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

The gastric HK-ATPase: structure, function, and inhibition

The gastric H,K-ATPase, a member of the P(2)-type ATPase family, is the integral membrane protein responsible for gastric acid secretion. It is an alpha,beta-heterodimeric enzyme that exchanges cytoplasmic hydronium with extracellular potassium. The catalytic alpha subunit has ten transmembrane segments with a cluster of intramembranal carboxylic amino acids located in the middle of the transmembrane segments TM4, TM5,TM6, and TM8. Comparison to the known structure of the SERCA pump, mutagenesis, and molecular modeling has identified these as constituents of the ion binding domain. The beta subunit has one transmembrane segment with N terminus in cytoplasmic region. The extracellular domain of the beta subunit contains six or seven N-linked glycosylation sites. N-glycosylation is important for the enzyme assembly, maturation, and sorting. The enzyme pumps acid by a series of conformational changes from an E(1) (ion site in) to an E(2) (ion site out) configuration following binding of MgATP and phosphorylation. Several experimental observations support the hypothesis that expulsion of the proton at 160 mM (pH 0.8) results from movement of lysine 791 into the ion binding site in the E(2)P configuration. Potassium access from the lumen depends on activation of a K and Cl conductance via a KCNQ1/KCNE2 complex and Clic6. K movement through the luminal channel in E(2)P is proposed to displace the lysine along with dephosphorylation to return the enzyme to the E(1) configuration. This enzyme is inhibited by the unique proton pump inhibitor class of drug, allowing therapy of acid-related diseases.

Binding of ethylenediamine to phosphatidylserine is inhibitory to Na+/K(+)-ATPase

Covalent linkage of ethylenediamine with the Na+/K(+)-ATPase complex from rabbit kidney outer medulla by the use of the water-soluble carbodiimide, N-ethyl,N'-(3-dimethylaminopropyl)carbodiimide, resulted in a 73% reaction with phosphatidylserine and only 27% with carboxylic groups in the proteic component of the enzyme. Condensation products from the reaction between phosphatidylserine and ethylenediamine, N-(O-phosphatidylseryl)ethylenediamine, N,N'-bis(O-phosphatidylseryl)ethylenediamine and its intermediary product O-phosphatidyl-[N,N'-bis(seryl)]ethylenediamine, were synthesised. Symmetrically substituted ethylenediamine was the most likely condensation product of ethylenediamine with endogenous phosphatidylserine. The synthesised lipids were incorporated in proteoliposomes containing Na+/K(+)-ATPase and only the addition of the phospholipid phosphatidylcholine. The ratio of phospholipid to protein was 52 (w/w). These proteoliposomes were perforated by the addition of 0.5% cholate and both the Na(+)-dependent phosphorylation level and its dependence on Na+, Mg2+ and ATP were measured. Phosphatidylcholine alone increased the half-maximal activation concentration for Na+ ([Na+]0.5) from 0.2 to 1-2 mM, for Mg2+ from 0.1 to 0.8 microM and for ATP from 0.02 to 0.3 microM. The Ki for K+ (in the absence of Na+) was unaffected: 12.8 microM vs. 12.5 microM in the non-reconstituted system. Replacing 10 mol% of phosphatidylcholine by phosphatidylethanolamine: or phosphatidylserine had no significant effect on [Na+]0.5: 1.1 and 0.7 mM, respectively. Replacing 5 mol% phosphatidylcholine by the bis(phosphatidylseryl) substituent of ethylenediamine further increased [Na+]0.5 to 13.7 mM, while half-maximal activation concentrations for Mg2+ and ATP were unaltered. The mono-phosphatidylseryl derivatives of ethylenediamine, each 5 mol%, also increased [Na+]0.5, but to a lesser extent (3.2-3.8 mM). In addition to their competitive effects, the phosphatidylseryl-substituted ethylenediamine compounds exerted a slowly-increasing non-competitive inhibition, not only in phosphorylation, but also in overall ATPase activity, which was reduced, although not abolished, by exogenous protein (bovine serum albumin). A detergent-like action in the usual sense is unlikely since liposomes containing these lipids remained intact. These studies prove that phospholipids are not only required for optimal activity of this transport enzyme, but in excess or in compositions deviating from the normal, may also be inhibitory.

Studies on the mechanism of action of A80915A, a semi-naphthoquinone natural product, as an inhibitor of gastric (H(+)-K+)-ATPase

A semi-naphthoquinone natural product, A80915A, produced by Streptomyces aculeolatus was found to be a potent inhibitor of gastric (H(+)-K+)-ATPase, the enzyme responsible for acid secretion in the stomach. Enzyme activity was measured by potassium-stimulated hydrolysis of ATP or p-nitrophenolphosphate with enzyme prepared from the stomach fundic mucosa of pigs. Concentration-dependent inhibition was observed with an IC50 of about 2-3 microM for both ATPase and p-nitrophenylphosphatase. A Hill plot indicated that the enzyme has two binding sites for A80915A. Inhibition was not affected by the presence of the reducing agent dithiothreitol, indicating a lack of involvement of enzyme sulfhydryl groups. A 30-min incubation of enzyme with increasing drug concentrations followed by a 10-fold dilution did not alter the IC50, indicating that A80915A does not covalently modify the enzyme. Coincubation of enzyme with 3.8 microM A80915A resulted in time-dependent inhibition. The rate of inhibition was slowed significantly by the presence of 20 mM potassium, rubidium and ammonium but not by 20 mM sodium, lithium and choline, or by 40 mM sucrose. The level of inhibition was influenced by the order of addition of potassium and drug to the enzyme. Taken together, these studies indicate that inhibition by A80915A is dependent on the conformation of gastric (H(+)-K+)-ATPase and that potassium slows the rate of inhibition by converting the enzyme to a conformation where the drug binding site is not as accessible. The mode of action of A80915A is distinct from that of two well characterized proton pump inhibitors, omeprazole and SCH 28080.

Voltage-dependent pump currents of the sarcoplasmic reticulum Ca2(+)-ATPase in planar lipid membranes

Sarcoplasmic reticulum vesicles containing largely Ca2(+)-ATPase were incorporated into planar lipid membranes. The ATPase was activated by a UV flash-induced concentration jump of ATP from a photolabile caged ATP. Under these conditions stationary pump currents were observed. The dependence of these pump currents on applied voltages was investigated. The current-voltage curve of the Ca2(+)-ATPase shows monotonously increasing pump currents with increasing positive potentials of the ATP containing compartment. This indicates the existence of electrogenic steps in the direction of the transported Ca2+ ions. From the extrapolated reversal potentials of the curve is concluded that less than four positive net charges are transported per hydrolyzed ATP.

Sidedness of the effect of amines on the steady-state phosphorylation level of reconstituted Na+/K+-ATPase

The sidedness of the effects of several amines on the steady-state phosphorylation level of rabbit kidney Na+/K+-ATPase has been studied with the enzyme incorporated in phosphatidylcholine-cholesterol containing proteoliposomes. The presence of ouabain prevented phosphorylation of non-incorporated or rightside-out incorporated enzyme, so that only the inside-out incorporated Na+/K+-ATPase molecules were studied. Addition of either Na+ or several amines to the extracellular side of the enzyme led to an enhancement of the steady-state phosphorylation level obtained with optimal concentrations of Na+, Mg2+ and ATP at the cytosolic side. The series imidazole greater than Na+ greater than triallylamine greater than Tris greater than ethylenediamine showed a decrease in affinity. Histidine, sorbitol and choline chloride had no effect at the extracellular side. This means that in addition to the well-known cytosolic ligands either Na+ or a positively charged amine buffer has to be present extracellularly in order to obtain an optimal phosphorylation level. At the cytoplasmic side the tested amines exerted different effects. (i) Imidazole and triallylamine enhanced the steady-state phosphorylation level when the extracellular conditions were optimal (saturating amine concentration). (ii) Tris and ethylenediamine decreased the steady-state phosphorylation level and (iii) histidine had no effect. The cytoplasmic effects of the amine compounds correlate with those described by Schuurmans Stekhoven et al. (Biochim. Biophys. Acta 937 (1988) 161-171) for the unsided preparation. The extracellular effects, however, are apparently masked in experiments with fragmented enzyme preparations and are assumed to be potentiating effects which make the enzyme ready for phosphorylation upon a cytoplasmic trigger (e.g. Na+).

Cation sidedness in the phosphorylation step of Na+/K+-ATPase

Na+/K+ -ATPase, reconstituted into phospholipid vesicles, has been used to study the localisation of binding sites of ligands involved in the phosphorylation reaction. Inside-out oriented Na+/K+ -ATPase molecules are the only population in this system, which can be phosphorylated, as the rightside-out oriented as well as the non-incorporated enzyme molecules are inhibited by ouabain. In addition, the right-side-out oriented Na+/K+ -ATPase molecules have their ATP binding site intravesicularly and are thus not accessible to substrate added to the extravesicular medium. Functional binding sites for the following ligands have been demonstrated: (i) Potassium, acting at the extracellular side with high affinity (stimulating the dephosphorylation rate of the E2P conformation) and low affinity (inducing the non-phosphorylating E2K complex). (ii) Potassium, acting at the cytoplasmic side with both high and low affinity. The latter sites are also responsible for the formation of an E2K complex and complete with Na+ for its binding sites. (iii) Sodium at the cytoplasmic side responsible for stimulation of the phosphorylation reaction. (iv) Sodium (and amine buffers) at the extracellular side enhancing the phosphorylation level of Na+/K+ -ATPase where choline chloride has no effect. (v) Magnesium at the cytoplasmic side, stimulating the phosphorylation reaction and inhibiting it above optimal concentrations.

Transport ratios of reconstituted (H+ + K+)-ATPase

Gastric (H+ + K+)-ATPase was reconstituted into artificial phosphatidylcholine/cholesterol vesicles by means of a freeze-thaw-sonication procedure. The passive and active transport mediated by these vesicles were measured (Skrabanja, A.T.P., Asty, P., Soumarmon, A., De Pont, J.J.H.H.M. and Lewin, M.J.M. (1986) Biochim. Biophys. Acta 860, 131-136). To determine real initial velocities, the proteoliposomes were separated from non-incorporated enzyme, by means of centrifugation on a sucrose gradient. The purified proteoliposomes were used to measure active H+ and Rb+ transport, giving at room-temperature velocities of 46.3 and 42.5 mumol per mg per h, respectively. A transport ratio of two cations per ATP hydrolyzed was also measured. These figures indicate that the enzyme catalyzes an electroneutral H+-Rb+ exchange.

Gastric (H+,K+)-ATPase

Gastric acid secretion results from the activity of a specific ATPase, the (H+,K+)-ATPase. This enzyme, discovered in 1973, exchanges H+ for K+. It has two ATP binding sites, both involved in enzyme activity, whose affinities vary as a function of the H+ and K+ concentrations. Hydrolysis of ATP at the highest affinity site leads to the synthesis of a covalent aspartyl phosphate which accumulates in the absence of K+. The presence of this cation accelerates dephosphorylation resulting in the stimulation of ATPase (and PNPPase) activity. The structure of membranous (H+,K+)-ATPase is poorly defined. n-Octylglucoside solubilizes an active enzyme of 390-420 kDa which can be partly depolymerized using cholate. The monomer, characterized in SDS has a 95 kDa molecular mass and is inactive. In the presence of magnesium, (H+,K+)-ATPase catalyzes the active and neutral exchange of H+ for K+ at the expense of ATP. In the absence of ATP, (H+,K+)-ATPase acts as a passive transporter exchanging K+ for K+ at maximal rate and H+ for K+ at a 20 times slower rate.