Role of protein conformation changes and transphosphorylations in the function of Na+/K(+)-transporting adenosine triphosphatase: an attempt at an integration into the Na+/K+ pump mechanism

Regulation of Electrolyte Permeability by Herbal Monomers in Edematous Disorders

Edema is a gradual accumulation of fluid in the interstitial tissues or luminal cavities, which is regulated by ion transport pathways and reflects dysfunction of fluid and salt homeostasis. Increasing evidence suggests that some herbal monomers significantly reduce organ/tissue edema. In this review, we briefly summarized the electrolyte permeability involved in pathomechanisms of organ edema, and the benefits of herbal monomers on ionic transport machinery, including Na+-K+-ATPase, Na+ and Cl- channels, Na+-K+-2Cl- co-transporter, etc. Pharmaceutical relevance is implicated in developing advanced strategies to mitigate edematous disorders. In conclusion, the natural herbal monomers regulate electrolyte permeability in many edematous disorders, and further basic and clinical studies are needed.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

Are pyridoxal and fluorescein probes in lysine residues of alpha-chain in Na+,K(+)-ATPase sensing ATP binding?

Na+,K(+)-ATPase preparations from pig kidneys were treated with 50 microM pyridoxal 5'-diphospho-5'-adenosine (AP2PL) in the presence of NaCl. The resulting preparations contained 0.5 mol of the AP2PL probe at the Lys-480/mol alpha-chain. This modification reduced both Na+,K(+)-ATPase activity and the amount of Na(+)-dependent phosphoenzyme from ATP to around 50% but not that from acetyl phosphate (AcP). The addition of 1 mM AcP to the modified enzyme in the presence of Mg2+ and Na+ induced phosphorylation (3.0/s) followed by an AP2PL fluorescence increase (1.2/s). The addition of 10 microM ATP instead of AcP induced rapid phosphorylation (28/s) followed by a slow increase in fluorescence (1.0/s). When modified enzyme preparations were treated with fluorescein 5'-isothiocyanate (FITC), the phosphorylation capacity from ATP was reduced to around 5% with little influence on either the AP2PL fluorescence change by ATP or phosphorylation from AcP. The addition of increasing concentrations of ATP with 160 mM NaCl to the K(+)-bound AP2PL-FITC-labeled enzyme showed different rates for each fluorescence change and different affinities for ATP of the changes. These data and others indicate that the AP2PL probe at Lys-480 can monitor ATP binding to high- and low-affinity sites and suggest the simultaneous presence of two different low-affinity sites for ATP detected by an AP2PL probe at Lys-480 and an FITC probe at Lys-501.

Reinstatement of the ATP high energy paradigm

The paradigm that the hydrolysis of ATP releases high Gibbs energy able to perform work has increasingly been questioned over the last two decades. Results from theoretical and experimental studies have been interpreted to indicate that the synthesis of ATP from ADP and P(i) does not require energy supply and that binding of ATP per se can transmit utilizable energy to an enzyme. As has recently been concluded, all this has led to a change of the ATP high energy paradigm in bioenergetics. Starting from this challenge, the present review singles out the striking sources of the apparent dichotomy in bioenergetics, and endeavours to eliminate the apparent contradictions by the application of the prior knowledge on both the participation of the enzyme protein in energy exchange processes and the particular reactivities of phosphorus that make it an outstanding element for functionally variable work assignments in enzymatic systems.

Modeling of the three-dimensional structure of the digitalis intercalating matrix in Na+/K(+)-ATPase protodimer

Based on the knowledge that the digitalis receptor site in Na+/K(+)-ATPase is the interface between two interacting alpha-subunits of the protodimer (alpha beta)2, the present review makes an approach towards modeling the three-dimensional structure of the digitalis intercalating matrix by exploiting the information on: the primary structure and predicted membrane topology of the catalytic alpha-subunit; the determinants of the secondary, tertiary and quaternary structure of the membrane-spanning protein domains; the impact of mutational amino acid substitutions on the affinity of digitalis compounds, and the structural characteristics in potent representatives. The designed model proves its validity by allowing quantitative interpretations of the contributions of distinct amino acid side chains to the special bondings of the three structural elements of digitalis compounds.

Location and properties of the digitalis receptor site in Na+/K(+)-ATPase

Since 1985, several research groups have shown that a number of amino acids in the catalytic alpha-subunit of Na+/K(+)-ATPase more or less strongly modulate the affinity of a digitalis compound like ouabain to the enzyme. However, scrutiny of these findings by means of chimeric Na+/K(+)-ATPase constructs and monoclonal antibodies has recently revealed that the modulatory effect of most of these amino acids does not at all result from direct interaction with ouabain, but rather originates from long-range effects on the properties of the digitalis binding matrix. Starting from this knowledge, the present review brings together the various pieces of evidence pointing to the conclusion that the interface between two interacting alpha-subunits in the Na+/K(+)-ATPase protodimer (alpha beta)2 provides the cleft for inhibitory digitalis intercalation.

Interaction of hypothalamic Na,K-ATPase inhibitor with isolated human peripheral blood mononuclear cells

A ligand for the digitalis receptor located on the membrane-embedded Na,K-ATPase (NKA; EC 3.6.1.37) has been isolated from bovine hypothalamus (hypothalamic inhibitory factor; HIF) and identified as isomeric ouabain (Tymiak et al., 1993, Proc. Natl. Acad. Sci. 90: 8189-8193). In analogy to cardioactive steroids (CS) derived from plants or from toad, HIF inhibits the Na/K-exchange process and the ATPase activity of isolated Na,K-ATPase although by a different molecular action mechanism. In the present work we show that, as plant-derived ouabain, HIF inhibits 86Rb-uptake by isolated human lymphocytes with an IC50 of about 20 nM; above this concentration HIF reduces cell viability in contrast to ouabain. The decrease in cell viability by excess HIF is accompanied by discrete morphological alterations (mitochondrial swelling) visible by transmission electron microscopy of ultra-thin sectioned peripheral blood mononuclear cells. Taken together the results show that the hypothalamic NKA inhibitor blocks NKA of isolated human lymphocytes with high potency at nanomolar concentrations without toxicity; concentrations exceeding the ones required to block 86Rb-uptake reduce cell viability, probably due to leak formation across the NKA molecule. Thus, lymphocytes constitute a potential target for HIF action and by their altered NKA status a possible messenger between the nervous and the immune system.

Na,K-ATPase characterized in artificial membranes. 2. Successive measurement of ATP-driven Rb-accumulation, ouabain-blocked Rb-flux and palytoxin-induced Rb-efflux

The Na,K-ATPase is a multifunctional system anchored in the membrane of eukaryotic cells; it is responsible for the establishment and regulation of the Na/K balance of cell and organism by a stoichiometric mechanism linking Na extrusion to K uptake and ATP hydrolysis. The receptor for cardioactive steroids such as digoxin and ouabain is located at the extracellular surface of the system. Conversely, palytoxin, the most potent animal toxin, exerts its toxic effect by creating nonspecific leaks in the cell membrane leading to K-efflux and influx of Na and Ca ions. Ouabain prevents the pore-forming action of palytoxin in cells and therefore Na,K-ATPase is suspected to be the common receptor of ouabain and palytoxin. We have developed an artificial membrane system to determine structure-function relationships and ligand interactions of purified Na,K-ATPase: two-sided, bi-directional ATP-filled liposomes. In this system, ATP-driven 86Rb accumulation, arrest of 86Rb-uptake by ouabain, and palytoxin-induced 86Rb-leak were measured successively in the same preparation. Ouabain prevented the leak when the enzyme was ouabain-sensitive (rabbit kidney) but not when it was ouabain-resistant (rat kidney). On the basis of these data in conjunction with conformational analyses, allosteric conformational competition for the ouabain-palytoxin antagonism is proposed.

Biomechanical model of the P-type ion pumps of the cell

The mechanisms of the Na/K pump and of the primary Ca pumps of the cell have not yet been clarified. A biomechanical model of these so-called p-type ion pumps is proposed here. It is based on the assumption that the Na+ and Ca2+ ions are occluded by a contracting protein chain cooperating with the ATPase section of the pump. After transfer of the chain into the region of high Na+ or Ca2+ concentrations, the ions are released through stretching of the chain by the ATPase. In the backward transfer of the chain, a retrograde transport of Na+ ions is prevented through occlusion of K+ ions by another region of the same chain. In the case of Ca2+ ions, a similar effect is expected from hydrated Mg2+ ions. The two sections of the chain discriminate between the electrical field strength at the surface and the polarizability of the ions. The most likely mechanism for the transfer of the ion-binding chain is considered to involve a thermally induced transition of a pump dimer between two almost equivalent stable orientations in the membrane.