastric LMMs related to gastric acid secretion is discussed.

Pharmacological properties of blood pressure and heart rate control in suncus

Blood pressure and heart rate and responses to various physiological substances in suncus were characterized and compared with those in mice. The blood pressures of the two species were similar, but the heart rate of suncus (about 400 beat/min) was significantly lower than that of mice. Norepinephrine increased the blood pressure but decreased the heart rate in suncus. The latter was blocked by cervical vagotomy. Sensitivities to acetylcholine and isoproterenol were lower in suncus. These results suggest that regulation of blood pressure and heart rate in suncus is very unique and different from the well-defined system of the rodents.

Studies on carbonic anhydrase (CA) of light microsomal membranes isolated from bovine and pig gastric mucosa

1. The occurrence and characteristics of carbonic anhydrase (CA) activity were studied in light microsomal membranes (LMM) purified from bovine gastric mucosa. 2. Bovine gastric LMM contained a high activity of CA ranging from 170 to 400 mumol.H+/min/mg protein when assayed at 0 degree C by pH-stat technique. 3. The addition of 2mM EDTA to the assay mixture increased the enzyme activity. Lower concentrations (0.5-1 mM) had no effect. 4. The enzyme activity was dose-dependently inhibited by acetazolamide and furosemide (I50: 5 x 10(-10) M and 4.8 x 10(-7) M, respectively) and by chloride ion (Ki 85 mM) and appeared to be quite stable to treatment with alkaline Triton X-100. 5. Most of the CA activity is loosely associated with the LMM since it was removed by different washing treatments. Nevertheless, after extensive washes, gastric LMM still contained CA activity suggesting the existence of a firmly membrane-associated form of CA. 6. Values of CA activity higher than those reported previously were found in pig gastric LMM. Furthermore, the washing treatments described in this work were more effective in washing CA activity off pig gastric LMM than procedures described previously.

Kinetics of transient pump currents generated by the (H,K)-ATPase after an ATP concentration jump

(H,K)-ATPase containing membranes from hog stomach were attached to black lipid membranes. Currents induced by an ATP concentration jump were recorded and analyzed. A sum of three exponentials (tau 1(-1) approximately 400 sec-1, tau 2(-1) approximately 100 sec-1, tau 3(-1) approximately 10 sec-1; T = 300 K, pH 6, MgCl2 3 mM, no K+) was fitted to the transient signal. The dependence of the resulting time constants and the peak current on electrolyte composition, ATP conversion rate, temperature, and membrane conductivity was recorded. The results are consistent with a reaction scheme similar to that proposed by Albers and Post for the NaK-ATPase. Based on this model the following assignments were made: tau 2 corresponds to ATP binding and exchange with caged ATP. tau 1 describes the phosphorylation reaction E1 x ATP-->E1P. The third, slowest time constant tau 3 is tentatively assigned to the E1P-->E2P transition. This is the first electrogenic step and is accelerated at high pH and by ATP via a low affinity binding site. The second electrogenic step is the transition from E2K to E1H. The E2K<==>E1H equilibrium is influenced by potassium with an apparent K0.5 of 3 mM and by the pH. Low pH and low potassium concentration stabilize the E1 conformation.

Charge transport by ion translocating membrane proteins on solid supported membranes

A new method for the investigation of ion translocating membrane proteins is presented. Protein containing membrane fragments or vesicles are adsorbed to a solid supported membrane. The solid supported membrane consists of a lipid monolayer on a gold evaporated or gold sputtered glass substrate which is coated with a long chained mercaptan (CH3(CH2)mSH, m = 15, 17). Specific conductance and specific capacitance of the solid supported membrane are comparable to those of a black lipid membrane. However, the solid supported membrane has the advantage of a much higher mechanical stability. The electrical activity of bacteriorhodopsin, Na,K-ATPase, H,K-ATPase, and Ca-ATPase on the solid supported membrane is measured and compared to signals obtained on a conventionally prepared black lipid membrane. It is shown that both methods yield similar results. The solid supported membrane therefore represents an alternative method for the investigation of electrical properties of ion translocating transmembrane proteins.

Gastric H+, K(+)-ATPase inhibition by catechins

Five catechins, (+)-catechin, (-)-epicatechin, (-)-epicatechin gallate, (-)-epigallocatechin and (-)-epigallocatechin gallate, inhibited gastric H+, K(+)-ATPase activity with IC50 values ranging from 1.7 x 10(-4) to 6.9 x 10(-8) M, with (-)-epigallocatechin gallate as the most potent inhibitor. The intensity of inhibitor activity paralleled the number of phenolic hydroxy groups in the molecule. The inhibition of the enzyme by (-)-epicatechin was competitive with respect to ATP and noncompetitive with respect to K+. These findings suggest that the anti-secretory and anti-ulcerogenic effects of catechins previously reported, are due to their inhibitory activity on gastric H+, K(+)-ATPase.

Gastric parietal cell H(+)-K(+)-ATPase microsomes are associated with isoforms of ankyrin and spectrin

Stimulation of HCl secretion by gastric parietal cells requires the fusion of cytoplasmic H(+)-K(+)-ATPase-bearing tubulovesicles with the apical membrane. This insertion of membrane results in a dramatic increase in apical surface area through the formation of microvilli. To elucidate the elements that may stabilize the newly inserted H(+)-K(+)-ATPase within the apical membrane, we searched for specific cytoskeletal proteins associating with the gastric enzyme. We document by immunoblot analysis that ankyrin, spectrin, and actin copurify with H(+)-K(+)-ATPase microsomes prepared from gastric parietal cells. Coprecipitation of 125I-labeled native erythrocyte ankyrin with the H(+)-K(+)-ATPase from gastric microsomes using anti-H(+)-K(+)-ATPase antibodies suggests that ankyrin associates with the H(+)-K(+)-ATPase. Indirect immunofluorescence and confocal microscopy show that ankyrin and H(+)-K(+)-ATPase cosegregate within resting and secreting parietal cells. Taken together, these data suggest that the association of the gastric H(+)-K(+)-ATPase with spectrin and actin is mediated by ankyrin and that this interaction contributes to the maintenance of the polarized distribution of the enzyme to the apical domain of gastric parietal cells during acid secretion.

Effect of stilbene derivatives on gastric H+, K(+)-ATPase

The effect of naturally occurring hydroxystilbene, 3,3',4,5-tetrahydroxystilbene (piceatanol), and its derivatives on gastric H+, K(+)-ATPase was studied. Piceatanol inhibited H+, K(+)-ATPase in a dose-dependent manner. The 50% inhibition value was 4.3 x 10(-6) M. It was found from the kinetic study that the inhibition of the enzyme by piceatanol was competitive with respect to ATP and was noncompetitive with respect to K+. Piceatanol also effectively inhibited gastric acid secretion. However, methylation of phenolic hydroxy groups of piceatanol resulted in a complete loss of inhibition of the enzyme and acid secretion, suggesting the role of phenolic hydroxy groups in the inhibition. The study on hydroxystilbene derivatives also showed that phenolic hydroxy groups are important in the interaction with H+, K(+)-ATPase and that stilbenes with neighbouring hydroxy groups are the most effective inhibitors.

Site-directed antibodies as topographical probes of the gastric H,K-ATPase alpha-subunit

Gastric acid is secreted by an ATP-driven H+ and K+ exchanger (H,K-ATPase), an integral apical membrane protein of parietal cells. Although the primary structure of the enzyme is known, its higher order structure is uncertain. In order to acquire topographical probes of native, microsomal H,K-ATPase, synthetic peptides corresponding to the 17 amino-terminal (N-peptide) and 16 carboxyl-terminal (C-peptide) residues of pig gastric H,K-ATPase alpha-subunit were coupled to keyhole limpet hemocyanin (KLH). Rabbits were immunized with peptide-KLH conjugates and their sera were tested for specificity by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and immunocytochemistry. All sera showed high ELISA reactivities with synthetic peptides, peptide-BSA conjugates, and microsomal H,K-ATPase adsorbed to microtiter wells (some titers greater than 1:10(4)). Immunoblots of H,K-ATPase resolved by SDS-PAGE showed both N-peptide and C-peptide antibodies reacting with a single 94 kDa band. All sera selectively stained parietal cells in pig gastric mucosal sections. Preimmune sera gave negative or weak signals in all assays. In competition ELISAs, N-peptide antibodies, but not C-peptide antibodies, were displaced from the corresponding bound synthetic peptides by added microsomal H,K-ATPase. One of the N-peptide antibodies inhibited H,K-ATPase activity by more than 50%; binding of this antibody was decreased when ATP or K+ were bound to the enzyme. These results indicate a cytoplasmically-oriented alpha-subunit N-terminus which may participate conformationally in the H,K-ATPase catalytic cycle, and suggest that antibodies against synthetic H,K-ATPase peptides are potentially useful probes of native microsomal H,K-ATPase topography.

Inhibition of gastric H+,K(+)-ATPase and acid secretion by cassigarol A, a polyphenol from Cassia garrettiana Craib

The effects of cassigarol A, a naturally occurring polyphenol, on gastric H+,K(+)-ATPase and gastric acid secretion were studied. Cassigarol A inhibited H+,K(+)-ATPase and K-stimulated p-nitrophenyl phosphatase from hog gastric mucosa with 50% inhibition of 1.2 x 10(-6) and 6.3 x 10(-6) M, respectively. The kinetic study showed that the inhibition of H+,K(+)-ATPase by cassigarol A was competitive with respect to ATP and non-competitive with respect to K+. Cassigarol A inhibited both H+,K(+)-ATPase-mediated proton transport and 2-deoxy-D-glucose-induced acid secretion. On the other hand, cassigarol A acetate, in which phenolic hydroxy groups are acetylated, was not effective in the inhibition of enzyme activity and acid secretion. These results indicate that cassigarol A is a potent inhibitor of gastric H+,K(+)-ATPase, that the anti-secretory activity of cassigarol A is related to the inhibition of H+,K(+)-ATPase and that an important moiety of cassigarol A in the interaction with the enzyme is the phenolic hydroxy groups.

Involvement of the H+/K(+)-ATPase alpha subunit as a major antigenic protein in autoimmune gastritis induced by neonatal thymectomy in mice

Autoimmune gastritis develops spontaneously in approximately 60% of BALB/c mice thymectomized neonatally. Histologically and clinically it is similar to the atrophic gastritis associated with pernicious anaemia in humans. Here we identified antigenic protein relating to the pathogenesis of autoimmune gastritis in these mice. All sera from 32 thymectomized mice with gastritis contained autoantibodies to the vesicular fraction prepared from rat gastric parietal cells. Immunoblot analysis revealed all of these to react with a 94-kD protein corresponding in molecular mass with the H+/K(+)-ATPase alpha subunit. Some sera were also reactive with 65-85-kD and/or 60-kD proteins, whose sizes correspond to the H+/K(+)-ATPase beta subunit and intrinsic factor, respectively. The finding that immuno-adsorption with these sera resulted in reduction of H+/K(+)-ATPase activity in the vesicular fraction, supported a conclusion of H+/K(+)-ATPase alpha and/or beta subunits as the antigenic proteins. After immunization of normal syngeneic mice with various doses of gastric parietal cells or their vesicular fraction, all sera from animals demonstrating atrophic gastric mucosa with lymphocyte infiltration reacted with the H+/K(+)-ATPase alpha subunit. No antibodies to other proteins were induced even in mice immunized with higher doses of antigen. We therefore conclude that H+/K(+)-ATPase alpha subunit is important as the target antigen in pathogenesis of autoimmune gastritis in neonatally thymectomized mice, probably due to a high affinity for the MHC molecule.

Rapid purification of the gastric H+/K(+)-ATPase complex by tomato-lectin affinity chromatography

We have previously shown that tomato lectin binds specifically to the 60-90 kDa membrane glycoprotein of parietal cell tubulovesicles, the beta-subunit of the gastric H+/K(+)-ATPase (proton pump) [Callaghan, Toh, Pettitt, Humphris & Gleeson (1990) J. Cell Sci. 95, 563-576; Toh, Gleeson, Simpson, Mortiz, Callaghan, Goldkorn, Jones, Martinelli, Mu, Humphris, Pettitt, Mori, Masuda, Sobieszczuk, Weinstock, Mantamadiotis & Baldwin (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 6418-6422]. Here we have exploited this interaction for the development of a rapid single-step chromatography procedure for the purification of an active pig gastric proton pump complex. Initially, H+/K(+)-ATPase-enriched membranes, prepared from pig gastric microsomes by density-gradient centrifugation, were extracted in 1% Triton X-100 and passed through a 1 ml tomato lectin-Sepharose 4B column. The bound material, eluted with 20 mM-chitotriose, showed a major band with an apparent molecular mass of 95 kDa, and a faint broad band of 60-90 kDa, by SDS/PAGE. N-Glycanase treatment of the bound material resulted in the appearance of a 35 kDa band, the size of the protein core of the 60-90 kDa glycoprotein beta-subunit. The two components were identified as the 95 kDa alpha-subunit and the 60-90 kDa beta-subunit of the gastric H+/K(+)-ATPase, by immunoreactivity with monospecific antibodies, and by tryptic peptide sequences of the tomato-lectin-bound material. The beta-subunit was present in approximately equimolar amounts to the catalytic alpha-subunit. Whereas the gastric H+/K(+)-ATPase was not active after solubilization in 1% Triton X-100, solubilization of density-gradient-purified membranes in the non-ionic detergent, C12E8, followed by chromatography of the extract on tomato lectin-Sepharose 4B, resulted in the purification of the gastric H+/K(+)-ATPase complex which exhibited K(+)-dependent phosphatase activity. This is the first report of a rapid purification of a partially active solubilized gastric H+/K(+)-ATPase complex.

Immunopurification of gastric parietal cell tubulovesicles

1. The tubulovesicles of hog and rabbit gastric parietal cells were immunopurified from microsomes using monoclonal antibodies against the (H+, K+)-ATPase. 2. The best yields of immunoprecipitation were obtained with an ATPase/mAb molar ratio of 0.3: the immunoprecipitate contained 79 and 90% of the hog and rabbit microsomal PNPPase activity respectively and K(+)-stimulated ATPase specific activity was 221 +/- 29 mumoles Pi per hr and per mg of membrane protein. 3. The immunoprecipitate contained vesicles that were 85% cytoplasmic-side out, like tubulovesicles in vivo, demonstrating that the epitopes were cytoplasmic. 4. The alpha-beta protomer of (H+, K+)-ATPase accounted for 80 +/- 12% of the immunopurified proteins. 5. The major other proteins ran at 80, 75, 69, 57, 47, 44, 39, 34 and 32 kDa on the SDS-PAGE. 6. Comparative analysis between sucrose-gradient purified fractions and immunopurified tubulovesicles demonstrated that carbonic anhydrase and actin were contaminants and that the 53 kDa and presumably the 50 kDa bands of the gradient fraction were alpha and beta subunits of F1 ATPase.

Effect of free fatty acids and detergents on H,K-ATPase. The steady-state ATP phosphorylation level and the orientation of the enzyme in membrane preparations

The effects of detergents and free fatty acids on the K(+)-activated ATPase activity and on the steady-state phosphorylation level of pig gastric H,K-ATPase were studied. Unsaturated free fatty acids inhibited the K(+)-activated ATPase activity, due to inactivation of the enzyme (long-term effects) and to a decrease in the K(+)-sensitive dephosphorylation rate (short-term effects). The degree of inhibition depended on the reaction conditions: the protein concentration, the temperature and the ligands used. No effect was observed when saturated- or methylated unsaturated fatty acids were tested. Free fatty acids and the detergent C12E8 increased the steady-state ATP phosphorylation level, indicating the presence of vesicular structures in the H,K-ATPase preparations. At higher concentrations these compounds inactivated H,K-ATPase, which was measured as a decrease in phosphorylation capacity. By combining the data from the ATP phosphorylation level in the absence and presence of C12E8 (without inactivation) and the data from the K(+)-activated ATPase activity with and without ionophore the tightness of vesicular preparations and the orientation of H,K-ATPase was determined. A rather simple method for the isolation of H,K-ATPase is reported, which yields highly purified H,K-ATPase preparations with a ATP phosphorylation capacity of 3.9 nmol P per mg protein or 0.57 mol P per mol alpha beta protomer. This number suggests that each alpha-subunit H,K-ATPase can be phosphorylated at the same time.

Inhibition of gastric H+,K(+)-ATPase by the anti-ulcer agent, sofalcone

Effects of the anti-ulcer agent, sofalcone, on gastric H+,K(+)-ATPase were studied as well as those of other chalcone derivatives, chalcone and sophoradin. These drugs inhibited pig gastric H+,K(+)-ATPase in a dose-dependent manner. They were 5-10-fold less inhibitory toward Na+,K(+)-ATPase than H+,K(+)-ATPase. The potencies of these drugs on the inhibition of enzymes were as follows: sophoradin greater than sofalcone greater than chalcone. Kinetic studies showed that the inhibition of H+,K(+)-ATPase by sofalcone was competitive with respect to ATP and was non-competitive with respect to K+. Sofalcone also inhibited H+,K(+)-ATPase mediated proton transport and reduced the phosphoenzyme level. These results suggest that sofalcone inhibits gastric H+,K(+)-ATPase competitively with ATP at the ATP site and thereby blocks the phosphorylation of the enzyme. This may be the cause of the anti-secretory activity of sofalcone.

SK&F 96067 is a reversible, lumenally acting inhibitor of the gastric (H+ + K+)-ATPase

SK&F 96067 [3-butyryl-4-(2-methylphenylamino)-8-methoxyquinoline] has been identified, from a novel class of 4-aminoquinolines, as a reversible inhibitor of the gastric (H+ + K+)-ATPase. This compound has been studied in gastric membrane vesicle preparations enriched in the (H+ + K+)-ATPase. At pH 7.0, SK&F 96067 inhibited K(+)-stimulated ATPase activity competitively with respect to the activating cation K+, with a Ki value of 0.39 +/- 0.05 microM. Under comparable conditions, SK&F 96067 was 32 times more potent as an inhibitor of the gastric (H+ + K+)-ATPase relative to the closely related (Na+ + K+)-ATPase. Studies in intact gastric vesicles showed that SK&F 96067 also inhibited hydrogen ion transport. Using the initial rate of acridine orange quenching as the index of acidification, an IC50 of 0.84 +/- 0.24 microM was observed. Steady state acidification, as measured by aminopyrine accumulation, was inhibited with greater potency (IC50 = 0.06 +/- 0.01 microM) consistent with the accumulation of this inhibitor into the intravesicular acidic space to a site of action on the inside (lumenal) face of the enzyme. Inhibition of ATPase activity in the presence of both SK&F 96067 and the K(+)-competitive (H+ + K+)-ATPase inhibitor, SCH 28080, indicated that their binding was mutually exclusive, consistent with SK&F 96067 acting at the same lumenal binding site as does SCH 28080. The steady-state inhibition kinetics of SK&F 96067 against K(+)-stimulated ATPase activity were followed as a function of pH. At pH 6.6 and 7.0 the inhibition was competitive with respect to the activating cation K+. At pH 7.5 and 8.1 a mixed pattern of inhibition was detected. Thus, at alkaline pH values, the binding of SK&F 96067 and K+ were no longer mutually exclusive. The potency of SK&F 96067 decreased as pH rose, consistent with the protonated form of the inhibitor being the preferred inhibitory species. A kinetic model is discussed, in which, at acidic pH, the protonated form of SK&F 96067 binds to the enzyme competitively with respect to K+, whereas, at alkaline pH, the neutral form of SK&F 96067 can bind simultaneously with K+.

Omeprazole and bafilomycin, two proton pump inhibitors: differentiation of their effects on gastric, kidney and bone H(+)-translocating ATPases

The effects of omeprazole and bafilomycin on processes dependent on two different types of H(+)-translocating ATPases were compared. A H(+)-ATPase of the E1E2-type, the H+,K(+)-ATPase, was purified from gastric mucosa. Vacuolar type H(+)-ATPases were prepared both from kidney medulla and from osteoclast-containing medullary bone. H+,K(+)-ATPase-mediated proton transport in gastric vesicles was selectively inhibited by omeprazole with a high potency (inhibitory concentrations greater than or equal to 3 microM) and in time- and pH-dependent manner. This result is consistent with the mechanism of action of omeprazole, which is dependent on acid-induced transformation of the drug into an active inhibitor reacting with luminally accessible sulfhydryl groups of the enzyme. Accordingly, the presence of the membrane-impermeable mercaptane glutathione did not affect the inhibitory action of omeprazole on the H+,K(+)-ATPase. Proton transport in kidney- and bone-derived membrane vesicles was also inhibited by omeprazole, but with a lower potency (inhibitory concentrations greater than or equal to 100 microM). Furthermore, the presence of glutathione totally abolished this inhibition, indicating that cytosolic, rather than luminal, SH-groups of the respective vacuolar H(+)-ATPase were interacting with omeprazole at high concentrations. In line with these results, it was found that omeprazole was much more potent in inhibiting acid production in isolated gastric glands (IC50 approximately 0.25 microM) than in inhibiting osteoclast-mediated 45Ca-release in isolated mouse calvaria (IC50 approximately 200 microM). Bafilomycin, on the other hand, was much more effective in inhibiting proton transport mediated by the vacuolar H(+)-ATPases in the kidney- and bone-derived membrane vesicles (IC50 approximately 2 nM) than in inhibiting H+,K(+)-ATPase-mediated proton transport in gastric membrane vesicles (IC50 approximately 50 microM). Thus, approximately 10(4) times higher concentrations of bafilomycin were needed to inhibit the H+,K(+)-ATPase to the same extent as the vacuolar H(+)-ATPase. A similar difference in potency of bafilomycin was found when its inhibitory effect was determined in isolated mouse calvaria (IC50 approximately 2.5 nM) and in isolated gastric glands (IC50 approximately 5 microM). Hence, omeprazole was found to be a specific inhibitor of the H+,K(+)-ATPase under physiological conditions, i.e. in the presence of glutathione, while bafilomycin was found to be selective towards vacuolar H(+)-ATPases.

Monoclonal antibodies specific for the core protein of the beta-subunit of the gastric proton pump (H+/K+ ATPase). An autoantigen targetted in pernicious anaemia

The gastric H+/K(+)-transporting adenosine triphosphatase (H+/K+ ATPase) (proton pump) consists of a catalytic alpha-subunit and a recently proposed 60-90-kDa glycoprotein beta-subunit. Using dog gastric membranes as the antigen, we have produced two murine monoclonal antibodies, 4F11 (IgG1) and 3A6 (IgA), which are specific for the 60-90-kDa glycoprotein. The monoclonal antibodies (1) specifically stained the cytoplasm of unfixed and formalin-fixed dog gastric parietal cells; (2) specifically reacted by ELISA with gastric tubulovesicular membranes; (3) recognised epitopes located on the luminal face of parietal cell tubulovesicular membranes, the site of the proton pump, by immunogold electron microscopy; (4) immunoblotted a 60-90-kDa molecule from tubulovesicular membranes and a 35-kDa component from peptide N-glycosidase-F-treated membrane extracts; (5) immunoblotted the 60-90-kDa parietal cell autoantigen associated with autoimmune gastritis and pernicious anemia, purified by chromatography on parietal cell autoantibody- or tomato-lectin-Sepharose 4B affinity columns, and the 35-kDa protein core of this autoantigen; this autoantigen has amino acid sequence similarity to the beta-subunit of the related Na+/K(+)-transporting adenosine triphosphatase (Na+/K+ ATPase) [Toh et al. (1990) Proc. Natl Acad. Sci. 87, 6418-6422]; (6) co-precipitated a molecule of 95 kDa with the 60-90-kDa molecule from 125I-labelled detergent extracts of dog tubulovesicular membranes; and (7) co-purified the catalytic alpha-subunit of the H+/K+ ATPase with the 60-90-kDa molecule by immunoaffinity chromatography of tubulovesicular membrane extracts on a monoclonal antibody 3A6-Sepharose 4B column, indicating a physical association between the two molecules. These results provide further evidence that the 60-90-kDa glycoprotein is the beta-subunit of the gastric H+/K+ ATPase. We conclude that the monoclonal antibodies specifically recognise luminal epitopes on the 35-kDa core protein of the 60-90-kDa beta-subunit of the gastric proton pump, a major target molecule in autoimmune gastritis and pernicious anaemia. These monoclonal antibodies will be valuable probes to study the structure and function of this associated beta-subunit, as well as the ontogeny of the gastric proton pump.

Inhibition of gastric H+, K(+)-ATPase by quercetin

The effects of the naturally occurring flavonoid, quercetin, on gastric H+, K(+)-ATPase were investigated. Quercetin inhibited hog gastric H+, K(+)-ATPase and K(+)-stimulated p-nitrophenyl phosphatase (K(+)-pNPPase) activity in a dose dependent manner with IC50 values of 2.3 microM, and 6.0 microM respectively. The inhibition of H+, K(+)-ATPase by quercetin is competitive with ATP and is noncompetitive with K+. The steady-state phosphorylation level of the enzyme was also dose-dependently reduced by quercetin with an IC50 value of 4.5 microM. These results suggest that quercetin reduces the phosphorylated enzyme level by competition with ATP, and thereby inhibits the H+, K(+)-ATPase activity.

The beta-subunit of the gastric H+/K(+)-ATPase can occur without the alpha-subunit

The renal Na+/K(+)-ATPase and the gastric H+/K(+)-ATPase both contain alpha- and beta-subunits. We report here the identification and partial purification of a second population of the beta-subunit of the gastric H+/K(+)-ATPase, which has no accompanying alpha-subunit detectable by Coomassie blue staining or by Western blotting with monoclonal antibodies specific for the alpha-subunit.

Inhibition of gastric H+, K(+)-ATPase by chalcone derivatives, xanthoangelol and 4-hydroxyderricin, from Angelica keiskei Koidzumi

Two chalcone derivatives, xanthoangelol (1) and 4-hydroxyderricin (II) isolated from Angelica keiskei Koidzumi, inhibited pig gastric H+, K(+)-ATPase with IC50 values of 1.8 and 3.3 microM, respectively. The inhibition by I or II was competitive with respect to ATP and was non-competitive with respect to K+ I and II also inhibited K+, stimulated p-nitrophenyl phosphatase, with IC50 values of 1.3 and 3.5 microM, respectively. Proton transport in-vitro was inhibited by I or II, in a dose-dependent manner, 1 at 100 mg kg-1, i.p. significantly inhibited acid secretion and the formation of stress-induced gastric lesions. These results suggest that the antisecretory effect of 1 is due to the inhibition of gastric H+, K(+)-ATPase.

Evidence for the presence of a proton pump of the vacuolar H(+)-ATPase type in the ruffled borders of osteoclasts

Microsomal membrane vesicles prepared either from chicken medullary bone or isolated osteoclasts were shown to have ATP-dependent H(+)-transport activity. This activity was N-ethylmaleimide-sensitive but resistant to oligomycin and orthovanadate, suggesting a vacuolar-type ATPase. Furthermore, immunological cross-reactivity of 60- and 70-kD osteoclast membrane antigens with Neurospora crassa vacuolar ATPase was observed when analyzed by immunoblotting. Same antibodies labeled only osteoclasts in chicken and rat bone in immunohistochemistry. Immunoelectronmicroscopy localized these antigens in apical membranes of rat osteoclasts and kidney intercalated cells of inner stripe of outer medulla. Pretreatment of animals with parathyroid hormone enhanced the immunoreaction in the apical membranes of osteoclasts. No immunoreaction was seen in osteoclasts when antibodies against gastric H+,K(+)-ATPase were used. These results suggest that osteoclast resorbs bone by secreting protons through vacuolar H(+)-ATPase.