Kinetic characterization of tetrapropylammonium inhibition reveals how ATP and Pi alter access to the Na+-K+-ATPase transport site

FXYD protein isoforms differentially modulate human Na/K pump function

Tight regulation of the Na/K pump is essential for cellular function because this heteromeric protein builds and maintains the electrochemical gradients for Na+ and K+ that energize electrical signaling and secondary active transport. We studied the regulation of the ubiquitous human α1β1 pump isoform by five human FXYD proteins normally located in muscle, kidney, and neurons. The function of Na/K pump α1β1 expressed in Xenopus oocytes with or without FXYD isoforms was evaluated using two-electrode voltage clamp and patch clamp. Through evaluation of the partial reactions in the absence of K+ but presence of Na+ in the external milieu, we demonstrate that each FXYD subunit alters the equilibrium between E1P(3Na) and E2P, the phosphorylated conformations with Na+ occluded and free from Na+, respectively, thereby altering the apparent affinity for Na+. This modification of Na+ interaction shapes the small effects of FXYD proteins on the apparent affinity for external K+ at physiological Na+. FXYD6 distinctively accelerated both the Na+-deocclusion and the pump-turnover rates. All FXYD isoforms altered the apparent affinity for intracellular Na+ in patches, an effect that was observed only in the presence of intracellular K+. Therefore, FXYD proteins alter the selectivity of the pump for intracellular ions, an effect that could be due to the altered equilibrium between E1 and E2, the two major pump conformations, and/or to small changes in ion affinities that are exacerbated when both ions are present. Lastly, we observed a drastic reduction of Na/K pump surface expression when it was coexpressed with FXYD1 or FXYD6, with the former being relieved by injection of PKA's catalytic subunit into the oocyte. Our results indicate that a prominent effect of FXYD1 and FXYD6, and plausibly other FXYDs, is the regulation of Na/K pump trafficking.

External Ion Access in the Na/K Pump: Kinetics of Na+, K+, and Quaternary Amine Interaction

Na/K pumps build essential ion gradients across the plasmalemma of animal cells by coupling the extrusion of three Na⁺, with the import of two K⁺ and the hydrolysis of one ATP molecule. The mechanisms of selectivity and competition between Na⁺, K⁺, and inhibitory amines remain unclear. We measured the effects of external tetrapropylammonium (TPA⁺) and ethylenediamine (EDA²⁺) on three different Na/K pump transport modes in voltage-clamped Xenopus oocytes: 1) outward pump current (I_P), 2) passive inward H⁺ current at negative voltages without Na⁺ or K⁺ (I_H), and 3) transient charge movement reporting the voltage-dependent extracellular binding/release of Na⁺ (Q_Na). Both amines competed with K⁺ to inhibit I_P. TPA⁺ inhibited I_H without competing with H⁺, whereas EDA²⁺ did not alter I_H at pH 7.6. TPA⁺ competed with Na⁺ in Q_Na measurements, reducing Na⁺-apparent affinity, evidenced by a ∼-75 mV shift in the charge-voltage curve (at 20 mM TPA⁺) without reduction of the total charge moved (Q_tot). In contrast, EDA²⁺ and K⁺ did not compete with Na⁺ to inhibit Q_Na; both reduced Q_tot without decreasing Na⁺-apparent affinity. EDA²⁺ (15 mM) right-shifted the charge-voltage curve by ∼+50 mV. Simultaneous occlusion of EDA²⁺ and Na⁺ by an E2P conformation unable to reach E1P was demonstrated by voltage-clamp fluorometry. Trypsinolysis experiments showed that EDA²⁺-bound pumps are much more proteolysis-resistant than Na⁺-, K⁺-, or TPA⁺-bound pumps, therefore uncovering unique EDA²⁺-bound conformations. K⁺ effects on Q_Na and I_H were also evaluated in pumps inhibited with beryllium fluoride, a phosphate mimic. K⁺ reduced Q_tot without shifting the charge-voltage curve, indicating noncompetitive effects, and partially inhibited I_H to the same extent as TPA⁺ in non-beryllium-fluorinated pumps. These results demonstrate that K⁺ interacts with beryllium-fluorinated pumps inducing conformational changes that alter Q_Na and I_H, suggesting that there are two external access pathways for proton transport by I_H.

Probing the extracellular access channel of the Na,K-ATPase

When the Na,K-ATPase pumps at each turnover two K(+) ions into the cytoplasm, this translocation consists of several reaction steps. First, the ions diffuse consecutively from the extracellular phase through an access pathway to the binding sites where they are coordinated. In the next step, the enzyme is dephosphorylated and the ions are occluded inside the membrane domain. The subsequent transition to the E1 conformation produces a deocclusion of the binding sites to the cytoplasmic side of the membrane and allows in the last steps ion dissociation and diffusion to the aqueous phase. The interaction and competition of K(+) with various quaternary organic ammonium ions have been used to gain insight into the molecular mechanism of the ion binding process from the extracellular side in the P-E2 conformation of the enzyme. Using the electrochromic styryl dye RH421, evidence has been obtained that the access pathway consists of a wide and water-filled funnel-like part that is accessible also for bulky cations such as the benzyltriethylammonium ion, and a narrow part that permits passage only of small cations such as K(+) and NH4(+) in a distinct electrogenic way. Benzyltriethylammonium ions inhibit K(+) binding in a competitive manner that can be explained by a stopper-like function at the interface between the wide and narrow parts of the access pathway. In contrast to other quaternary organic ammonium ions, benzyltriethylammonium ions show a specific binding to the ion pump in a position inside the access pathway where it blocks effectively the access to the binding sites.

A review of MR spectroscopy studies of pediatric bipolar disorder

Pediatric bipolar disorder is a severe mental illness whose pathophysiology is poorly understood and for which there is an urgent need for improved diagnosis and treatment. MR spectroscopy is a neuroimaging method capable of in vivo measurement of neurochemicals relevant to bipolar disorder neurobiology. MR spectroscopy studies of adult bipolar disorder provide consistent evidence for alterations in the glutamate system and mitochondrial function. In bipolar disorder, these 2 phenomena may be linked because 85% of glucose in the brain is consumed by glutamatergic neurotransmission and the conversion of glutamate to glutamine. The purpose of this article is to review the MR spectroscopic imaging literature in pediatric bipolar disorder, at-risk samples, and severe mood dysregulation, with a focus on the published findings that are relevant to glutamatergic and mitochondrial functioning. Potential directions for future MR spectroscopy studies of the glutamate system and mitochondrial dysfunction in pediatric bipolar disorder are discussed.

Synergistic stimulation by potassium and ammonium of K(+)-phosphatase activity in gill microsomes from the crab Callinectes ornatus acclimated to low salinity: novel property of a primordial pump

We provide an extensive characterization of the modulation by p-nitrophenylphosphate, Mg²⁺, Na⁺, K(+), Rb⁺, NH(4)(+) and pH of gill microsomal K⁺-phosphatase activity in the posterior gills of Callinectes ornatus acclimated to low salinity (21‰). The synergistic stimulation by K⁺ and NH(4)(+) of the K⁺-phosphatase activity is a novel finding, and may constitute a species-specific feature of K(+)/NH(4)(+) interplay that regulates crustacean gill (Na⁺, K⁺)-ATPase activity. p-Nitrophenylphosphate was hydrolyzed at a maximum rate (V) of 69.2 ± 2.8nmolPimin⁻¹mg⁻¹ with K(0.5)=2.3 ± 0.1mmolL(-1), obeying cooperative kinetics (n(H)=1.7). Stimulation by Mg²⁺ (V=70.1 ± 3.0nmolPimin⁻¹mg⁻¹, K(0.5)=0.88 ± 0.04mmolL⁻¹), K⁺ (V=69.6 ± 2.7nmolPimin⁻¹mg⁻¹, K(0.5)=1.60 ± 0.07mmolL⁻¹) and NH(4)(+) (V=90.8 ± 4.0nmolPimin⁻¹mg⁻¹, K(0.5)=9.2 ± 0.3mmol L⁻¹) all displayed site-site interaction kinetics. In the presence of NH(4)(+), enzyme affinity for K⁺ unexpectedly increased by 7-fold, while affinity for NH(4)(+) was 28-fold greater in the presence than absence of K⁺. Ouabain partially inhibited K⁺-phosphatase activity (K(I)=320 ± 14.0μmolL⁻¹), more effectively when NH(4)(+) was present (K(I)=240 ± 12.0μmolL⁻¹). We propose a model for the synergistic stimulation by K⁺ and NH(4)(+) of the K⁺-phosphatase activity of the (Na⁺, K⁺)-ATPase from C. ornatus posterior gill tissue.

Frontal lobe bioenergetic metabolism in depressed adolescents with bipolar disorder: a phosphorus-31 magnetic resonance spectroscopy study

OBJECTIVES

To compare the concentrations of high-energy phosphorus metabolites associated with mitochondrial function in the frontal lobe of depressed adolescents with bipolar disorder (BD) and healthy controls (HC).

METHODS

We used in vivo phosphorus-31 magnetic resonance spectroscopy ((31) P-MRS) at 3 Tesla to measure phosphocreatine (PCr), beta-nucleoside triphosphate (β-NTP), inorganic phosphate (Pi), and other neurometabolites in the frontal lobe of eight unmedicated and six medicated adolescents with bipolar depression and 24 adolescent HCs.

RESULTS

Analysis of covariance, including age as a covariate, revealed differences in PCr (p=0.037), Pi (p=0.017), and PCr/Pi (p=0.002) between participant groups. Percentage neurochemical differences were calculated with respect to mean metabolite concentrations in the HC group. Post-hoc Tukey-Kramer analysis showed that unmedicated BD participants had decreased Pi compared with both HC (17%; p=0.038) and medicated BD (24%; p=0.022). The unmedicated BD group had increased PCr compared with medicated BD (11%; p=0.032). The PCr/Pi ratio was increased in unmedicated BD compared with HC (24%; p=0.013) and with medicated BD (39%; p=0.002). No differences in β-NTP or pH were observed.

CONCLUSIONS

Our results support the view that frontal lobe mitochondrial function is altered in adolescent BD and may have implications for the use of Pi as a biomarker. These findings join volumetric studies of the amygdala, and proton MRS studies of n-acetyl aspartate in pointing to potential differences in neurobiology between pediatric and adult BD.

Selectivity of externally facing ion-binding sites in the Na/K pump to alkali metals and organic cations

The Na/K pump is a P-type ATPase that exchanges three intracellular Na(+) ions for two extracellular K(+) ions through the plasmalemma of nearly all animal cells. The mechanisms involved in cation selection by the pump's ion-binding sites (site I and site II bind either Na(+) or K(+); site III binds only Na(+)) are poorly understood. We studied cation selectivity by outward-facing sites (high K(+) affinity) of Na/K pumps expressed in Xenopus oocytes, under voltage clamp. Guanidinium(+), methylguanidinium(+), and aminoguanidinium(+) produced two phenomena possibly reflecting actions at site III: (i) voltage-dependent inhibition (VDI) of outwardly directed pump current at saturating K(+), and (ii) induction of pump-mediated, guanidinium-derivative-carried inward current at negative potentials without Na(+) and K(+). In contrast, formamidinium(+) and acetamidinium(+) induced K(+)-like outward currents. Measurement of ouabain-sensitive ATPase activity and radiolabeled cation uptake confirmed that these cations are external K(+) congeners. Molecular dynamics simulations indicate that bound organic cations induce minor distortion of the binding sites. Among tested metals, only Li(+) induced Na(+)-like VDI, whereas all metals tested except Na(+) induced K(+)-like outward currents. Pump-mediated K(+)-like organic cation transport challenges the concept of rigid structural models in which ion specificity at site I and site II arises from a precise and unique arrangement of coordinating ligands. Furthermore, actions by guanidinium(+) derivatives suggest that Na(+) binds to site III in a hydrated form and that the inward current observed without external Na(+) and K(+) represents cation transport when normal occlusion at sites I and II is impaired. These results provide insights on external ion selectivity at the three binding sites.

Quaternary benzyltriethylammonium ion binding to the Na,K-ATPase: a tool to investigate extracellular K+ binding reactions

This study examined how the quaternary organic ammonium ion, benzyltriethylamine (BTEA), binds to the Na,K-ATPase to produce membrane potential (V(M))-dependent inhibition and tested the prediction that such a V(M)-dependent inhibitor would display electrogenic binding kinetics. BTEA competitively inhibited K(+) activation of Na,K-ATPase activity and steady-state (86)Rb(+) occlusion. The initial rate of (86)Rb(+) occlusion was decreased by BTEA to a similar degree whether it was added to the enzyme prior to or simultaneously with Rb(+), a demonstration that BTEA inhibits the Na,K-ATPase without being occluded. Several BTEA structural analogues reversibly inhibited Na,K-pump current, but none blocked current in a V(M)-dependent manner except BTEA and its para-nitro derivative, pNBTEA. Under conditions that promoted electroneutral K(+)-K(+) exchange by the Na,K-ATPase, step changes in V(M) elicited pNBTEA-activated ouabain-sensitive transient currents that had similarities to those produced with the K(+) congener, Tl(+). pNBTEA- and Tl(+)-dependent transient currents both displayed saturation of charge moved at extreme negative and positive V(M), equivalence of charge moved during and after step changes in V(M), and similar apparent valence. The rate constant (k(tot)) for Tl(+)-dependent transient current asymptotically approached a minimum value at positive V(M). In contrast, k(tot) for pNBTEA-dependent transient current was a "U"-shaped function of V(M) with a minimum value near 0 mV. Homology models of the Na,K-ATPase alpha subunit suggested that quaternary amines can bind to two extracellularly accessible sites, one of them located at K(+) binding sites positioned between transmembrane helices 4, 5, and 6. Altogether, these data revealed important information about electrogenic ion binding reactions of the Na,K-ATPase that are not directly measurable during ion transport by this enzyme.

P-type ATPases as drug targets: tools for medicine and science

P-type ATPases catalyze the selective active transport of ions like H+, Na+, K+, Ca2+, Zn2+, and Cu2+ across diverse biological membrane systems. Many members of the P-type ATPase protein family, such as the Na+,K+-, H+,K+-, Ca2+-, and H+-ATPases, are involved in the development of pathophysiological conditions or provide critical function to pathogens. Therefore, they seem to be promising targets for future drugs and novel antifungal agents and herbicides. Here, we review the current knowledge about P-type ATPase inhibitors and their present use as tools in science, medicine, and biotechnology. Recent structural information on a variety of P-type ATPase family members signifies that all P-type ATPases can be expected to share a similar basic structure and a similar basic machinery of ion transport. The ion transport pathway crossing the membrane lipid bilayer is constructed of two access channels leading from either side of the membrane to the ion binding sites at a central cavity. The selective opening and closure of the access channels allows vectorial access/release of ions from the binding sites. Recent structural information along with new homology modeling of diverse P-type ATPases in complex with known ligands demonstrate that the most proficient way for the development of efficient and selective drugs is to target their ion transport pathway.

Red blood cell Na pump: Insights from species differences

The red blood cell membrane is specialized to exchange chloride and bicarbonate; usually the pH gradient, the chloride ratio, and the membrane potential are tightly coupled. We review the evidence that led to the ability to separately vary inside and outside pH in red cells. The effect of pH on Na pump activity and on the selectivity of the inside and the outside transport sites is reviewed. In red blood cells, at high pH, the outside site is not selective. An increase in protons leads to an increase in K(+) affinity, thus making the site more selective. The pK for this site is different in rats and humans; because of the high conservation of residues in these two species, there are only a few possible residues that can account for this difference. On the inside, work from unsided preparations suggests that, at high pH, the transport site is highly selective for Na(+). Once again, an increase in protons leads to an increase in K(+) affinity, but now the result is a less selective site. During their maturation, reticulocytes lose many membrane proteins. The type and fractional loss is species dependent. For example, most reticulocytes lose most of their Na pumps, retaining about 100 pumps per cell, but animals from the order Carnivora lose all their pumps. We review some of the evidence that PKC phosphorylation of N-terminus serines is responsible for endocytosis in other cell types and species variation in this region.

The reactive nitrogen species peroxynitrite is a potent inhibitor of renal Na-K-ATPase activity

Peroxynitrite is a reactive nitrogen species produced when nitric oxide and superoxide react. In vivo studies suggest that reactive oxygen species and, perhaps, peroxynitrite can influence Na-K-ATPase function. However, the direct effects of peroxynitrite on Na-K-ATPase function remain unknown. We show that a single bolus addition of peroxynitrite inhibited purified renal Na-K-ATPase activity, with IC50 of 107+/-9 microM. To mimic cellular/physiological production of peroxynitrite, a syringe pump was used to slowly release (approximately 0.85 microM/s) peroxynitrite. The inhibition of Na-K-ATPase activity induced by this treatment was similar to that induced by a single bolus addition of equal cumulative concentration. Peroxynitrite produced 3-nitrotyrosine residues on the alpha, beta, and FXYD subunits of the Na pump. Interestingly, the flavonoid epicatechin, which prevented tyrosine nitration, was unable to blunt peroxynitrite-induced ATPase inhibition, suggesting that tyrosine nitration is not required for inhibition. Peroxynitrite led to a decrease in iodoacetamidofluorescein labeling, implying that cysteine modifications were induced. Glutathione was unable to reverse ATPase inhibition. The presence of Na+ and low MgATP during peroxynitrite treatment increased the IC50 to 145+/-10 microM, while the presence of K+ and low MgATP increased the IC50 to 255+/-13 microM. This result suggests that the EPNa conformation of the pump is slightly more sensitive to peroxynitrite than the E(K) conformation. Taken together, these results show that peroxynitrite is a potent inhibitor of Na-K-ATPase activity and that peroxynitrite can induce amino acid modifications to the pump.

Gill (Na+,K+)-ATPase in diadromous, freshwater palaemonid shrimps: Species-specific kinetic characteristics and α-subunit expression

To better comprehend physiological adaptation to dilute media and the molecular mechanisms underlying ammonia excretion in palaemonid shrimps, we characterized the (Na+,K+)-ATPase from Macrobrachium amazonicum gills, disclosing high- (K(0.5) = 4.2+/-0.2 micromol L(-1); V = 33.9+/-1.9 U mg(-1)) and low-affinity (K(0.5) = 0.144+/-0.010 mmol L(-1); V = 232.9+/-15.3 U mg(-1)) ATP hydrolyzing sites. Stimulation by Na+ (K(0.5) = 5.5+/-0.3 mmol L(-1); V = 275.1+/-15.1 U mg(-1)), Mg2+ (K(0.5) = 0.79+/-0.06 mmol L(-1); V = 261.9+/-18.3 U mg(-1)), K+ (K(M) = 0.88+/-0.04 mmol L(-1); V = 271.8+/-10.9 U mg(-1)) and NH4(+) (K(M) = 5.0+/-0.2 mmol L(-1); V = 385.9+/-15.8 U mg(-1)) obeys single saturation curves, activity being stimulated synergistically by NH4(+) and K+. There is a single K+ binding site, NH4(+) binding to a second, exclusive site, stimulating activity by 33%, modulating K+ affinity. (Na+,K+)-ATPase activity constitutes approximately 80% of total ATPase activity (K(Iouabain) = 147.5+/-8.9 micromol L(-1)); Na+-, K+-, Ca2+-, V- and F(o)F(1)-ATPases are also present. M. amazonicum microsomal fractions possess approximately 2-fold less (Na+,K+)-ATPase alpha-subunit than M. olfersi, consistent with a 2.6-fold lower specific activity. These differences in (Na+, K+)-ATPase stimulation by ATP and ions, and specific activities of other ATPases, suggest the presence of distinct biochemical adaptations to life in fresh water in these related species.

Divalent cation interactions with Na,K-ATPase cytoplasmic cation sites: implications for the para-nitrophenyl phosphatase reaction mechanism

The interactions of divalent cations with the adenosine triphosphatase (ATPase) and para-nitrophenyl phosphatase (pNPPase) activity of the purified dog kidney Na pump and the fluorescence of fluorescein isothiocyanate (FITC)-labeled pump were determined. Sr(2+) and Ba(2+) did not compete with K(+) for ATPase (an extracellular K(+) effect). Sr(2+) and Ba(2+) did compete with Na(+) for ATPase (an intracellular Na(+) effect) and with K(+) for pNPPase (an intracellular K(+) effect). These results suggest that Ba(2+) or Sr(2+) can bind to the intracellular transport site, yet neither Ba(2+) nor Sr(2+) was able to activate pNPPase activity; we confirmed that Ca(2+) and Mn(2+) did activate. As another measure of cation binding, we observed that Ca(2+) and Mn(2+), but not Ba(2+), decreased the fluorescence of the FITC-labeled pump; we confirmed that K(+) substantially decreased the fluorescence. Interestingly, Ba(2+) did shift the K(+) dose-response curve. Ethane diamine inhibited Mn(2+) stimulation of pNPPase (as well as K(+) and Mg(2+) stimulation) but did not shift the 50% inhibitory concentration (IC(50)) for the Mn(2+)-induced fluorescence change of FITC, though it did shift the IC(50) for the K(+)-induced change. These results suggest that the Mn(2+)-induced fluorescence change is not due to Mn(2+) binding at the transport site. The drawbacks of models in which Mn(2+) stimulates pNPPase by binding solely to the catalytic site vs. those in which Mn(2+) stimulates by binding to both the catalytic and transport sites are presented. Our results provide new insights into the pNPPase kinetic mechanism as well as how divalent cations interact with the Na pump.

Extracellular terbium and divalent cation effects on the red blood cell Na pump and chrysoidine effects on the renal Na pump

We examined the effect of extracellular terbium (Tb(3+)) and divalent metal cations (Ca(2+), Sr(2+), and Ba(2+)) on (86)Rb(+) influx into rabbit and human red blood cells. We found that Tb(3+) at 15 and 25 microM was a non-competitive inhibitor of (86)Rb(+) influx suggesting that Tb(3+) is not binding to the transport site. This result reduces the usefulness of Tb(3+) as a potential probe for the E(out) conformation (the conformation with the transport site facing extracellularly). Ba(2+), Sr(2+) and Ca(2+), at concentrations >50 mM, had minimal effects on Rb(+) influx into red blood cells (1 mM Rb-out). This suggests that the outside transport site is very specific for monovalent cations over divalent cations, in contrast to the inside transport site. We also found that chrysoidine (4-phenylazo-m-phenylenediamine) competes with Na(+) for ATPase activity and K(+) for pNPPase activity suggesting it is binding to the E(in) conformation. Chrysoidine and similar compounds may be useful as optical probes of the E(in) conformation.

Kinetic characterization of Na,K-ATPase inhibition by Eosin

Eosin is a probe for the Na pump nucleotide site. In contrast to previous studies examining eosin effects on Na only ATPase, we examined Na,K-ATPase- and K-activated pNPPase activity in red blood cell membranes and purified renal Na,K-ATPase. At saturating ATP (3 mM) the eosin IC(50) for Na pump inhibition was 19 microM. Increasing ATP concentrations (0.2-2.5 mM) did not overcome eosin-induced inhibition, thus eosin is a mixed-type inhibitor of ATPase activity. To test if eosin can bind to the high-affinity ATP site, purified Na,K-ATPase was labeled with 20 microM FITC. With increasing eosin concentrations (0.1 microM-10 microM) the incorporation of FITC into the ATP site significantly decreases suggesting that eosin prevents FITC reaction at the high-affinity ATP site. Eosin was a more potent inhibitor of K-activated phosphatase activity than of Na,K-ATPase activity. At 5 mM pNPP the eosin IC(50) for Na pump inhibition was 3.8+/-0.23 microM. Increasing pNPP concentrations (0.45-14.5 mM) did not overcome eosin-induced inhibition, thus eosin is a mixed-type inhibitor of pNPPase activity. These results can be fit by a model in which eosin and ATP bind only to the nucleotide site; in some pump conformations, this site is rigid and the binding is mutually exclusive and in other conformations, the site is flexible and able to accommodate both eosin and ATP (or pNPP). Interestingly, eosin inhibition of pNPPase became competitive after the addition of C(12)E(8) (0.1%) but the inhibition of ATPase remained mixed.

Similarities and differences between organic cation inhibition of the Na,K-ATPase and PMCA

The effects of three classes of organic cations on the inhibition of the plasma membrane Ca pump (PMCA) were determined and compared to inhibition of the Na pump. Quaternary amines (tetramethylammonium, tetraethylammonium, and tetrapropylammonium, TMA, TEA, and TPA, respectively) did not inhibit PMCA. This is not to imply that PMCA is inherently selective against monovalent cations because guanidine and tetramethylguanidine inhibited PMCA by competing with Ca(2+). The divalent organic cation, ethyl diamine, inhibited PMCA but was not competitive with Ca(2+). In contrast, propyl diamine did compete with Ca(2+) and was about 10-fold more potent than butyl diamine in inhibiting PMCA. For the Na pump, both TEA and TPA inhibited, but TMA did not. TEA, guanidine, and tetramethylguanidine inhibition was competitive with Na(+) for ATPase activation and with K(+) for pNPPase activation, both of which are cytoplasmic substrate cation effects. Thus, these findings are consistent with TEA, guanidine, and tetramethylguanidine inhibiting from the cytoplasmic side of the Na pump; in contrast, we have previously shown that TPA did not inhibit from the cytoplasmic side. The divalent alkane diamines ethyl, propyl, and butyl diamine all inhibited the Na pump and all competed at the intracellular surface. The order of potency was ED > PD > BD consistent with an optimal size for binding; similarly, for the quaternary amines TMA is apparently too small to make appropriate contacts, and TPA is too large. Homology models based upon the high-resolution SERCA structure are included to contextualize the kinetic observations.

Palytoxin-induced effects on partial reactions of the Na,K-ATPase

The interaction of palytoxin with the Na,K-ATPase was studied by the electrochromic styryl dye RH421, which monitors the amount of ions in the membrane domain of the pump. The toxin affected the pump function in the state P-E2, independently of the type of phosphorylation (ATP or inorganic phosphate). The palytoxin-induced modification of the protein consisted of two steps: toxin binding and a subsequent conformational change into a transmembrane ion channel. At 20 degrees C, the rate-limiting reaction had a forward rate constant of 10(5) M(-1)s(-1) and a backward rate constant of about 10(-3) s(-1). In the palytoxin-modified state, the binding affinity for Na+ and H+ was increased and reached values between those obtained in the E1 and P-E2 conformation under physiological conditions. Even under saturating palytoxin concentrations, the ATPase activity was not completely inhibited. In the Na/K mode, approximately 50% of the enzyme remained active in the average, and in the Na-only mode 25%. The experimental findings indicate that an additional exit from the inhibited state exists. An obvious reaction pathway is a slow dephosphorylation of the palytoxin-inhibited state with a time constant of approximately 100 s. Analysis of the effect of blockers of the extracellular and cytoplasmic access channels, TPA+ and Br2-Titu3+, respectively, showed that both access channels are part of the ion pathway in the palytoxin-modified protein. All experiments can be explained by an extension of the Post-Albers cycle, in which three additional states were added that branch off in the P-E2 state and lead to states in which the open-channel conformation is introduced and returns into the pump cycle in the occluded E2 state. The previously suggested molecular model for the channel state of the Na,K-ATPase as a conformation in which both gates between binding sites and aqueous phases are simultaneously in their open state is supported by this study.