Reconstitution of Active Transport Catalyzed by the Purified Sodium and Potassium Ion-stimulated Adenosine Triphosphatase from Canine Renal Medulla

The Effects of Ouabain and Potassium on Peritoneal Fluid and Solute Transport Characteristics

Background

We reported anomalous transport characteristics of potassium during experimental peritoneal dialysis in rats and suggested that mechanisms of peritoneal potassium transport could be other than simple passive transport. Intracellular transport of potassium in cultured human mesothelial cells was reported to be regulated by three different pathways, such as channels blocked by ouabain, channels blocked by furosemide, and other.

Objective

To investigate the effect of ouabain on peritoneal potassium and water transport characteristics.

Methods

A single 4-hour peritoneal dwell was performed in 28 5prague-Dawley rats. To minimize the diffusive transport of potassium, 4.5 mmol/L of KCI was added into conventional dialysis solution with 3.86% glucose [acidic peritoneal dialysis solution (APD)]. To evaluate the effect of the pH of dialysis solution on the transport of potassium and water, 4 mmol/L of NaOH was added into the potassium -containing study solutions [neutral peritoneal dialysis solution (NPD)]. To evaluate the effect of a potassium channel blocker on peritoneal potassium transport ATPase sensitive Na+-K+-transport inhibitor, ouabain (10–5 mmol/L) was added to dialysis solutions immediately before the dwell study in eight rats with APD (APD-O) and six rats with NPD (NPD-O). Ouabain was not added in eight and six rats with APD and NPD (APD-C and NPD-C, respectively). They were used as control. Infusion volume was 30 mL. The intraperitoneal volume (V D) was estimated by using a volume marker dilution method with corrections for the elimination of volume marker, radioiodinated human serum albumin (RI5A), from the peritoneal cavity (KE). The diffusive mass transport coefficient (KBD) and sieving coefficient (5) were estimated using the modified Babb-Randerson-Farrell model.

Results

VD was significantly higher (p < 0.05 from 90 min to 240 min) and KE (0.027 ± 0.018 mL/min for APD-O, 0.026 ± 0.017 mL/min for NPD-O, and 0.030 ± 0.022 mL/min for NPD-C, vs 0.058 ± 0.030 mL/min for APD-C, p < 0.05 for each) significantly lower during dialysis with APD -O, NPD -O, and NPD-C than with APD-C. The intraperitoneal glucose expressed as a percentage of the initial amount was significantly higher with APD-O, NPD-C, and NPD-Q than with APD-C (p < 0.05 from 90 min to 240 min). KBD for sodium was higher during dialysis with ouabain than without ouabain, while KBD for urea, glucose, and potassium, and 5 for urea, glucose, sodium, and potassium did not differ between the four groups.

Conclusions

The physiologic potassium concentration in neutral dialysis solutions and the use of ouabain decreased the intraperitoneal fluid absorption. The diffusive transport coefficient and sieving coefficient for potassium did not differ, while the diffusive transport coefficient for sodium increased during use of ouabain.

Assaying P-Type ATPases Reconstituted in Liposomes

Reconstitution of P-type ATPases in unilamellar liposomes is a useful technique to study functional properties of these active ion transporters. Experiments with such liposomes provide an easy access to substrate-binding affinities of the ion pumps as well as to the lipid and temperature dependence of the pump current. Here, we describe two reconstitution methods by dialysis and the use of potential-sensitive fluorescence dyes to study transport properties of two P-type ATPases, the Na,K-ATPase from rabbit kidney and the K(+)-transporting KdpFABC complex from E. coli. Several techniques are introduced how the measured fluorescence signals may be analyzed to gain information on properties of the ion pumps.

Decoupling the Na+–K+–ATPase in vivo: A possible new role in the gills of freshwater fishes

The literature suggests that when Na(+)-K(+)-ATPase has reduced access to its glycosphingolipid cofactor sulfogalactosyl ceramide (SGC), it is converted to a Na(+) uniporter. We recently showed that such segregation can occur within a single membrane when Na(+)-K(+)-ATPase is excluded from membrane microdomains or 'lipid rafts' enriched in SGC (D. Lingwood, G. Harauz, J.S. Ballantyne, J. Biol. Chem. 280, 36545-36550). Specifically we demonstrated that Na(+)-K(+)-ATPase localizes to SGC-enriched rafts in the gill basolateral membrane (BLM) of rainbow trout exposed to seawater (SW) but not freshwater (FW). We therefore proposed that since the freshwater gill Na(+)-K(+)-ATPase was separated from BLM SGC it should also transport Na(+) only, suggesting a new role for the pump in this epithelium. In this paper we discuss the biochemical evidence for SGC-based modulation of transport stoichiometry and highlight how a unique asparagine-lysine substitution in the FW pump isoform and FW gill transport energetics gear the Na(+)-K(+)-ATPase to perform Na(+) uniport.

Regulation of fish gill Na(+)-K(+)-ATPase by selective sulfatide-enriched raft partitioning during seawater adaptation

Na(+)-K(+)-ATPase is arguably the most important enzyme in the animal cell plasma membrane, but the role of the membrane in its regulation is poorly understood. We investigated the relationship between Na(+)-K(+)-ATPase and membrane microdomains or "lipid rafts" enriched in sulfatide (sulfogalactosylceramide/SGC), a glycosphingolipid implicated as a cofactor for this enzyme, in the basolateral membrane of rainbow trout gill epithelium. Our studies demonstrated that when trout adapt to seawater (33 ppt), Na(+)-K(+)-ATPase relocates to these structures. Arylsulfatase-induced desulfation of basolateral membrane SGC prevented this relocation and significantly reduced Na(+)-K(+)-ATPase activity in seawater but not freshwater trout. We contend that Na(+)-K(+)-ATPase partitions into SGC-enriched rafts to help facilitate the up-regulation of its activity during seawater adaptation. We also suggest that differential partitioning of Na(+)-K(+)-ATPase between these novel SGC-enriched regulatory platforms results in two distinct, physiological Na(+) transport modes. In addition, we extend the working definition of cholesterol-dependent raft integrity to structural dependence on the sulfate moiety of SGC in this membrane.

Sulfatide and Na+-K+-ATPase: A Salinity-sensitive Relationship in the Gill Basolateral Membrane of Rainbow Trout

We investigated the effect of salinity on the relationship between Na(+)-K(+)-ATPase and sulfogalactosyl ceramide (SGC) in the basolateral membrane of rainbow trout (Oncorhynchus mykiss) gill epithelium. SGC has been implicated as a cofactor in Na(+)-K(+)-ATPase activity, especially in Na(+)-K(+)-ATPase rich tissues. However, whole-tissue studies have questioned this role in the fish gill. We re-examined SGC cofactor function from a gill basolateral membrane perspective. Nine SGC fatty acid species were quantified by tandem mass spectrometry (MS/MS) and related to Na(+)-K(+)-ATPase activity in trout acclimated to freshwater or brackish water (20 ppt). While Na(+)-K(+)-ATPase activity increased, the total concentration and relative proportion of SGC isoforms remained constant between salinities. However, we noted a negative correlation between SGC concentration and Na(+)-K(+)-ATPase activity in fish exposed to brackish water, whereas no correlation existed in fish acclimated to freshwater. Differential Na(+)-K(+)-ATPase/SGC sensitivity is discussed in relation to enzyme isoform switching, the SGC cofactor site model and saltwater adaptation.

Functional genomic dissection of multimeric protein families in zebrafish

The study of multimeric protein function in the postgenomicera has become complicated by the discovery of multiple isoforms for each subunit of those proteins. A correspondingly large number of potential isoform combinations offer the multicellular organism a constellation of protein assemblies from which to generate a variety of functions across different cells, tissues, and organs. At the same time, the multiplicity of potential subunit isoform combinations presents a significant challenge when attempting to dissect the functions of particular isoform combinations. Biochemical and cell culture methods have brought us to a significant state of understanding of multimeric proteins but are unable to answer questions of function within the context of the many tissues and developmental stages of the multicellular organism. Answering those questions can be greatly facilitated in model systems in which expression can be determined over time, in the context of the whole organism, and in which hypomorphic function of each subunit can be studied individually and in combination. Fortunately, the potential for high-throughput in situ hybridization studies and antisense-based reverse genetic knockdowns in zebrafish offers exciting opportunities to meet this challenge. Some of these opportunities, along with cautions of interpretation and gaps in the existing technologies, are discussed in the context of ongoing investigations of the dimeric Na,K-ATPases and heterotrimeric G proteins.

Uptake of reconstituted Na,K-ATPase vesicles by isolated lymphocytes measured by FACS, confocal microscopy and spectrofluorometry

Na,K-ATPase (EC 3.6.1.37, Na,K-ATPase) is a fundamental vital membrane transport and receptor system which, after biosynthesis, is exported to the plasma membrane in inside-out vesicles. Na,K-ATPase can be extracted form the natural membrane and inserted into artificially formed phosphatidylcholine vesicles (liposomes). The ultrastructure of the reconstituted vesicles has been fully described. In the present work, the Na,K-ATPase-vesicles were labeled with fluorescent tracers either in their water or membrane phase, incubated with freshly isolated human lymphocytes, and the resulting cellular fluorescence measured with fluorescence activated cell sorting (FACS), confocal microscopy and spectrofluorometry. The FACS data show that all lymphocytes take up Na,K-ATPase-vesicles in a dose- and temperature-dependent fashion. Three-dimensional analysis of the fluorescence by confocal microscopy reveals that the fluorescence is contained within the cells. Quantitative determination by spectrofluorometry indicates that depending on the vesicle/cell ratio, a single lymphocyte takes up 650 to 36,500 vesicles within 30 min at 37 degrees C together with up to about 200,000 renal Na,K-ATPase molecules.

Two brain inhibitors inhibit renal Na,K-ATPases without recognizing the species-dependent variation of their ouabain-sensitivity

The membrane Na,K-ATPase is the driving force for sodium reabsorption in the kidney. Accordingly, Na,K-ATPase has been proposed to be a likely target for the action of a putative natriuretic hormone which would modulate sodium excretion by partial inhibition of renal Na,K-ATPase activity. To examine this hypothesis, it is necessary to isolate inhibitors from body fluids and tissues and to characterize their interaction with Na,K-ATPase in comparison to the plant inhibitors ouabain. Two inhibitors extracted from hypothalamus or hypothalamus-hypophysis have been compared to ouabain with regard to the shape of the dose-response curves and species-dependence. Ouabain inhibited renal Na,K-ATPase with dose-response curves spanning 3 to 5 orders of magnitude and marked species-dependence. By contrast, the brain inhibitors blocked the ATPase activity of isolated renal Na,K-ATPase with steep dose-response curves without species-dependence. Thus, the brain inhibitors are clearly distinct from plant ouabain; their chemical structures remain to be established.

Substantial glucose leakage from liposomes on filters and upon molecular-sieve chromatography in determinations of reconstituted glucose-transport activity and liposome volumes

Transport-protein activities are often determined by procedures that involve isolation of liposomes containing the transported radioactive solute. We determined the activity of the human red cell glucose transporter in liposomes and, by similar procedures, internal volumes of liposomes. For these purposes, we isolated freeze-thawed liposomes loaded with [14C]glucose, either by filtration on cellulose-nitrate and cellulose-acetate filters, or by chromatography on Sephadex. The interaction of liposomes with filters caused substantial leakage of [14C]glucose. About half of the internal [14C]glucose was released on the filters from glucose-transporter liposomes with inhibited transport. Chromatography at high flow rate provided higher and more accurate values than did the filtration procedure. Leakage corrections could be made by use of flow-cell scintillation elution profiles. The ratios between the corrected chromatographic volume values and the filtration values were 1.4-3.0 for liposomes without protein, 2.4-4.0 for glucose-transporter liposomes and 3.6-7.9 for liposomes with several human red cell integral membrane proteins. The D-glucose equilibrium exchange with glucose-transporter liposomes at 50 mM D-glucose was 2.0 nmol D-glucose per microgram transporter per second as determined by use of chromatography at high flow rate. The filtration procedure gave only 0.6 nmol.microgram-1.s-1 due to the [14C]glucose leakage. In our experiments, the chromatographic procedure thus proved superior.

Demonstration of the electrogenicity of proton translocation during the phosphorylation step in gastric H+K(+)-ATPase

Membrane fragments containing the H+K(+)-ATPase from parietal cells have been adsorbed to a planar lipid membrane. The transport activity of the enzyme was determined by measuring electrical currents via the capacitive coupling between the membrane sheets and the planar lipid film. To initiate the pump currents by the ATPase a light-driven concentration jump of ATP from caged ATP was applied as demonstrated previously for Na+K(+)-ATPase (Fendler, K., Grell, E., Haubs, M., Bamberg, E. 1985. EMBO J. 4:3079-3085). Since H+K(+)-ATPase is an electroneutrally working enzyme no stationary pump currents were observed in the presence of K+. By separation of the H+ and K+ transport steps of the reaction cycle, however, the electrogenic step of the phosphorylation could be measured. This was achieved in the absence of K+ or at low concentrations of K+. The observed transient current is ATP dependent which can be assigned to the proton movement during the phosphorylation. From this it was concluded that the K+ transport during dephosphorylation is electrogenic, too, in contrast to the Na+K(+)-ATPase where the K+ step is electroneutral. The transient current was measured at different ionic conditions and could be blocked by vanadate and by the H+K(+)-ATPase specific inhibitor omeprazole. An alternative mechanism for activation of this inhibitor is discussed.

Isolation and characterization of gastric microsomal glycoproteins. Evidence for a glycosylated beta-subunit of the H+/K(+)-ATPase

Detergent-solubilization of hog gastric microsomal membrane proteins followed by affinity chromatography using wheat germ agglutinin or Ricinus communis I agglutinin resulted in the isolation of five glycoproteins with the apparent molecular masses on sodium dodecyl sulfate polyacrylamide gels of (in kDa): 60-80 (two glycoproteins sharing this molecular mass); 125-150; and 190-210. In the nonionic detergent Nonidet P-40 (NP-40), the 94 kDa H+/K(+)-ATPase was recovered exclusively in the lectin-binding fraction; however, in the cationic detergent dodecyltrimethylammonium bromide, most of the ATPase was recovered in the nonbinding fraction. Detection of glycoproteins either by periodic acid-dansyl hydrazine staining of carbohydrate in polyacrylamide gels or by Western blots probed with lectins indicated that the majority of the ATPase molecules are not glycosylated. In addition, in the absence of microsomal glycoproteins, the NP-40-solubilized ATPase does not bind to a lectin column. Taken together, these results suggest that the recovery of NP-40-solubilized ATPase in the lectin-binding fraction is due to its noncovalent interaction with a gastric microsomal glycoprotein. Immunoprecipitation of the ATPase from NP-40-solubilized microsomal membrane proteins resulted in the co-precipitation of a single 60-80 kDa glycoprotein. Characterization of the 60-80 kDa glycoprotein associated with the ATPase revealed that: it is a transmembrane protein; it has an apparent core molecular mass of 32 kDa; and, it has five asparagine-linked oligosaccharide chains. Given its similarity to the glycosylated beta-subunit of the Na+/K(+)-ATPase, this 60-80 kDa gastric microsomal glycoprotein is suggested to be a beta-subunit of the H+/K(+)-ATPase.

Ouabain-insensitive Na-ATPase activity in the basolateral membrane from rat jejunum

1. In the basolateral membrane preparation of the rat enterocyte (jejunal tract) there is not only the well-known (Na,K)-ATPase activity, but also a ouabain-insensitive Na-ATPase. 2. The Na-ATPase is not activated by anions or other monovalent cations. As a substrate, ATP cannot be replaced by other nucleotides. 3. The Na-ATPase is insensitive to ouabain and bumetanide, inhibited partially by furosemide and totally by ethacrynate. 4. The activation of Na-ATPase at different Na concentrations shows an hyperbolic curve (Km = 15.7 +/- 2.3 mM and Vmax = 204 +/- 19 nmoles Pi/mg protein per min) different from the sigmoidal curve (Km = 9.8 +/- 1.2 mM and Vmax = 640 +/- 15 nmoles Pi/mg protein per min) shown by (Na,K)-ATPase. 5. These results are compared with the corresponding ones found in other animals and tissues in which the Na-ATPase was found. 6. The Na-ATPase activity can be interpreted as the enzymatic correspondent of a ouabain-insensitive Na pump, present in the basolateral membrane of the enterocyte different in behaviour with respect to the known Na pump.

Characterization of (Na+ + K+)-ATPase-liposomes. III. Controlled activation and inhibition of symmetric pumps by timed asymmetric ATP, RbCl, and cardiac glycoside addition

Inside-out as well as right-side-out oriented (Na+ + K+)-ATPase molecules reconstituted in liposomes are activated successively by timed asymmetric addition of ATP to the internal and external liposome compartment; this presents the first functional confirmation of the symmetric pump-orientation in cholate-dialysed preparations revealed previously by the equal distribution of intramembrane particles on the concave and convex surface of freeze-fractured (Na+ + K+)-ATPase-liposomes. The initial transport rates of the symmetrically oriented pump populations are regulated by varying the bilateral K or Rb ion concentrations; ATP, ouabain, digoxin or vanadate are used to activate or block selectively the right-side-out, inside-out or both (Na+ + K+)-ATPase populations. Finally, these liposomes of the second generation present a new tool to evaluate the membrane-permeability as well as the effects of receptor-ligands or other probes in a single preparation.

(Na+ + K+)-ATPase in artificial lipid vesicles: influence of the concentration of mono- and divalent cations on the pumping rate

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. Transport activity was induced by adding ATP to the external medium. A voltage-sensitive dye was used to detect the ATP-driven potassium extrusion in the presence of valinomycin. The observed substrate-protein interactions of the reconstituted (Na+ + K+)-ATPase largely agree with that from native tissues. An agreement between ATP hydrolysis and transport activity is given for concentration dependences of sodium, potassium, magnesium and calcium ions. The only significant deviations were observed in the influence of pH. Protons were found to have different influence on transport, enzymatic activity and phosphorylation of the enzyme. The transport studies showed a twofold interaction of protons with the protein: competition with sodium at the cytoplasmic ion binding sites, a non competitive inhibition of transport which is not correlated with protein phosphorylation.

Inside-out basolateral plasma membrane vesicles from rat kidney proximal tubular cells

A method for preparation of highly purified basolateral plasma membranes from rat kidney proximal tubular cells is reported. These membranes were assayed for the presence of vesicles as well as for their orientation. (Na+ + K+)-ATPase activity and [3H]ouabain binding studies with membranes treated with or without SDS revealed that the preparation consisted of almost 100% vesicles. The percentage of inside-out vesicles was found to be approx. 70%. This percentage was determined measuring the (Na+ + K+)-ATPase activity in K+-loaded vesicles and in membranes treated with or without trypsin and SDS. These membranes represent a very efficient tool to assay the correlation between active transport and ATPase activities in basolateral plasma membranes from rat kidney proximal tubular cells.

(Na+ + K+)-ATPase in artificial lipid vesicles: influence of lipid structure on pumping rate

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into tightly-sealed, single-shelled lipid vesicles by a detergent-dialysis procedure. The rate of ATP-driven potassium extrusion from vesicles formed from different phosphatidylcholines (PC) was measured optically, using a voltage-sensitive dye in the presence of valinomycin. High transport rates were observed for di(18:1)PC, di(20:1)PC and di(22:1)PC, whereas vesicles formed from di(14:1)PC and di(16:1)PC were virtually inactive. The variation of pumping activity with lipid structure mainly results from differences in the amount of enzyme incorporated with the correct orientation into the vesicle membrane, and to a lesser extent from lipid-dependent variations of the intrinsic turnover rate of the enzyme. The activation energy of ion transport decreases in the order di(16:1)PC, di(18:1)PC, di(20:1)PC approximately equal to di(22:1)PC.

Optical study of active ion transport in lipid vesicles containing reconstituted Na,K-ATPase

A fluorescence method is described for the measurement of ATP-driven ion fluxes in lipid vesicles containing purified Na,K-ATPase. The membrane voltage of enzyme containing vesicles was measured by using a voltage-sensitive indocyanine dye. By addition of valinomycin the vesicle membrane is made selectively permeable to K+ so that the membrane voltage approaches the Nernst potential for K+. With constant external K+ concentration, the time course of internal K+ concentration can be continuously measured as change of the fluorescence signal after activation of the pump. The optical method has a higher time resolution than tracer-flux experiments and allows an accurate determination of initial flux rates. From the temperature dependence of active K+ transport its activation energy was determined to be 115 kJ/mol. ATP-stimulated electrogenic pumping can be measured as fast fluorescence change when the membrane conductance is low (i.e., at low or zero valinomycin concentration). In accordance with expectation, the amplitude of the fast signal change increases with decreasing passive ion permeability of the vesicle membrane. The resolution of the charge movement is so high that a few pump turnovers can be easily detected.

Reconstitution of (Na+ + K+)-ATPase proteoliposomes having the same turnover rate as the membranous enzyme

Membranous (Na+ + K+)-ATPase from the electric eel was solubilized with 3-[3-cholamidopropyl)-dimethylammonio)-1-propanesulfonate (Chaps). 50 to 70% of the solubilized enzyme was reconstituted in egg phospholipid liposomes containing cholesterol by using Chaps. The obtained proteoliposomes consisted of large vesicles with a diameter of 134 +/- 24 nm as the major component, and their protein/lipid ratio was 1.25 +/- 0.07 g protein/mol phospholipid. The intravesicular volume of these proteoliposomes is too small to consistently sustain the intravesicular concentrations of ligands, especially K+, during the assay. The decrease in K+ concentration was cancelled by the addition of 20 microM valinomycin in the assay medium. The low value of the protein/lipid ratio suggests that these proteoliposomes contain one Na+/K+-pump particle with a molecular mass of 280 kDa per one vesicle as the major component. In these proteoliposomes, the specific activity of the (Na+ + K+)-ATPase reaction was 10 mumol Pi/mg protein per min, and the turnover rate of the ATP-hydrolysis was 3500 min-1, the same as the original enzyme under the same assay condition. The ratio of transported Na+ to hydrolyzed ATP was 3, the same as that in the red cell. The proteoliposomes could be disintegrated by 40-50 mM Chaps without any significant inactivation. This disintegration of proteoliposomes nearly tripled the ATPase activity compared to the original ones and doubled the specific ATPase activity compared to the membranous enzyme, but the turnover rate was the same as the original proteoliposomes and the membranous enzyme. This disintegration of proteoliposomes by Chaps suggests the selective incorporation of the (Na+ + K+)-ATPase particle into the liposomes and the asymmetric orientation of the (Na+ + K+)-ATPase particle in the vesicle.

Characterization of (Na+ + K+)-ATPase liposomes. I. Effect of enzyme concentration and modification on liposome size, intramembrane particle formation and Na+,K+-transport

Rabbit renal (Na+ + K+)-ATPase (EC 3.6.1.3) was purified and incorporated into phosphatidylcholine liposomes. Freeze-fracture analysis of the reconstituted system reveals intramembrane particles formed by (Na+ + K+)-ATPase molecules which are randomly distributed on concave and convex fracture faces. The reconstituted (Na+ + K+)-ATPase performs active Na+,K+-transport. The distribution of particles as well as the rate of active transport are directly proportional to the (Na+ + K+)-ATPase protein concentration used for reconstitution, while the total amount of sodium and potassium ions exchanged by ATP per volume vesicle suspension reaches maximum when each vesicle contains on the average more than two particles. (Na+ + K+)-ATPase pretreated with ouabain or vanadate yields the same particle density and vesicle size as control enzyme. However, detergent-denatured enzyme loses its ability to form intramembrane particles or to increase the vesicle size indicating that the lipids surrounding the protein part of the molecule are essential for the reconstitution process. The vesicle diameter increases as a function of the number of particles per vesicle. Histograms of the size distribution become wider with increasing intramembrane particle density and tend to show more than one maximum.

Reconstitution of (Na+ + K+)-ATPase into phospholipid vesicles with full recovery of its specific activity

(Na+ + K+)-ATPase from rectal glands of the spiny dogfish has been reconstituted into phospholipid vesicles. The nonionic detergent octaethyleneglycoldodecyl monoether ( C12E8 ) is used to dissolve both the enzyme and the lipids and reconstitution is accomplished by subsequent removal of the detergent by adsorption to polystyrene beads. About 60% of the enzyme incorporates in the right-side-out orientation (r/o). The fraction of molecules in the inside-out orientation (i/o) increases from about 10% to about 30% with a parallel decrease in the fraction of 'non-oriented' (n-o) molecules (both sides exposed) when the protein/lipid ratio decreases from 1:10 to 1:75. The orientation of enzyme molecules detected from vanadate binding is the same as measured from activity, i.e., the turnover of the enzyme molecule in the different orientations is the same. The recovery of the specific activity of the incorporated enzyme increases with an increase in the protein/lipid ratio and is 100% with a protein/lipid ratio of about 1:20 or higher. Full recovery is only obtained provided a proper lipid composition is chosen which includes both negatively charged phospholipids, preferably phosphatidylinositol, and cholesterol. The ATP-dependent, K+-stimulated Na+-influx is found to be about 35 mumol Na+ per mg (i/o)-protein per min at 22 degrees C in 1:10 protein/lipid liposomes. The specific activity corresponds to 3 Na+ transported per ATP molecule hydrolyzed.

Incorporation of Na+ - Ca2+ antiporter and of (Na+ + K+)-ATPase into liposomes and demonstration of their non-identity

(Na+ + K+)-ATPase was isolated from the grey matter of brain and incorporated into liposomes. Most of the reconstituted enzyme was oriented 'inside-out' with respect to its in vivo orientation and externally added ATP promoted Na+ uptake that was inhibitable by internally trapped ouabain. Using the same proteoliposomes, an Na+ - Ca2+ exchange system was observed as indicated by the following pieces of evidence. (1) The Na+ gradient provided the only readily apparent driving force for acceleration of Ca2+ accumulation into proteoliposomes. (2) The antiporter was specific for Ca2+, high Mg2+ excess did not inhibit Ca2+ antiport. (3) The Na+ efflux was dependent on the extravesicular Ca2+ concentration. (4) The Na+ efflux was not inhibited by tetrodotoxin. The demonstrated Na+ - Ca2+ exchange could not be related to (Na+ + K+)-ATPase protein, since it was not purified with (Na+ + K+)-ATPase, as followed from transport studies with liposomes containing (Na+ + K+)-ATPase of different specific activity. The results strongly indicate that plasma membranes isolated from the grey matter of brain contain an Na+ - Ca2+ exchange system and that the proteoliposomes are suitable for further purification of the carrier molecule.

Preparation of Na,K-ATPase-containing liposomes with predictable transport properties by a procedure relating the Na,K-transport capacity to the ATPase activity

A microprocedure for the preparation of Na,K-ATPase-containing liposomes with a minimal starting material (200 microgram) of purified Na,K-ATPase is presented. Phosphatidylcholine is added gradually to cholate-solubilized Na,K-ATPase of various concentrations and the lipid-induced decrease in enzyme activity is monitored. After removal of the detergent by dialysis, the transport parameters of the resulting Na,K-ATPase-liposomes are established by a microassay. By relating the transport properties to the Na,K-ATPase activity preset before dialysis, a procedure is developed which allows to prepare standardized Na,K-ATPase-liposomes with predictable transport properties.

Partial purification of the Na+-dependent D-glucose transport system from renal brush border membranes

A membrane extract enriched with the Na+ -dependent D-glucose transport system was obtained by differential cholate solubilization of rat renal brush border membranes in the presence of 120 mM Na+ ions. Sodium ions were essential in stabilizing the transport system during cholate treatment. This membrane extract was further purified with respect to its Na+-coupled D-glucose transport activity and protein content by the use of asolectin-equilibrated hydroxylapatite. The reconstituted proteoliposomes prepared from this purified fraction showed a transient accumulation of D-glucose in response to a Na+ gradient. The observed rate of Na+-coupled D-glucose uptake by the proteoliposomes represented about a sevenfold increase as compared to that of the reconstituted system derived from an initial 1.2% cholate extract of the membranes. Other Na+-coupled transport systems such as L-alanine, alpha -ketoglutarate and phosphate were not detected in these reconstituted proteoliposomes.

Membrane charge as effector of cytochrome P-450LM2 catalyzed reactions in reconstituted liposomes

The phospholipid specificity of rabbit liver microsomal cytochrome P-450LM2 catalyzed hydroxylation reactions was examined in reconstituted phospholipid vesicles. An apparent linear relationship between the negative charge of the vesicles and the rate of P-450LM2-catalyzed O-dealkylation of p-nitroanisole or 7-ethoxycoumarin was obtained. The membrane charge-mediated increase in hydroxylation activities was found not to be due to (i) an altered lipid/water partition coefficient of the substrate, (ii) a change in the apparent Michaelis constant of P-450LM2 for the substrate, (iii) a different activation energy of the O-demethylation of p-nitroanisole, (iv) different spin states of P-450LM2 or (v) an altered secondary structure of this enzyme as monitored by circular dichroism. However, when the formation of the ferrous carbonyl complex of P-450LM2 was followed under aerobic or anaerobic conditions after the addition of NADPH to the vesicles, an increased negative charge of the membrane was accompanied by an increased reducibility of P-450LM2. A similar linear relationship between the reducibility of cytochrome b5 and the negative charge of the liposomes was also evident in membranes containing NADPH-cytochrome P-450 reductase and cytochrome b5. It is proposed that the interaction of the reductase with P-450LM2 is inefficient in neutral vesicles and thus rate determining for the overall hydroxylation activities.

A transport model for the (Na+/K+)ATPase in liposomes including the (Na+/K+)-channel function

(Na+/K+)ATPase liposomes of various degrees of reconstitution are formed by varying the amount of phosphatidylcholine added to the soluble (Na+/K+)ATPase before vesicles are formed by cholate removal. In the presence of ATP, the reconstituted sodium pump effectuates (Na+/K+) antiport. In the absence of ATP, the reconstituted sodium pump forms a (Na+/K+) channel. The stable plateaus formed by (1) the active Na+ transport, (2) the active K+ transport, (3) the 'passive' Na+ flux, and (4) the 'passive' K+ flux are determined in the optimally and the partially reconstituted liposomes. The activities of all four vectorial functions vary in a tightly correlated fashion, suggesting that they are mediated by the same transport-active configuration of (Na+/K+)ATPase. A transport model which includes the active and the passive (Na+/K+) fluxes mediated by the sodium pump in liposomes is outlined.

A relationship between anion transport and a structural transition of the human erythrocyte membrane

Scanning microcalorimetry was employed as an aid in examining some structural features of the anion transport system in red blood cell vesicles. Two structural transitions were previously shown to be sensitive to several covalent and non-covalent inhibitors of anion transport in red cells. In this study, these transitions were selectively removed, either thermally or enzymatically, and the subsequent effect on 35SO2- 4 efflux in red cell vesicles was determined. It is shown that removal of one of these transitions (B2) has a negligible inhibitory effect on anion transport. Cytoplasmic, intermolecular disulfide linkages between band 3 dimers are known to form during the B2 transition. The integrity of the 4,4-diisothiocyanostilbene-2,2-disulfonate-sensitive C transition, on the other hand, is shown to be a requirement for anion transport. The localized region of the membrane giving rise to this transition contains the transmembrane segment of band 3, as well as membrane phospholipids. The calorimetric results suggest a structure of band 3 which involves independent structural domains, and are consistent with the transmembrane segment playing a direct role in the transport process.

Structural analysis by mass spectrometry and NMR spectroscopy of the glycolipid sulfate from Halobacterium salinarium and a note on its possible function

The major glycolipid sulfate of the extreme halophile Halobacterium salinarium was isolated and characterised mainly by mass spectrometry and NMR spectroscopy. The mass spectrum of the permethylated, desulfated and trimethylsilylated derivative showed the molecule to be a trihexosyl glycerol C20-diether with the sulfate group on the terminal hexose. A 3-position of the sulfate was indicated by the mass spectrum obtained after acetylation and trimethylsilylation (solvolysis of sulfate and replacement by a trimethylsilyl group). The NMR spectrum of the desulfated permethylated glycolipid gave conclusive evidence for the presence of one beta and two alpha anomeric protons. With the knowledge of degradation data it was possible to assign the beta signal to galactose (terminal hexose), and the alpha signals to glucose and mannose. These data together make it likely that the glycolipid sulfate is identical in structure with the glycolipid from Halobacterium cutirubrum characterised previously (M. Kates and P.W. Deroo, J. Lipid Res., 14 (1973) 438). On the basis of a suggested function of cerebroside sulfate of animal origin (identical polar end with the bacterial glycolipid: beta-galactopyranose-3-sulfate) and the present knowledge of ion transport in Halobacteria, it is proposed that the bacterial glycolipid may function as a selective K+ receptor for the K+ transport from a high-Na+ and low-K+ outside medium.