ATP binding to solubilized (Na+ + K+)-ATPase. The abolition of subunit-subunit interaction and the maximum weight of the nucleotide-binding unit

Effects of palytoxin on cation occlusion and phosphorylation of the (Na+,K+)-ATPase

Palytoxin (PTX) inhibits the (Na(+) + K+)-driven pump and simultaneously opens channels that are equally permeable to Na+ and K+ in red cells and other cell membranes. In an effort to understand the mechanism by which PTX induces these fluxes, we have studied the effects of PTX on: 1) K+ and Na+ occlusion by the pump protein; 2) phosphorylation and dephosphorylation of the enzyme when a phosphoenzyme is formed from ATP and from P(i); and 3) p-nitro phenyl phosphatase (p-NPPase) activity associated with the (Na+, K+)-ATPase. We have found that palytoxin 1) increases the rate of deocclusion of K+(Rb+) in a time- and concentration-dependent manner, whereas Na+ occluded in the presence of oligomycin is unaffected by the toxin; 2) makes phosphorylation from P(i) insensitive to K+, and 3) stimulates the p-NPPase activity. The results are consistent with the notion that PTX produces a conformation of the Na+, K(+)-pump that resembles the one observed when ATP is bound to its low-affinity binding site. Further, they suggest that the channels that are formed by PTX might arise as a consequence of a perturbation in the ATPase structure, leading to the loss of control of the outside "gate" of the enzyme and hence to an uncoupling of the ion transport from the catalytic function of the ATPase.

protomers of Na+,K+-ATPase from microsomes of duck salt gland are mostly monomeric: Formation of higher oligomers does not modify molecular activity

The distance that separates alphabeta protomers of the Na(+), K(+)-ATPase in microsomes and in purified membranes prepared from duck nasal salt glands was estimated by measuring fluorescence resonance energy transfer between anthroylouabain bound to a population of alphabeta protomers and either N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain or 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain bound to the rest. Energy transfer between probes bound in the microsomal preparation was less than in the purified membranes. The efficiency of energy transfer between anthroylouabain and N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain was 29.2% in the microsomes compared with 62.6% in the purified preparation. Similar results were obtained with 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain as acceptor. We calculate that either the protomer bound probes were on the average 13 A farther apart in the microsomes than in the purified membranes, or that 53% of the protomers are monomeric in the microsome preparation. Microsomes prepared in the presence of phalloidin (a toxin that binds to F actin and stabilizes the actin-based cytoskeleton) showed less quench than those prepared in its absence. The data support the hypothesis that protomers are kept apart by their association with the cytoskeleton. The turnover rate while hydrolyzing ATP is the same in the microsomal and purified preparations; higher oligomer formation has no significant effect on the enzyme reaction mechanism.

Alphabeta protomers of Na+,K+-ATPase from microsomes of duck salt gland are mostly monomeric: formation of higher oligomers does not modify molecular activity

The distance that separates alphabeta protomers of the Na(+), K(+)-ATPase in microsomes and in purified membranes prepared from duck nasal salt glands was estimated by measuring fluorescence resonance energy transfer between anthroylouabain bound to a population of alphabeta protomers and either N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain or 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain bound to the rest. Energy transfer between probes bound in the microsomal preparation was less than in the purified membranes. The efficiency of energy transfer between anthroylouabain and N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain was 29.2% in the microsomes compared with 62.6% in the purified preparation. Similar results were obtained with 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain as acceptor. We calculate that either the protomer bound probes were on the average 13 A farther apart in the microsomes than in the purified membranes, or that 53% of the protomers are monomeric in the microsome preparation. Microsomes prepared in the presence of phalloidin (a toxin that binds to F actin and stabilizes the actin-based cytoskeleton) showed less quench than those prepared in its absence. The data support the hypothesis that protomers are kept apart by their association with the cytoskeleton. The turnover rate while hydrolyzing ATP is the same in the microsomal and purified preparations; higher oligomer formation has no significant effect on the enzyme reaction mechanism.

The effect of ionic strength and specific anions on substrate binding and hydrolytic activities of Na,K-ATPase

The physiological ligands for Na,K-ATPase (the Na,K-pump) are ions, and electrostatic forces, that could be revealed by their ionic strength dependence, are therefore expected to be important for their reaction with the enzyme. We found that the affinities for ADP3-, eosine2-, p-nitrophenylphosphate, and V(max) for Na,K-ATPase and K+-activated p-nitrophenylphosphatase activity, were all decreased by increasing salt concentration and by specific anions. Equilibrium binding of ADP was measured at 0-0.5 M of NaCl, Na2SO4, and NaNO3 and in 0.1 M Na-acetate, NaSCN, and NaClO4. The apparent affinity for ADP decreased up to 30 times. At equal ionic strength, I, the ranking of the salt effect was NaCl approximately Na2SO4 approximately Na-acetate < NaNO3 < NaSCN < NaCl04. We treated the influence of NaCl and Na2SO4 on K(diss) for E x ADP as a "pure" ionic strength effect. It is quantitatively simulated by a model where the binding site and ADP are point charges, and where their activity coefficients are related to I by the limiting law of Debye and Hückel. The estimated net charge at the binding site of the enzyme was about +1. Eosin binding followed the same model. The NO3- effect was compatible with competitive binding of NO3- and ADP in addition to the general I-effect. K(diss) for E x NO3 was approximately 32 mM. Analysis of V(max)/K(m) for Na,K-ATPase and K+-p-nitrophenylphosphatase activity shows that electrostatic forces are important for the binding of p-nitrophenylphosphate but not for the catalytic effect of ATP on the low affinity site. The net charge at the p-nitrophenylphosphate-binding site was also about +1. The results reported here indicate that the reversible interactions between ions and Na,K-ATPase can be grouped according to either simple Debye-Hückel behavior or to specific anion or cation interactions with the enzyme.

Binding of ADP to sarcoplasmic reticulum Ca(2+)-ATPase in the absence of Mg2+ is specifically inhibited by thapsigargin: observations on the ligand stoichiometry

The conditions of nucleotide binding to native, though partly purified, Ca(2+)-ATPase from SR as well as the stoichiometry of nucleotide and strontium binding and the phosphorylation capacity was reevaluated. Binding of MgADP appeared to be aberrant whereas even high-affinity binding of [14C]-ADP took place in the absence of Mg2+. Also low-affinity ATP binding was possible in the absence of divalent cations. A heterogeneity in ADP binding compatible with a two-component model in the absence of thapsigargin was changed to an apparent homogeneity of low-affinity receptors following a mole:mole interaction of enzyme and thapsigargin. Since the affinity of both components was reduced by thapsigargin, high- as well as low-affinity ADP binding seem to be specific and probably to the substrate receptor proper. Analysis of ADP binding isotherms in the absence of Mg2+ according to a model of two independent populations of sites was compatible with a binding capacity of 8.49 +/- 0.43 nmoles/mg protein corresponding to a molecular mass of 118 +/- 6 kD per ADP site. The same total binding capacity was found for ATP. The phosphorylation capacity corresponded to more than one and less than two approximately P per two 110-kD peptides (formally one approximately P per 154 kD protein). Specific binding of Ca2+ and the congener Sr2+ to SR Ca(2+)-ATPase was compatible with their interaction with a single population of sites. The binding capacity was equal to one divalent cation per nucleotide binding peptide. The binding of one nucleotide and one divalent cation per approximately 110 kD peptide and the absence of cooperativity in divalent cation binding might imply that Ca(2+)-ATPase works as a monomer.

Quantification of the Na+/K(+)-pump in solubilized tissue by the ouabain binding method coupled with high-performance gel chromatography

Membrane-bound Na+/K(+)-ATPase purified from dog kidney outer medulla was solubilized with octaethylene glycol n-dodecyl ether (C12E8) and incubated with [3H]ouabain in the presence of NaCl. ATP and MgCl2 for 10 min at 0 degrees C. The resulting enzyme was separated, by high-performance gel chromatography executed at 0.2 degrees C. Mainly into its (alpha beta)2-diprotomer and alpha beta-protomer, which both bound stoichiometrically to [3H]ouabain. The amounts of ouabain that bound to the tissue itself and its microsomes could be estimated in the same way, as [3H]ouabain was found to bind only to the diprotomer and protomer they possessed. The amounts of ouabain that bound to them in the solubilized state were at least 5-times higher than those that did so when they were non-solubilized, suggesting that the surfactant rendered the enzyme accessible to ouabain. When the solubilized tissue (138 mg ml-1 wet tissue) was reacted with ouabain in the presence of 0.1 M NaCl and 4.8 mM MgCl2 for 10 min at 0 degrees C, maximal ouabain binding was attained in the presence of 18.3 microM [3H]ouabain, 1.2 mM ATP and 3 to 5 mg ml-1 C12E8, which was common to the outer medulla and human colon cancer cells. The present method enabled the pump number in protein and tissue samples in the range 7.2 x 10(-9) (purified pump) to 1.5 x 10(-12) (cancer tissue) mol/mg protein to be estimated within 2 h.

Phosphorylation/dephosphorylation of reconstituted shark Na+,K(+)-ATPase: one phosphorylation site per alpha beta protomer

In the present investigation reconstitution of Na+,K(+)-ATPase increases the number of phosphorylation sites (EP) of solubilized enzyme from 4.2 +/- 0.3 nmol/mg to 6.9 +/- 0.6 nmol/mg. The latter figure corresponds to one phosphorylation site per alpha beta-promoter. A cholesterol content > 10 mol% in the liposome bilayer and a high extracellular [Na+] are necessary to obtain this high value. Spontaneous dephosphorylation after maximum phosphorylation in Na+ is biphasic both in solubilized enzyme and after reconstitution. The rate of dephosphorylation compares with the specific hydrolytic Na(+)-ATPase activity measured at exactly identical conditions for all three preparations assuming parallel dephosphorylation of at least two phosphointermediates. The distribution of EP-species is found to vary among the three enzyme preparation used, i.e., membrane bound, solubilized, and reconstituted Na+,K(+)-ATPase, however in all the equilibrium is strongly poised away from the E1P-form.

Influence of intramembrane electric charge on Na,K-ATPase

Effects of lipophilic ions, tetraphenylphosphonium (TPP+) and tetraphenylboron (TPB-), on interactions of Na+ and K+ with Na,K-ATPase were studied with membrane-bound enzyme from bovine brain, pig kidney, and shark rectal gland. Na+ and K+ interactions with the inward-facing binding sites, monitored by eosin fluorescence and phosphorylation, were not influenced by lipophilic ions. Phosphoenzyme interactions with extracellular cations were evaluated through K(+)-, ADP-, and Na(+)-dependent dephosphorylation. TPP+ decreased: 1) the rate of transition of ADP-insensitive to ADP-sensitive phosphoenzyme, 2) the K+ affinity and the rate coefficient for dephosphorylation of the K-sensitive phosphoenzyme, 3) the Na+ affinity and the rate coefficient for Na(+)-dependent dephosphorylation. Pre-steady state phosphorylation experiments indicate that the subsequent occlusion of extracellular cations was prevented by TPP+. TPB- had opposite effects. Effects of lipophilic ions on the transition between phosphoenzymes were significantly diminished when Na+ was replaced by N-methyl-D-glucamine or Tris+, but were unaffected by the replacement of Cl- by other anions. Lipophilic ions affected Na-ATPase, Na,K-ATPase, and p-nitrophenylphosphatase activities in accordance with their effects on the partial reactions. Effects of lipophilic ions appear to be due to their charge indicating that Na+ and K+ access to their extracellular binding sites is modified by the intramembrane electric field.

The role of (alpha beta) protomer interaction in determining functional characteristics of red cell Na,K-ATPase

We have examined the possibility that interaction of (alpha beta) protomers within a diprotomer is responsible for some anomalous characteristics of red cell Na,K-ATPase by examining their response to two inhibitors, FITC and H2DIDS, which bind covalently, and to ouabain, which debinds slowly from red cell pumps. The phenomena we examined were: (1) the biphasic curve relating Na,K-ATPase activity to ATP concentration, and (2) protection of Na pumps against vanadate inhibition by external Na. If interaction of (alpha beta) protomers within a diprotomer were responsible for these phenomena, random inactivation of (alpha beta) protomers should have resulted in a high proportion of (alpha beta) promtomers with an inhibited protomer as a partner, and therefore should have significantly altered the consequences of subunit interaction. With each inhibitor, 60-70% inhibition of ATPase activity did not alter the functional characteristics of the residual activity. We conclude that interaction of functional (alpha beta) protomers does not explain the phenomena which we investigated. This is consistent with our previous observation that Na,K pumps of red cell membranes exist as monomeric (alpha beta) protomers (Martin, D.W. and Sachs, V.R. (1992) J. Biol. Chem. 267, 23922-23929).

Solubilized alpha beta Na,K-ATPase remains protomeric during turnover yet shows apparent negative cooperativity toward ATP

A prominent feature of the Na,K-ATPase reaction is an ATP dependence that suggests high- and low-affinity ATP requirements during the enzymic cycle. As only one ATP-binding domain has been identified in the alpha subunit and none has been identified in the beta subunit, it has seemed likely that the apparent negative cooperativity results from subunit interactions in an (alpha beta)2 diprotomer. To test this possibility, we have examined the behavior of solubilized alpha beta protomers of Na,K-ATPase down to 50 nM [gamma-32P]ATP. Active-enzyme analytical ultracentrifugation shows that the protomer is the active species and that no oligomerization occurs during turnover. However, we find that dual ATP effects can be clearly demonstrated and that nonhydrolyzable ATP analogs can stimulate the Na,K-ATPase activity of the soluble protomer. We conclude that the apparent negative cooperativity is inherent to the alpha beta protomer and that this should explain some of the complexities found with membrane-bound Na,K-ATPase and, perhaps, other P-type cation pumps.

Heterogeneity of pig kidney Na,K-ATPase as indicated by ADP- and ouabain-binding stoichiometry

A centrifugation method has been used for determination of [14C]ADP and [3H]ouabain binding to Na,K-ATPase from pig kidney with high specific activity. In the presence of K+, the fit of the [14C]ADP binding data to a two-site model gives a component with high affinity which accounts for 12 +/- 2% of the total sites. The figure is significantly different from 50%, i.e., two components of equal size cannot be assumed. This contrasts with a ratio between the sites of 1:1 obtained by the rate dialysis technique. The discrepancy may be due to the fact that the centrifugation method enables bound ADP to be determined at lower concentrations of free ligand. [3H]Ouabain binding in the absence of Na+ is compatible with a straight line in a Scatchard plot if the isotope is purified shortly before use. An unspecific binding of ouabain can be neglected if the concentration of free ouabain is not too high. In the presence of Na+, the isotherms become upward concave. An analysis of the binding data gives a 19:81% division, although equilibrium is not quite attained. This is a maximum value because the lack in equilibrium will be most pronounced at the small values of free ouabain. Thus the ADP-binding studies are supported. The finding here is in some agreement with the semiquantitative immunoassay showing that pig kidney enzyme contains the isoenzymes alpha 1, alpha 2 and alpha 3 in a proportion of 84:12:4, respectively. Determination of ADP- and ouabain-binding site stoichiometry favours a theory with one substrate site per (alpha beta)2.

Study of ATP binding in the active site of Na+,K(+)-ATPase as probed by ultraviolet resonance Raman spectroscopy

The ultraviolet resonance Raman (UV RR) spectra of functional ATP/membrane-bound Na+,K+-ATPase complexes have been obtained. The substrate binding in the enzyme active site has been shown to be accompanied with significant changes in the electronic vibrational structure of the adenine ring. From the spectral analysis of ATP, 8-Br-ATP and 6-NHMe-adenine at various pH values the conclusion was made that N1 and the NH2 group and, probably, N7 of the substrate adenine part, interact with the protein surroundings via hydrogen bonds.

Thallium binding to native and radiation-inactivated Na+/K+-ATPase

The number of high-affinity K+-binding sites on purified Na+/K+-ATPase from pig kidney outer medulla has been assessed by measurement of equilibrium binding of thallous thallium, Tl+, under conditions (low ionic strength, absence of Na+ and Tris+) where the enzyme is in the E2-form. Na+/K+-ATPase has two identical Tl+ sites per ADP site, and the dissociation constant varies between 2 and 9 microM. These values are identical to those for Tl+ occlusion found previously by us, indicating that all high-affinity binding leads to occlusion. The specific binding was obtained after subtraction of a separately characterized unspecific adsorption of Tl+ to the enzyme preparations. Radiation inactivation leads to formation of modified peptides having two Tl+-binding sites with positive cooperativity, the second site-dissociation constant approximating that for the native sites. The radiation inactivation size (RIS) for total, specific Tl+ binding is 71 kDa, and the RIS for Tl+ binding with original affinity is approx. 190 kDa, equal to that of Na+/K+-ATPase activity and to that for Tl+ occlusion with native affinity. This latter RIS value confirms our recent theory that in situ the two catalytic peptides of Na+/K+-ATPase are closely associated. The 71 kDa value obtained for total Tl+ sites is equal to that for total binding of ATP and ADP and it is clearly smaller than the molecular mass of one catalytic subunit (112 kDa). The Tl+-binding experiments reported thus supports the notion that radiation inactivation of Na+/K+-ATPase is a stepwise rather than an all or none process.

A novel spin-label for study of membrane protein rotational diffusion using saturation transfer electron spin resonance. Application to selectively labelled class I and class II-SH groups of the shark rectal gland Na+/K+-ATPase

Na+/K+-ATPase in membranous preparations from the rectal gland of Squalus acanthias has been spin-labelled either on Class I -SH groups, which maintain overall ATPase activity, or on Class II -SH groups, for which only phosphorylation activity is preserved. Labelling of the Class I groups requires solubilization of the membranes and subsequent reconstitution by precipitation with Mn2+ in order to remove contaminating peripheral proteins, which are also labelled. Control experiments with preparations in which the Class II groups are labelled demonstrate that the mobility and aggregation state of the enzyme in the reconstituted membranes are similar to those in the native membrane. Both the conventional maleimide nitroxide derivative and a new benzoylvinyl nitroxide derivative have been used for the labelling. The segmental mobility of the labels and the overall rotational diffusion of the labelled protein have been investigated using saturation transfer ESR spectroscopy. The benzoylvinyl spin-label derivative offers particular advantages for the study of the protein rotational mobility in that the segmental mobility is considerably reduced relative to that observed with the maleimide derivative. This is especially the case for the Class I groups, where the maleimide label exhibits pronounced segmental mobility. Comparison of the results from the two labels indicates that the integral of the saturation-transfer spectrum is much more sensitive to segmental motion than are the diagnostic line-height ratios. This fact allows a better level of discrimination between the two types of motion. The results from the benzoylvinyl nitroxide-labelled Class I groups suggest that the Na+/K+-ATPase is probably present as an (alpha beta)2-diprotomer (or higher oligomer) in the native membrane.

The molecular size required varies according to the reaction step round the sodium pump cycle

Progress along the path of the sodium pump cycle requires a stepwise recruitment of additional subunits for maximal activity. These results show that whereas a particle the size of the alpha beta protomer presents Na+,K+-ATPase activity at 10 microM ATP, an additional subunit, perhaps a second alpha-chain, is required to obtain the much greater Na+,K+-ATPase activity resulting from the occupation of low-affinity ATP sites at physiological ATP concentrations. A non-phosphorylating ATP analogue, however, will modestly stimulate the Na+,K+-ATPase activity acting at an alternative low-affinity site or step on the alpha beta protomer.

(Na+ + K+)-ATPase: on the number of the ATP sites of the functional unit

Questions concerning the number of the ATP sites of the functional unit of (Na+ + K+)-ATPase (i.e., the sodium pump) have been at the center of the controversies on the mechanisms of the catalytic and transport functions of the enzyme. When the available data pertaining to the number of these sites are examined without any assumptions regarding the reaction mechanism, it is evident that although some relevant observations may be explained either by a single site or by multiple ATP sites, the remaining data dictate the existence of multiple sites on the functional unit. Also, while from much of the data it is clear that the multiple sites of the unit enzyme represent the interacting catalytic sites of an oligomer, it is not possible to rule out the existence of a distinct regulatory site for ATP in addition to the interacting catalytic sites. Regardless of the ultimate fate of the regulatory site, any realistic approach to the resolution of the kinetic mechanism of the sodium pump should include the consideration of the established site-site interactions of the oligomer.

Thermoinactivation and aggregation of alpha beta units in soluble and membrane-bound (Na,K)-ATPase

Stability and conformational transitions of soluble and fully active alpha beta units of (Na,K)-ATPase in n-dodecyl octaethylene glycol monoether (C12E8) are examined. Sedimentation equilibrium centrifugation gave a molecular weight of 143 000 for the alpha beta unit eluting from TSK 3000 SW gel chromatography columns. Fluorescence analysis and phosphorylation experiments show that E1-E2 transitions between both dephospho and phospho forms of soluble (Na,K)-ATPase are similar to those previously observed in the membrane-bound state. The two conformations can also be identified by their different susceptibilities to irreversible temperature-dependent inactivation. E1 forms of both soluble and membrane-bound (Na,K)-ATPase are more thermolabile than E2 forms. Gel chromatography on TSK 3000 SW and 4000 SW columns shows that thermal inactivation of soluble (Na,K)-ATPase at 40 degrees C is accompanied by aggregation of alpha beta units to (alpha beta)2 units and higher oligomers. The aggregates are stable in C12E8 but dissolve in sodium dodecyl sulfate. Similar aggregation accompanies inactivation of membrane-bound (Na,K)-ATPase at 55-60 degrees C. These data suggest that inactivation both in the soluble and in the membrane-bound state involves exposure of hydrophobic residues to solvent. The instability of the soluble E1 form may be related to inadequate length of the dodecyl alkyl chain of C12E8 for stabilization of hydrophobic protein domains that normally associate with alkyl chains of phospholipids in the membrane. Interaction between alpha beta units-does not seem to be required for the E1-E2 conformational change, but irreversible aggregation appears to be a consequence of denaturation of (Na,K)-ATPase in both soluble and membranous states.

Solubilization and further chromatographic purification of highly purified, membrane-bound Na,K-ATPase

Highly purified membrane-bound Na,K-ATPase from pig kidney outer medulla was dissolved in the non-ionic detergent C12E8. Chromatography of the dissolved material on a DEAE matrix yielded enzymatical material having a ouabain-binding capacity of 6.9 nmoles per mg protein (measured according to Lowry et al., with bovine serum albumin as standard). This material, which after addition of lipids had the same K+-phosphatase turnover as the membrane-bound enzyme, could consist entirely of live molecules with a molecular weight of 145 kDa, a value close to that expected for alpha beta-promoters of Na,K-ATPase.

Application of the principle of linked functions to ATP-driven ion pumps: kinetics of activation by ATP

If a ligand binds with unequal affinity to two distinct states of a protein, then the equilibrium between the two states becomes a function of the concentration of the ligand. A necessary consequence is that the ligand must also affect the forward and/or reverse rate constants for transition between the two states. For an enzyme or transport protein with such a transition as a slow step in the catalytic cycle, the overall rate also becomes a function of ligand concentration. These conclusions are independent of whether or not the ligand is a direct participant in the reaction. If it is a direct participant, then the kinetic effect arising from the principle of linked functions is distinct from the direct catalytic effect. These principles suffice to account for the biphasic response of the hydrolytic activity of ATP-driven ion pumps to the concentration of ATP, without the need to invoke more than one ATP binding site per catalytic center.

Kinetic properties of C12E8-solubilized (Na+ + K+)-ATPase

The properties of the rectal gland (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.8) solubilized in octaethyleneglycol dodecylmonoether ( C12E8 ) have been investigated. The kinetic properties of the solubilized enzyme resemble those of the membrane-bound enzyme to a large extent. The main difference is that Km for ATP for the (Na+ + K+)-ATPase is about 30 microM for the solubilized enzyme and about 100 microM for the membrane-bound enzyme. The Na+-form (E1) and the K+-form (E2) can also be distinguished in the solubilized enzyme, as seen from tryptic digestion, the intrinsic fluorescence and eosin fluorescence responses to Na+ and K+. The number of vanadate-binding sites is unchanged upon solubilization, and it is shown that vanadate binding is much more resistant to detergent inactivation than the enzymatic activities. The number of phosphorylation sites on the 95-100% pure supernatant enzyme is about 3.8 nmol/mg, and is equal to the number of vanadate sites. Inactivation of the enzyme by high concentrations of detergent can be shown to be related to the C12E8 /protein ratio, with a weight ratio of about 4 being a threshold for the onset of inactivation at low ionic strength. At high ionic strength, more C12E8 is required both for solubilization and inactivation. It is observed that the commercially available detergent polyoxyethylene 10-lauryl ether is much less deleterious than C12E8 , and its advantages in the assay of detergent-solubilized (Na+ + K+)-ATPase are discussed. The results show that (Na+ + K+)-ATPase can be solubilized in C12E8 in an active form, and that most of the kinetic and conformational properties of the membrane-bound enzyme are conserved upon solubilization. C12E8 -solubilized (Na+ + K+)-ATPase is therefore a good model system for a solubilized membrane protein.

Kinetic studies on interaction of (Na,K)-ATPase with Cibacron Blue F3GA as probe of the nucleotide fold

Cibacron Blue F3GA (Cb) effectively and reversibly inhibits the activity of (Na,K)-ATPase. Its inhibitory effect does not occur through occupation of the ouabain binding site, but presumably results from Cb-occupation of one catalytic site not competitively attracting ATP. Cb also inhibits ouabain binding to (Na,K)-ATPase. Its inhibitory effect is competitively antagonized by ATP proving accommodation of Cb in the ATP binding site. - If one admits Cb as a suitable analytical tool for the detection of a supersecondary structure folding pattern, the findings suggest that the ATP binding site is lined by beta-pleated sheets flanked by alpha-helices thus providing an environment that funnels ATP to the catalytic site.

Binding of sodium and potassium to the sodium pump of pig kidney evaluated from nucleotide-binding behaviour

Using a rate-dialysis technique at 0-2 degrees C, the affinities of Na+ and K+ for the sodium pump of pig kidney outer medulla were determined from their effects on the binding of ADP to the enzyme. Since all experiments were carried out in the presence of Tris, the enzyme in absence of its specific ligands was assumed to be in a 'sodium-like' conformation. The model used in the analysis of the results assumed the enzyme to be a dimeric structure with two identical high-affinity nucleotide-binding sites. It is concluded from the data that the effects of Na+ and K+ on the binding of nucleotide to either subunit of a nucleotide-free enzyme are identical. The two subunits, taken together, have five identical and non-interacting K+-binding sites (Kdiss = 0.5 mM) whose occupation antagonizes nucleotide binding. The binding of a nucleotide molecule to a nucleotide-free enzyme results in the abolition of K+ binding to two of the five K+-binding sites. The binding of the second molecule of nucleotide prevents the binding of three more K+ ions to the enzyme. These results can explain the K+-induced curvature observed in nucleotide-binding isotherms in Scatchard plots. The two subunits, taken together, have five identical and non-interacting Na+-binding sites (Kdiss = 0.5 mM) whose occupation antagonizes the effects of K+ on nucleotide binding, but does not affect nucleotide binding directly. A few experiments carried out at 18 degrees C indicate that the model applies also at this temperature. It is likely that the cation sites investigated are intracellular ones and it is concluded that the binding of each cation to its site induces a specific conformational change in the neighbourhood of the site itself without affecting the regions around the remaining cation binding sites.

The molecular weight of the calcium-transport-ATPase of the human red blood cell determined by radiation inactivation

Radiation inactivation was applied to analyze the molecular weight of the functional unit of (Ca2+ + Mg2+)-ATPase in human erythrocyte membranes. The enzyme activity was stable for at least 7 days at room temperature in membranes lyophilized in the presence of sucrose (150-300 mM). The enzyme activity in the lyophilized membranes and remaining after irradiation from a 60Co source was activated by calmodulin. A Mr of 290,000 +/- 15,000 was determined for (Ca2+ + Mg2+)-ATPase activity. Since the Mr by SDS-polyacrylamide gel electrophoresis is approximately 138,000 (Niggli et al. (1979) J. Biol. Chem. 254, 9955-9958), our results suggest that the Ca2+ pump ATPase functions as a dimer in the native human erythrocyte membrane.

Soluble and enzymatically stable (Na+ + K+)-ATPase from mammalian kidney consisting predominantly of protomer alpha beta-units. Preparation, assay and reconstitution of active Na+, K+ transport

Soluble (Na+ + K+)-ATPase consisting predominantly of alpha beta-units with Mr below 170 000 was prepared by incubating pure membrane-bound (Na+ + K+)-ATPase (35-48 mumol Pi/min per mg protein) from the outer renal medulla with the non-ionic detergent dodecyloctaethyleneglycol monoether (C12E8). (Na+ + K+)-ATPase and potassium phosphatase remained fully active in the detergent solution at C12E8/protein ratios of 2.5-3, at which 50-70% of the membrane protein was solubilized. The soluble protomeric (Na+ + K+)-ATPase was reconstituted to Na+, K+ pumps in phospholipid vesicles by the freeze-thaw sonication procedure. Protein solubilization was complete at C12E8/protein ratios of 5-6, at the expense of partial inactivation, but (Na+ + K+)-ATPase and potassium phosphatase could be reactivated after binding of C12E8 to Bio-Beads SM2. At C12E8/protein ratios higher than 6 the activities were irreversibly lost. Inactivation could be explained by delipidation. It was not due to subunit dissociation since only small changes in sedimentation velocities were seen when the C12E8/protein ratio was increased from 2.9 to 46. As determined immediately after solubilization, S20,w was 7.4 S for the fully active (Na+ + K+)-ATPase, 7.3 S for the partially active particle, and 6.5 S for the inactive particle at high C12E8/protein ratios. The maximum molecular masses determined by analytical ultracentrifugation were 141 000-170 000 dalton for these protein particles. Secondary aggregation occurred during column chromatography, with formation of enzymatically active (alpha beta)2-dimers or (alpha beta)3-trimers with S20,w = 10-12 S and apparent molecular masses in the range 273 000-386 000 daltons. This may reflect non-specific time-dependent aggregation of the detergent micelles.