Effect of lysophosphatidylcholine on K+ transport in rat heavy gastric membranes enriched with (H+-K+)-ATPase

K+- and ATP-dependent H+-accumulation in rat heavy gastric membrane vesicles enriched with (H+-K+)-ATPase was markedly stimulated by amphiphiles like lysophosphatidylcholine and Zwittergent 3-14 at concentrations of 10(-5) M. Their stimulatory effect was dependent on K+-concentration in the medium and was abolished by SCH 28,080, a specific inhibitor of (H+-K+)-ATPase. Lysophosphatidylcholine at the optimal dose (3 X 10(-5) M) showed dual effects on K+-dependent membrane functions; it stimulated the rate of K+-uptake by nearly 60%, but partially inhibited SCH 28,080-sensitive and K+-dependent ATP-hydrolysis (about 20% reduction). These data indicate that H+-pumping through (H+-K+)-ATPase in the inside-out gastric membrane vesicles was facilitated by the stimulatory effect of lysophosphatidylcholine on membrane K+-transport in spite of its partial inhibition of ATP-hydrolysis. It appears that the rate limiting step for operation of the ATPase is the availability of K+ ions in the luminal side of the pump. We propose that ionic amphiphiles may modulate K+-transport in rat heavy gastric membranes through specific interactions with the putative K+-transporter.     

Protective effect of EGTA on rat gastric membranes enriched in (H+-K+)-ATPase

Rat gastric membranes enriched in (H⁺-K⁺)-ATPase, when prepared in the presence of 1 mM ethyleneglycol-bis-(β-aminoethyl ether)N,N′-tetraacetic acid, showed the ability to accumulate H⁺ ions upon addition of ATP, KCl, and valinomycin. The membranes were largely impermeable to K⁺ and Cl^?. In contrast, the rat membranes prepared without the Ca²⁺ chelator lost the ability to develop a pH gradient because of the membrane leakiness to H⁺. A majority of these membrane vesicles became also permeable to K⁺. We suggest that the calcium chelator preserved the gastric membrane permeability barrier during isolation by inhibiting various Ca²⁺-dependent phospholipases in rat gastric mucosa.     

Effect of carbachol or histamine stimulation on rat gastric membranes enriched in (H+-K+)-ATPase

We have examined histamine- or carbachol-induced changes in rat gastric membranes enriched in K+-stimulated ATPase. Stimulation of secretion by both secretagogues in vivo produced a class of microsomal membranes which exhibited valinomycin-independent, KCl-dependent H+ transport. In contrast, membrane vesicles isolated from cimetidine inhibited resting mucosa exhibited largely the ionophore-dependent H+ transport. In addition, only in the carbachol-stimulated membranes a portion of the ionophore-independent H+ transport was refractory to cimetidine pretreatment. The gastric microsomal membranes were resolved into light and heavy fractions by centrifugation over isotonic 2H2O media. The ionophore-independent H+ transport was almost exclusively associated with the heavy microsomal fraction while the ionophore-dependent H+ transport was detected in the light fraction. Also, these fractions were considerably different from each other in their appearance in electron micrographs and SDS gel electrophoresis patterns. Secretagogue stimulation increased the population of the heavy microsomal membrane vesicles exhibiting the valinomycin-independent, K+-dependent H+ transport and their overall content of K+-stimulated ATPase. Cimetidine treatment, on the other hand, increased the ATPase activity associated with the light microsomes, and produced the heavy microsomal membranes showing only a marginal degree of the ionophore independent H+ accumulation, even though they were very similar to the carbachol-stimulated heavy membranes in the specific activity of K+-stimulated ATPase. SDS gel patterns and appearance in electron micrograph. These observations suggest that activation of secretion involves at least two distinctive events; transformation of the light to the heavy gastric membranes containing a K+-dependent H+ pump and an increased KCl permeability in the latter.     

A protein phosphatase associated with rat heavy gastric membranes enriched with (H+-K+)-ATPase influences membrane K+ transport activity

Rat stimulated heavy gastric membranes enriched with (H+-K+)-ATPase, a marker for the apical membrane of the parietal cell, displayed a 32P-histone-dephosphorylating activity which appeared to be physically copurified with, but functionally independent of, the ATPase. The protein phosphatase activity was optimal at pH 7.5 and was inhibited by fluoride (50 mM), inorganic phosphate (50 mM), and p-chloromercuribenzoate (0.1 mM), but was insensitive to vanadate (1 mM). The 32P-phosphoproteins in the heavy gastric membranes were also dephosphorylated, apparently by their own membrane-bound phosphatase in the presence of Mg2+ at millimolar concentrations, which is likely to enhance membrane-membrane interaction. Heavy gastric membrane vesicles incubated with Mg2+ (2 mM) exhibited no alterations in K+-dependent ATP-hydrolyzing activity, Cl permeability, and protein and lipid compositions, but irreversibly lost the ATP, K+-dependent H+-pumping activity. Since valinomycin, a K+-specific ionophore, restored the intravesicular acidifying activity and an inhibitor of the protein phosphatase, inorganic phosphate, largely blocked the Mg2+-induced change in the membrane transport function, it is reasonable to propose that the phosphatase action on certain membrane proteins, possibly the putative K+ transporter or regulatory proteins, selectively decreases K+-conductance in the apical membranes of gastric parietal cells.     

Thiophosphorylation of hog gastric (H+ + K+)-ATPase membranes by endogenous protein kinases

(H+ + K+)-ATPase-enriched membranes from hog stomachs were tested for their capacity to autophosphorylate using [gamma-32P]ATP or [gamma-35S]ATP[S] as phosphate donors. The radioactive polypeptides were characterized by SDS-PAGE. In the presence of Mg2+ and 5 microM [gamma-32P]ATP, rapid and transient incorporation of 32P occurred at 0 degrees C. Radioactivity was essentially found in the major polypeptide of the material, the 95 kDa subunit of (H+ + K+)-ATPase. Under the same experimental conditions, thiophosphorylation was slower and reached a plateau within 1 h. Incorporation levels were higher with manganese than with magnesium. After one hour at 0 degrees C, and in the presence of 10 mM manganese and 5 microM ATP[S], 0.58 +/- 0.06 nmoles of thiophosphate were incorporated per mg of protein. Twenty seven percent of the thiophosphorylated amino acids were acylphosphates i.e. likely to be the ATPase thiophosphointermediate. The remaining thiophosphorylated amino acids (73%) were thought to be produced by protein kinases. This was supported by the autoradiographies of membrane SDS-PAGE which indicated that, in addition to the 95 kDa ATPase subunit, other polypeptides were thiophosphorylated especially at 108, 58, 47, 45 and 36-40 kDa. A previous study had provided strong evidence that chloride transport in gastric microsomes, is modulated by a protein kinase-dependent phosphorylation (Soumarmon, A., Abastado, M., Bonfils, S. and Lewin M.J.M. (1980) J. Biol. Chem. 255, 11682-11687). In the present work, we demonstrate that the peptidic inhibitor of cAMP-dependent protein kinases decreased thiophosphorylation of a 45 kDa polypeptide. We suggest that this polypeptide could be regarded as a candidate for the role of chloride transporter or chloride transport regulator.     

H+ transport by reconstituted gastric (H+ + K+)-ATPase

Gastric (H+ + K+)-ATPase was reconstituted into artificial phosphatidylcholine/cholesterol liposomes by means of a freeze-thaw-sonication technique. Upon addition of MgATP, active H+ transport was observed, with a maximal rate of 2.1 mumol X mg-1 X min-1, requiring the presence of 100 mM K+ at the intravesicular site. However, in the absence of ATP an H+-K+ exchange with a maximal rate of 0.12 mumol X mg-1 X min-1 was measured, which could be inhibited by the well-known ATPase inhibitors vanadate and omeprazole, giving the first evidence of a passive K+-H+ exchange function of gastric (H+ + K+)-ATPase. An Na+-H+ exchange activity was also measured, which was fully inhibited by 1 mM amiloride. Simultaneous reconstitution of Na+/H+ antiport and (H+ + K+)-ATPase could explain why reconstituted ATPase appeared less cation-specific than the native enzyme (Rabon, E.C., Gunther, R.B., Soumarmon, A., Bassilian, B., Lewin, M.J.M. and Sachs, G. (1985) J. Biol. Chem. 260, 10200-10212).     

Characterization of gastric mucosal membranes. X. Immunological studies of gastric (H+ + K+)-ATPase

Gastric mucosal homogenates from hog were fractionated by differential and density gradient centrifugation and free-flow electrophoresis. The two major membrane fractions (FI and FII) thus obtained are distinct both enzymically and in terms of transport reactivity. This heterogenicity extends to their antigenic activity. Purified antibodies which were raised against the K+-ATPase-containing H+ transport fraction FI were of two types: inhibitory and non-inhibitory. Inhibitory antibodies reduced the K+-ATPase activity by approximately 80% and the K+-p-nitro-phenylphosphatase activity by approximately 40% in a concentration-dependent manner, while the small Mg++-dependent component of the enzyme activity was unaffected. Antibodies inhibiting the K+-ATPase also inhibited H+ transport. These antibodies did not cross-react with the other major membrane fraction isolated by free- flow electrophoresis, FII, and gave a single band on rocket immunoelectrophoresis. Antibodies against this FII fraction also did not react with the K+-ATPase and were heterogeneous, giving at least four bands with rocket immunoelectrophoresis and inhibiting both the 5'- nucleotidase and Mg++-ATPase of this fraction. Immunofluorescent staining of tissue sections showed that the FI was derived from the parietal cell of gastric tissue and was localized to the supranuclear area of the cell. Staining of isolated rat gastric cell suspensions by FI antibodies confirmed the selectivity of the antibody and showed a polar, plasma membrane localization. FII antibodies also largely stained the parietal cells in tissue sections. In the 16 hog tissues tested, FI antibodies cross-reacted only with gastric fundus, thyroid and weakly with thymus. Immunoelectronmicroscopy showed that FI antibodies reacted strongly with the secretory membrane at the apical cell surface of the parietal cells and at the secretory canaliculi, weakly with the apical surface of the zymogen cell, and not with the basal-lateral surface of the cells. Thus, the protontranslocating ATPase is localized in the parietal cells and in the region postulated to be the site of acid secretion.     

Evidence for the presence of a carbohydrate moiety in fluorescein isothiocyanate labeled fragments of rat gastric (H+-K+)-ATPase

Limited tryptic digestion of fluorescein isothiocyanate (FITC)-labeled (H+-K+)-ATPase from rat resting light gastric membranes produced a soluble 27-kDa polypeptide which retained the fluorescence of the parent enzyme. Its production was markedly enhanced in the presence of an amphiphilic detergent, Zwittergent 3-14, which potently inhibits the ATPase activity. This increase is probably due to protection of certain tryptic cleavage sites through conformational changes of the membrane enzyme by the detergent. The NH2-terminal sequence of the 27-kDa polypeptide corresponded exactly to that beginning at Asn-369 of the cDNA-deduced primary structure of the rat ATPase. The presence of the phosphorylation site, Asp-385, and FITC-labeled Lys-517, which is known to be a part of the ATP-binding site, indicates that the 27-kDa polypeptide contains a major cytoplasmic portion of (H+-K+)-ATPase. Interestingly, the polypeptide was stained with periodate-Schiff's base, indicating its glycoprotein nature. The carbohydrate group attached to the polypeptide seems to include at least an N-linked high-mannose moiety, since the polypeptide showed Con A binding activity as detected with a Con A-biotin/avidin-peroxidase assay on nitrocellulose transblots. Also, its Con A binding activity was inhibited by excess methyl alpha-D-mannopyranoside and disappeared upon treatment of the polypeptide with endoglycosidase H and N-glycanase. Further tryptic action converted the 27-kDa polypeptide to 2 smaller FITC-labeled polypeptides of 25 and 15 kDa, which lost 18 and 96 amino acid residues, respectively, from the NH2 terminus of the parent polypeptide.(ABSTRACT TRUNCATED AT 250 WORDS)     

Papain fragmentation of the gastric (H+ + K+)-ATPase

Membrane-bound (H+ + K+)-ATPase purified from hog gastric mucosa was exposed to limited papain digestion. Such treatment resulted in a rapid inhibition of the K+-stimulated adenosine triphosphatase and p-nitrophenyl phosphatase activities, with about 90% of these activities lost after 3 min incubation at 37 degrees C with 0.1 units of papain per mg of enzyme protein. Parallel to the inhibition of the enzyme activities, there was a production of a 77 kDa membrane-bound fragment containing the aspartyl phosphate residue of the phospho-intermediate. This fragment accounted for about 45% of the total enzyme protein after the 3 min papain treatment. The digestion barely affected the steady-state level of phosphorylation, allowed the aspartyl phosphate of the 77 kDa fragment to undergo the transition to the E2P form, and did not significantly alter the fraction of ADP-sensitive phosphoenzyme. The presence of KCl, however, depressed the steady-state level of phosphoenzyme formed from [gamma-32P]ATP considerably less than that of the control enzyme. With further exposure to papain the 77 kDa peptide became fragmented into a 28 kDa soluble peptide that retained the phosphorylating site. Binding of fluorescein 5'-isothiocyanate (FITC) to the native enzyme did not affect the sites of papain hydrolysis because the same peptide fragments were obtained. The FITC reaction site was also in the 28 kDa soluble peptide fragment.     

The expression of gastric H+-K+-ATPase mRNA and protein in developing rat fundic gland

The proton pump H⁺-K⁺-ATPase is the final common pathway mediating the production and secretion of hydrochloric acid by gastric parietal cells. The present studies were undertaken to examine whether the expression of gastric H⁺-K⁺-ATPase mRNA and protein changes are associated with the development of H⁺-K⁺-ATPase activity in the rat fundic gland. H⁺-K⁺-ATPase activity was examined in rat fundic gland at different stages from gestational day 18.5 to postnatal 8 weeks. The expression of H⁺-K⁺-ATPase mRNA was detected by in situ hybridization using a digoxigenin-labelled RNA probe with a tyramide signal amplification system. The expression of H⁺-K⁺-ATPase protein was evaluated by immunoblotting and immunohistochemistry using antibodies against H⁺-K⁺-ATPase - and -subunits. We found that H⁺-K⁺-ATPase enzyme activity was detectable from the onset of gland formation (day 19.5 of gestation) and increased with age in the developing rat fundic gland. Expression of mRNA and protein was also discernible at the same time, and a progressive increase in expressions was observed as rats developed. Our results suggested that in developing rat fundic gland, the expression of both mRNA and protein of H⁺-K⁺-ATPase increased with age in a manner that parallels the development of H⁺-K⁺-ATPase enzymebreak activity.     

Purification of (Ca2+-Mg2+)-ATPase from rat liver plasma membranes

The Ca2+-stimulated, Mg2+-dependent ATPase from rat liver plasma membranes was solubilized using the detergent polyoxyethylene 9 lauryl ether and purified by column chromatography using Polybuffer Exchanger 94, concanavalin A-Sepharose 4B, and Sephadex G-200. The molecular weight of the enzyme, estimated by gel filtration in the presence of the detergent on a Sephadex G-200 column, was 200,000 +/- 15,000. The enzyme was purified at least 300-fold from rat liver plasma membranes and had a specific activity of 19.7 mumol/mg/min. Polyacrylamide gel electrophoresis under nondenaturing conditions of the purified enzyme indicated that the enzymatic activity correlated with the major protein band. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis, one major band in the molecular weight range of 70,000 +/- 5,000 was seen. The isoelectric point of the purified enzyme was 6.9 +/- 0.2 as determined by analytical isoelectric focusing. The enzyme was activated by Ca2+ with an apparent half-saturation constant of 87 +/- 2 nM for Ca2+. Calmodulin and trifluoperazine at the concentration of 1 microgram/ml and 100 microM, respectively, had no effect on the enzymatic activity.     

(Ca2+ + Mg2+)-ATPase in enriched sarcolemma from dog heart

An enriched fraction of plasma membranes was prepared from canine ventricle by a process which involved thorough disruption of membranes by vigorous homogenization in dilute suspension, sedimentation of contractile proteins and mitochondria at 3000 X g followed by sedimentation of a microsomal fraction at 200 000 X g. The microsomal suspension was then fractionated on a discontinuous sucrose gradient. Particles migrating in the density range 1.0591--1.1083 were characterized by (Na+ + K+)-ATPase activity and [3H]ouabain binding as being enriched in sarcolemma and were comprised of nonaggregated vesicles of diameter approx. 0.1 micron. These fractions contained (Ca2+ + Mg2+)-ATPase which appreared endogenous to the sarcolemma. The enzyme was solubilized using Triton X-100 and 1 M KCl and partially purified. Optimal Ca2+ concentration for enzyme activity was 5--10 microM. Both Na+ and K+ stimulated enzyme activity. It is suggested that the enzyme may be involved in the outward pumping of Ca2+ from the cardiac cell.     

Redistribution and characterization of (H+ + K+)-ATPase membranes from resting and stimulated gastric parietal cells.

When isolated from resting parietal cells, the majority of the (H+ + K+)-ATPase activity was recovered in the microsomal fraction. These microsomal vesicles demonstrated a low K+ permeability, such that the addition of valinomycin resulted in marked stimulation of (H+ + K+)-ATPase activity, and proton accumulation. When isolated from stimulated parietal cells, the (H+ + K+)-ATPase was redistributed to larger, denser vesicles: stimulation-associated (s.a.) vesicles. S.a. vesicles showed an increased K+ permeability, such that maximal (H+ + K+)-ATPase and proton accumulation activities were observed in low K+ concentrations and no enhancement of activities occurred on the addition of valinomycin. The change in subcellular distribution of (H+ + K+)-ATPase correlated with morphological changes observed with stimulation of parietal cells, the microsomes and s.a. vesicles derived from the intracellular tubulovesicles and the apical plasma membrane, respectively. Total (H+ + K+)-ATPase activity recoverable from stimulated gastric mucosa was 64% of that from resting tissue. Therefore, we tested for latent activity in s.a. vesicles. Permeabilization of s.a. vesicles with octyl glucoside increased (H+ + K+)-ATPase activity by greater than 2-fold. Latent (H+ + K+)-ATPase activity was resistant to highly tryptic conditions (which inactivated all activity in gastric microsomes). About 20% of the non-latent (H+ + K+)-ATPase activity was also resistant to trypsin digestion. We interpret these results as indicating that, of the s.a. vesicles, approx. 55% have a right-side-out orientation and are impermeable to ATP, 10% right-side-out and permeable to ATP, and 35% have an inside-out orientation.     

Solubilization of active (H+ + K+)-ATPase from gastric membrane

(H+ + K+)-ATPase-enriched membranes were prepared from hog gastric mucosa by sucrose gradient centrifugation. These membranes contained Mg2+-ATPase and p-nitrophenylphosphatase activities (68 +/- 9 mumol Pi and 2.9 +/- 0.6 mumol p-nitrophenol/mg protein per h) which were insensitive to ouabain and markedly stimulated by 20 mM KCl (respectively, 2.2- and 14.8-fold). Furthermore, the membranes autophosphorylated in the absence of K+ (up to 0.69 +/- 0.09 nmol Pi incorporated/mg protein) and dephosphorylated by 85% in the presence of this ion. Membrane proteins were extracted by 1-2% (w/v) n-octylglucoside into a soluble form, i.e., which did not sediment in a 100 000 X g X 1 h centrifugation. This soluble form precipitated upon further dilution in detergent-free buffer. Extracted ATPase represented 32% (soluble form) and 68% (precipitated) of native enzyme and it displayed the same characteristic properties in terms of K+-stimulated ATPase and p-nitrophenylphosphatase activities and K+-sensitive phosphorylation: Mg2+-ATPase (mumol Pi/mg protein per h) 32 +/- 9 (basal) and 86 +/- 20 (K+-stimulated); Mg2+-p-nitrophenylphosphatase (mumol p-nitrophenol/mg protein per h) 2.6 +/- 0.5 (basal) and 22.2 +/- 3.2 (K+-stimulated); Mg2+-phosphorylation (nmol Pi/mg protein) 0.214 +/- 0.041 (basal) and 0.057 +/- 0.004 (in the presence of K+). In glycerol gradient centrifugation, extracted enzyme equilibrated as a single peak corresponding to an apparent 390 000 molecular weight. These findings provide the first evidence for the solubilization of (H+ + K+)-ATPase in a still active structure.