Prevention and recovery of CuSO4-induced inhibition of Na+/K+ -ATPase and Mg2+ -ATPase in rat brain synaptosomes by EDTA

The simultaneous action of acute paradoxical sleep deprivation and hypothyroidism modulates synaptosomal ATPases and acetylcholinesterase activities in rat brain

BACKGROUND

Thyroid dysfunctions as well as sleep abnormalities are usually followed by neurological, psychiatric and/or behavioral disorders. On the other hand, changes in the brain adenosine triphosphatases (ATPases) and acetylcholinesterase (AChE) activities show significant importance in pathogenetic pathways in the evolution of numerous neuropsychiatric diseases.

METHODS

This study aimed to evaluate the in vivo simultaneous effects of hypothyroidism and paradoxical sleep deprivation for 72 h on synaptosomalATPases and AChE activities of whole rat brains. In order to induce hypothyroidism, 6-n-propyl-2-thiouracil was administrated in drinking water during 21 days. The modified multiple platform method was used to induce paradoxical sleep deprivation. The AChE and ATPases activities were measured using spectrophotometric methods.

RESULTS

Hypothyroidism significantly increased the activity of Na⁺/K⁺-ATPase compared to other groups, while at the same time significantly decreased AChE activity compared to the CT and SD groups. Paradoxical sleep deprivation significantly increased AChE activity compared to other groups. The simultaneous effect of hypothyroidism and sleep deprivation reduced the activity of all three enzymes (for Na⁺/K⁺-ATPase between HT/SD and HT group p < 0.0001, SD group p < 0.001,CT group p = 0.013; for ecto-ATPases between HT/SD and HT group p = 0.0034, SD group p = 0.0001, CT group p = 0.0007; for AChE between HT/SD and HT group p < 0.05, SD group p < 0.0001, CT group p < 0.0001).

CONCLUSIONS

The effect of simultaneous existence of hypothyroidism and paradoxical sleep deprivation reduces the activity of the Na⁺/K⁺-ATPase, ecto-ATPases, and AChE, what is different from individual effect of hypothyroidism and paradoxical sleep deprivation itself. This knowledge could help in the choice of appropriate therapy in such condition.

Analysis of mono-, di-, and triphosphates of thioguanosine and methylthioinosine in children with acute lymphoblastic leukemia by LC-MS/MS

Mercaptopurine (6-MP) is an indispensable, first-line, drug in the treatment of pediatric acute lymphoblastic leukemia (ALL). However, 6-MP has several intrinsic drawbacks, such as large individual variability in the drug response, undesirable adverse reactions, and drug resistance in patients with release ALL, which requires therapeutic drug monitoring (TDM). Several studies analyzed the total concentration of thiopurine nucleotides in red blood cells (RBCs) after hydrolysis, and two studies detected them separately and accurately by liquid chromatography-tandem mass spectrometry (LC-MS/MS). In this study, we developed a rapid and robust LC-MS/MS method for simultaneous quantitation of mono-, di-, and triphosphates of thioguanosine and methylthioinosine. Not only EDTA and DTT were added, but also EHT1864, a new Rac family small GTPases inhibitor, was innovatively added to ensure the stability of the analytes. Commercial availability and relatively low cost compound methotrexate-D3 was selected as internal standards. The linearity, accuracy, precision, recovery, matrix effect and stability of the method were all in line with the guidelines. This method provide an accurate and robust new solution for the determination of 6 metabolites of MP in RBCs from ALL patients with maintenance therapy.

Modelling chronic toxicokinetics and toxicodynamics of copper in mussels considering ionoregulatory homeostasis and oxidative stress

Chronic toxicity of copper (Cu) at sublethal levels is associated with ionoregulatory disturbance and oxidative stress. These factors were considered in a toxicokinetic-toxicodynamic model in the present study. The ionoregulatory disturbance was evaluated by the activity of the Na⁺/K⁺-ATPase enzyme (NKA), while oxidative stress was presented by lipid peroxidation (LPO) and glutathione-S-transferase (GST) activity. NKA activity was related to the binding of Cu²⁺ and Na ⁺ to NKA. LPO and GST activity were linked with the simulated concentration of unbound Cu. The model was calibrated using previously reported data and empirical data generated when zebra mussels were exposed to Cu. The model clearly demonstrated that Cu might inhibit NKA activity by reducing the number of functional pump sites and the limited Cu-bound NKA turnover rate. An ordinary differential equation was used to describe the relationship between the simulated concentration of unbound Cu and LPO/GST activity. Although this method could not explain the fluctuations in these biomarkers during the experiment, the measurements were within the confidence interval of estimations. Model simulation consistently shows non-significant differences in LPO and GST activity at two exposure levels, similar to the empirical observation.

Modulation of rat synaptosomal ATPases and acetylcholinesterase activities induced by chronic exposure to the static magnetic field

PURPOSE

It is considered that exposure to static magnetic fields (SMF) may have both detrimental and therapeutic effect, but the mechanism of SMF influence on the living organisms is not well understood. Since the adenosine triphosphatases (ATPases) and acetylcholinesterase (AChE) are involved in both physiological and pathological processes, the modulation of Na⁺/K⁺-ATPase, ecto-ATPases and AChE activities, as well as oxidative stress responses were followed in synaptosomes isolated from rats after chronic exposure toward differently oriented SMF.

MATERIAL AND METHODS

Wistar albino rats were randomly divided into three experimental groups (six animals per group): Up and Down group - exposed to upward and downward oriented SMF, respectively, and Control group. After 50 days, the rats were sacrificed, and synaptosomes were isolated from the whole rat brain and used for testing the enzyme activities and oxidative stress parameters.

RESULTS

Chronic exposure to 1 mT SMF significantly increased ATPases, AChE activities, and malondialdehyde (MDA) level in both exposed groups, compared to control values. The significant decrease in synaptosomal catalase activity (1.48 ± 0.17 U/mg protein) induced by exposure to the downward oriented field, compared to those obtained for Control group (2.60 ± 0.29 U/mg protein), and Up group (2.72 ± 0.21 U/mg protein).

CONCLUSIONS

It could be concluded that chronic exposure to differently oriented SMF increases ATPases and AChE activities in rat synaptosomes. Since brain ATPases and AChE have important roles in the pathogenesis of several neurological diseases, SMF influence on the activity of these enzymes may have potential therapeutic importance.

Interaction of Au(iii) and Pt(ii) complexes with Na/K-ATPase: experimental and theoretical study of reaction stoichiometry and binding sites

The present paper deals with investigation of the interaction between selected simple structure Au(iii) ([AuCl4]-, [AuCl2(dmso)2]+, [AuCl2(bipy)]+) and Pt(ii) ([PtCl2(dmso)2]) complexes with Na/K-ATPase as the target enzyme, using an experimental and theoretical approach. Reaction stoichiometries and binding constants for these enzyme/complex systems were determined, while kinetic measurements were used in order to reveal the type of inhibition. Based on the results obtained by quantum mechanical calculations (electrostatic surface potential (ESP), volume and surface of the complexes) the nature of the investigated complexes was characterized. By using the solvent accessible surface area (SASA) applied on specific inhibitory sites (ion channel and intracellular domains) the nature of these sites was described. Docking studies were used to determine the theoretical probability of the non-covalent metal binding site positions. Inhibition studies implied that all the investigated complexes decreased the activity of the enzyme while the kinetic analysis indicated an uncompetitive mode of inhibition for the selected complexes. Docking results suggested that the main inhibitory site of all these complexes is located in the ion translocation pathway on the extracellular side in the E2P enzyme conformation, similar to the case of cardiac glycosides, specific Na/K-ATPase inhibitors. Also, based on our knowledge, the hydrolyzed forms of [AuCl4]- and [PtCl2(dmso)2] complexes were investigated for the first time by theoretical calculations in this paper. Thereby, a new inhibitory site situated between the M2 and M4 helices was revealed. Binding in this site induces conformational changes in the enzyme domains and perturbs the E1-E2P conformational equilibrium, causing enzyme inhibition.

Neurotoxicity of Copper

Copper is an essential trace metal that is required for several important biological processes, however, an excess of copper can be toxic to cells. Therefore, systemic and cellular copper homeostasis is tightly regulated, but dysregulation of copper homeostasis may occur in disease states, resulting either in copper deficiency or copper overload and toxicity. This chapter will give an overview on the biological roles of copper and of the mechanisms involved in copper uptake, storage, and distribution. In addition, we will describe potential mechanisms of the cellular toxicity of copper and copper oxide nanoparticles. Finally, we will summarize the current knowledge on the connection of copper toxicity with neurodegenerative diseases.

The effect of subchronic supplementation with folic acid and l-arginine on homocysteine-induced seizures

The aim of the present study was to examine the effect of subchronic co-administration of folic acid (F) and l-arginine (A) on behavioural and electroencephalographic (EEG) characteristics of dl homocysteine thiolactone (H) induced seizures in adult rats. The activity of membrane ATPases in different brain regions were also investigated. Rats were treated with F, A, or vehicle for 15 days (regimen: F 5 mg/kg + A 500 mg/kg (F5A500); F 10 mg/kg + A 300 mg/kg (F10A300)). Seizures were elicited by convulsive dose of H (H, F5A500H, F10A300H) Subchronic supplementation with F and A did not affect seizure incidence, number of seizure episodes, and severity in F5A500H and F10A300H groups vs. H group. However, a tendency to increase latency and decrease the number of seizure episodes was noticed in the F10A300H group. EEG mean spectral power densities during ictal periods were significantly lower in F10A300H vs. H group. The activity of Na+/K+-ATPase and Mg2+-ATPase was significantly increased in almost all examined structures in rats treated with F and A. We can conclude that subchronic supplementation with folic acid and l-arginine has an antiepileptic effect in dl homocysteine thiolactone induced epilepsy.

A combined crystallographic analysis and ab initio calculations to interpret the reactivity of functionalized hexavanadates and their inhibitor potency toward Na(+)/K(+)-ATPase

In vitro influence of five synthesized functionalized hexavanadates (V6) on commercial porcine cerebral cortex Na(+)/K(+)-ATPase activity has been studied. Dose dependent Na(+)/K(+)-ATPase inhibition was obtained for all investigated compounds. Calculated half maximal inhibitory concentration IC50 values, in mol/L, for Na(+)/K(+)-ATPase were 7.6×10(-5), 1.8×10(-5), 2.9×10(-5), 5.5×10(-5) for functionalized hexavanadates (V6) with tetrabutylammonium (TBA) [V6-CH3][TBA]2, [V6-NO2][TBA]2, [V6-OH][TBA]2 and [V6-C3][TBA]2 respectively. [V6-OH][Na]2 inhibited Na(+)/K(+)-ATPase activity up to 30% at maximal investigated concentration 1×10(-3)mol/L. This reactivity has been interpreted using a study of the non-covalent interactions of functionalized hexavanadate hybrids through Cambridge Structural Database (CSD) analysis. Bibliographic searching has led to 18 different structures and 99 contacts. We have observed that C-H⋯O contacts consolidate the structures. We have also performed density functional theory (DFT) calculations and have determined electrostatic potential values at the molecular surface on a series of functionalized V6. These results enlightened their chemical reactivity and their potential biological applications such as the inhibition of the ATPase.

Inhibition of Na(+)/K(+)-ATPase and cytotoxicity of a few selected gold(III) complexes

Na(+)/K(+)-ATPase is in charge of maintaining the ionic and osmotic intracellular balance by using ATP as an energy source to drive excess Na(+) ions out of the cell in exchange for K(+) ions. We explored whether three representative cytotoxic gold(III) compounds might interfere with Na(+)/K(+)-ATPase and cause its inhibition at pharmacologically relevant concentrations. The tested complexes were [Au(bipy)(OH)2][PF6] (bipy=2,2'-bipyridine), [Au(py(dmb)-H)(CH3COO)2] (py(dmb)-H=deprotonated 6-(1,1-dimethylbenzyl)-pyridine), and [Au(bipy(dmb)-H)(OH)][PF6] (bipy(dmb)-H=deprotonated 6-(1,1-dimethylbenzyl)-2,2'-bipyridine). We found that all of them caused a pronounced and similar inhibition of Na(+)/K(+)-ATPase activity. Inhibition was found to be non-competitive and reversible. Remarkably, treatment with cysteine resulted in reversal or prevention of Na(+)/K(+)-ATPase inhibition. It is very likely that the described effects may contribute to the overall cytotoxic profile of these gold complexes.

In vitro effects of some gold complexes on Na(+)/K(+) ATPase activity and cell proliferation

The in vitro influence of gold(III) complexes, H[AuCl4], [Au(DMSO)2Cl2]Cl and [Au(bipy)Cl2]Cl (bipy = 2,2'-bipyridine), upon commercially available Na(+)/K(+) ATPase activity, purified from porcine brain cortex, was investigated. Additionally, the complexes were tested on human lymphocytes, and incidence of micronuclei and cell proliferation index was determined. Concentration-dependent inhibition of the enzyme for all three compounds was obtained, but with differing potencies. Calculated IC50 from Hill analysis were (in M): 5.75×10(-7), 5.50×10(-6) and 3.98×10(-5), for H[AuCl4], [Au(DMSO)2Cl2]Cl and [Au(bipy)Cl2]Cl, respectively, while Hill coefficient values, n, were above 1 in all cases. This inhibition can be prevented using -SH donating ligands such as L-Cys and glutathione, and these ligands can also cause a recovery of the enzyme activity after the induced inhibition. Kinetic analysis demonstrated that each of the studied gold(III) complexes affects Na(+)/K(+) ATPase reducing maximum enzymatic velocity, Vmax, but not significantly changing the affinity for the substrate (KM value), implying a noncompetitive mode of the interaction. Furthermore, among investigated gold(III) complexes, the [Au(bipy)Cl2]Cl complex exhibits a strong cytotoxic effect on human lymphocytes, which suggests its potential for use in antitumor therapy.

Inorganic mercury interacts with thiols at the nucleotide and cationic binding sites of the ouabain-sensitive cerebral electrogenic sodium pump

The molecular events leading to neuronal dysfunction often associated with mercury toxicity can be complex and is yet to be fully elucidated. Hence, the present study sought to evaluate the interaction of inorganic mercury (Hg(2+)) with the ouabain-sensitive electrogenic pump in partially purified mammalian brain membrane preparations. The results show that Hg(2+) significantly inhibited the transmembrane enzyme in a concentration dependent manner. In addition, Hg(2+) exerts its inhibitory effect on the activity of the enzyme by interacting with groups at the adenosine triphosphate (ATP), Na(+) and K(+) binding sites. However, preincubation of the enzyme with exogenous monothiols, cysteine, prevented the inhibition of Hg(2+) on the pump's activity suggesting that Hg(2+) may be interacting with the thiols at the nucleotide (ATP) and cationic (Na(+) and K(+)) binding sites. In fact, our data show that Hg(2+) oxidizes sulphydryl groups in cysteine in a time dependent fashion in vitro. Finally, we speculate that the small molecular volume of Hg(2+) in comparison with the substrates (ATP, Na(+) and K(+)) of sodium pump, its possibly high reactivity and strong affinity for thiols may account for its high toxicity towards the membrane bound ouabain-sensitive electrogenic pump.

Toxic effects of diazinon and its photodegradation products

The toxic effects of diazinon and its irradiated solutions were investigated using cultivated human blood cells (lymphocytes and erythrocytes) and skin fibroblasts. Ultra Performance Liquid Chromatography (UPLC)-UV/VIS system was used to monitor the disappearance of starting diazinon during 115-min photodegradation and formation of its by-products (diazoxon and 2-isopropyl-6-methyl-4-pyrimidinol (IMP)) as a function of time. Dose-dependent AChE and Na(+)/K(+)-ATPase inhibition by diazinon was obtained for all investigated cells. Calculated IC(50) (72 h) values, in M, were: 7.5x10(-6)/3.4x10(-5), 8.7x10(-5)/6.6x10(-5), and 3.0x10(-5)/4.6x10(-5) for fibroblast, erythrocyte and lymphocyte AChE/Na(+)/K(+)-ATPase, respectively. Results obtained for reference commercially purified target enzymes indicate similar sensitivity of AChE towards diazinon (IC(50) (20 min)-7.8x10(-5)M), while diazinon concentrations below 10mM did not noticeably affect Na(+)/K(+)-ATPase activity. Besides, diazinon and IMP induced increasing incidence of micronuclei (via clastogenic mode of action) in a dose-dependent manner up to 2x10(-6)M and significant inhibition of cell proliferation and increased level of malondialdehyde at all investigated concentrations. Although after 15-min diazinon irradiation formed products do not affect purified commercial enzymes activities, inhibitory effect of irradiated solutions on cell enzymes increased as a function of time exposure to UV light and resulted in significant reduction of AChE (up to 28-45%) and Na(+)/K(+)-ATPase (up to 35-40%) at the end of irradiation period. Moreover, photodegradation treatment strengthened prooxidative properties of diazinon as well as its potency to induce cytogenetic damage.

The activity of erythrocyte and brain Na+/K+ and Mg2+-ATPases in rats subjected to acute homocysteine and homocysteine thiolactone administration

Hyperhomocysteinemia is associated with various pathologies including cardiovascular disease, stroke, and cognitive dysfunctions. Systemic administration of homocysteine can trigger seizures in animals, and patients with homocystinuria suffer from epileptic seizures. Available data suggest that homocysteine can be harmful to human cells because of its metabolic conversion to homocysteine thiolactone, a reactive thioester. A number of reports have demonstrated a reduction of Na+/K+-ATPase activity in cerebral ischemia, epilepsy and neurodegeneration possibly associated with excitotoxic mechanisms. The aim of this study was to examine the in vivo effects of D,L-homocysteine and D,L-homocysteine thiolactone on Na+/K+- and Mg2+-ATPase activities in erythrocyte (RBC), brain cortex, hippocampus, and brain stem of adult male rats. Our results demonstrate a moderate inhibition of rat hippocampal Na+/K+-ATPase activity by D,L-homocysteine, which however expressed no effect on the activity of this enzyme in the cortex and brain stem. In contrast, D,L-homocysteine thiolactone strongly inhibited Na+/K+-ATPase activity in cortex, hippocampus and brain stem of rats. RBC Na+/K+-ATPase and Mg2+-ATPase activities were not affected by D,L-homocysteine, while D,L-homocysteine thiolactone inhibited only Na+/K+-ATPase activity. This study results show that homocysteine thiolactone significantly inhibits Na+/K+-ATPase activity in the cortex, hippocampus, and brain stem, which may contribute at least in part to the understanding of excitotoxic and convulsive properties of this substance.

Na⁺,K⁺-ATPase as the Target Enzyme for Organic and Inorganic Compounds

This paper gives an overview of the literature data concerning specific and non specific inhibitors of Na⁺,K⁺-ATPase receptor. The immobilization approaches developed to improve the rather low time and temperature stability of Na⁺,K⁺-ATPase, as well to preserve the enzyme properties were overviewed. The functional immobilization of Na⁺,K⁺-ATPase receptor as the target, with preservation of the full functional protein activity and access of various substances to an optimum number of binding sites under controlled conditions in the combination with high sensitive technology for the detection of enzyme activity is the basis for application of this enzyme in medical, pharmaceutical and environmental research.

Inhibition of Na+/K+-ATPase and Mg2+-ATPase by metal ions and prevention and recovery of inhibited activities by chelators

Kinetics and inhibition of Na(+)/K(+)-ATPase and Mg(2+)-ATPase activity from rat synaptic plasma membrane (SPM), by separate and simultaneous exposure to transition (Cu(2+), Zn(2+), Fe(2+) and Co(2+)) and heavy metals (Hg(2+) and Pb(2+)) ions were studied. All investigated metals produced a larger maximum inhibition of Na(+)/K(+)-ATPase than Mg(2+)-ATPase activity. The free concentrations of the key species (inhibitor, MgATP(2-), MeATP(2-)) in the medium assay were calculated and discussed. Simultaneous exposure to the combinations Cu(2+)/Fe(2+) or Hg(2+)/Pb(2+) caused additive inhibition, while Cu(2+)/Zn(2+) or Fe(2+)/Zn(2+) inhibited Na(+)/K(+)-ATPase activity synergistically (i.e., greater than the sum metal-induced inhibition assayed separately). Simultaneous exposure to Cu(2+)/Fe(2+) or Cu(2+)/Zn(2+) inhibited Mg(2+)-ATPase activity synergistically, while Hg(2+)/Pb(2+) or Fe(2+)/Zn(2+) induced antagonistic inhibition of this enzyme. Kinetic analysis showed that all investigated metals inhibited Na(+)/K(+)-ATPase activity by reducing the maximum velocities (V(max)) rather than the apparent affinity (Km) for substrate MgATP(2-), implying the noncompetitive nature of the inhibition. The incomplete inhibition of Mg(2+)-ATPase activity by Zn(2+), Fe(2+) and Co(2+) as well as kinetic analysis indicated two distinct Mg(2+)-ATPase subtypes activated in the presence of low and high MgATP(2-) concentration. EDTA, L-cysteine and gluthathione (GSH) prevented metal ion-induced inhibition of Na(+)/K(+)-ATPase with various potencies. Furthermore, these ligands also reversed Na(+)/K(+)-ATPase activity inhibited by transition metals in a concentration-dependent manner, but a recovery effect by any ligand on Hg(2+)-induced inhibition was not obtained.

The influence of potassium ion (K+) on digoxin-induced inhibition of porcine cerebral cortex Na+/K+-ATPase

The in vitro influence of potassium ion modulations, in the concentration range 2 mM-500 mM, on digoxin-induced inhibition of porcine cerebral cortex Na+ / K+-ATPase activity was studied. The response of enzymatic activity in the presence of various K+ concentrations to digoxin was biphasic, thereby, indicating the existence of two Na+ / K+-ATPase isoforms, differing in the affinity towards the tested drug. Both isoforms showed higher sensitivity to digoxin in the presence of K+ ions below 20 mM in the medium assay. The IC50 values for high/low isoforms 2.77 x 10(-6) M / 8.56 x 10(-5) M and 7.06 x 10(-7) M / 1.87 x 10(-5) M were obtained in the presence of optimal (20 mM) and 2 mM K+, respectively. However, preincubation in the presence of elevated K+ concentration (50-500 mM) in the medium assay prior to Na+ / K+-ATPase exposure to digoxin did not prevent the inhibition, i.e. IC50 values for both isoforms was the same as in the presence of the optimal K+ concentration. On the contrary, addition of 200 mM K+ into the medium assay after 10 minutes exposure of Na+ / K+-ATPase to digoxin, showed a time-dependent recovery effect on the inhibited enzymatic activity. Kinetic analysis showed that digoxin inhibited Na+ / K+-ATPase by reducing maximum enzymatic velocity (Vmax) and Km, implying an uncompetitive mode of interaction.

Interaction of some Pd(II) complexes with Na+ / K+-ATPase: inhibition, kinetics, prevention and recovery

The aim of this work was to investigate the influence of [PdCl4]2-, [PdCl(dien)]+ and [PdCl(Me4dien)]+ complexes on Na+ / K+-ATPase activity. The dose-dependent inhibition curves were obtained in all cases. IC50 values determined by Hill analysis were 2.25 x 10(-5) M, 1.21 x 10(-4) M and 2.36 x 10(-4) M, respectively. Na+ / K+-ATPase exhibited typical Michelis-Menten kinetics in the presence of Pd(II) complexes. Kinetic parameters (Vmax, Km) derived using Eadie-Hofstee transformation indicated a noncompetitive type of Na+ / K+-ATPase inhibition. The inhibitor constants (Ki) were determined from Dixon plots. The order of complex affinity for binding with Na+ / K+-ATPase, deducted from Ki values, was [PdCl4]2- > [PdCl(dien)]+ > [PdCl(Me4dien)]+. The results indicated that the potency of Pd(II) complexes to inhibit Na+/ K +-ATPase activity depended strongly on ligands of the related compound. Furthermore, the ability of SH-donor ligands, L-cysteine and glutathione, to prevent and recover the Pd(II) complexes-induced inhibition of Na+ / K+-ATPase was examined. The addition of 1 mM L-cysteine or glutathione to the reaction mixture before exposure to Pd(II) complexes prevented the inhibition by increasing the IC50 values by one order of magnitude. Moreover, the inhibited enzymatic activity was recovered by addition of SH-donor ligands in a concentration-dependent manner.

Prevention and recovery of (mu(3)-diethylentriamino)-chloro-palladium(II)-chloride induced inhibition of Na/K-ATPase by SH containing ligands--L-cysteine and glutathione

The effect of (mu(3)-diethylentriamino)-chloro-palladium(II)-chloride ([PdCl(dien)]Cl) on the activity of Na/K-ATPase from porcine cerebral cortex was studied in vitro, in the absence and presence of -SH containing ligands L-cysteine and glutathione (GSH). The aim of the study was to elucidate the mechanism of [PdCl(dien)](+) induced inhibition of the enzyme activity and to examine the ability of thiols to prevent and recover the inhibition. The coordinative interaction between [PdCl(dien)](+) and enzyme was verified by UV and (1)H NMR spectra. The semblance in the changes in absorption spectra of [PdCl(dien)](+) in the presence of Na/K-ATPase and thiols (L-cysteine and GSH) suggested that the complex ion interacts with enzymatic sulfhydryl groups. [PdCl(dien)](+) inhibited the enzyme activity in a dose-dependent manner. The Hill analysis of the inhibition curve yielded the half-maximum inhibitory activity value, IC(50)=1.21 x 10(-4)M, and Hill coefficient, n=0.7, suggesting the negative cooperation for binding of [PdCl(dien)](+) to the enzyme. Dependence of the initial reaction rate on the concentration of MgATP(2-) exhibited typical Michelis-Menten kinetics in the absence and presence of the inhibitor. Kinetic analysis showed that [PdCl(dien)](+) inhibited Na/K-ATPase by reducing the maximum reaction rate (V(max)), rather than changing the affinity to the substrate (K(m)). Kinetic parameters derived using Lineweaver-Burk transformation of experimental data indicated the non-competitive nature of Na/K-ATPase inhibition. The inhibitory constant, K(i)=1.05 x 10(-4)M, was determined from secondary replot of Lineweaver-Burk graph, and correlated with stability constants of [Pd(dien)(thiol)] complexes. 1 x 10(-3)M L-cysteine or GSH prevented the enzyme inhibition induced by Pd(II) complex cation when present below 1 x 10(-4)M. The both thiols completely reversed the inhibited activity in the concentration dependent manner, due to the complex formation with [PdCl(dien)](+).

Extracellular norepinephrine reduces neuronal uptake of norepinephrine by oxidative stress in PC12 cells

Cardiac norepinephrine (NE) uptake activity is reduced in congestive heart failure. Our studies in intact animals suggest that this effect on the cardiac sympathetic nerve endings is caused by oxidative stress and/or NE toxic metabolites derived from NE. In this study, we investigated the direct effects of NE on neuronal NE uptake activity and NE transporter (NET), using undifferentiated PC12 cells. Cells were incubated with NE (1–500 μM) either alone or in combination of Cu2+ sulfate (1 μM), which promotes free radical formation by Fenton reaction for 24 h. NE uptake activity was measured using [3H]NE. Cell viability was determined with the use of Trypan blue exclusion and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay, and cellular oxidative stress by dichlorodihydrofluorescein fluorescence and the GSH/GSSG ratio. Cell viability was reduced by NE >100 μM. At lower doses, NE produced oxidative stress and a dose-dependent reduction of NE uptake activity without affecting cell viability significantly. Cu2+, which has no direct effect on NE uptake activity, potentiated oxidative stress and reduction of NE uptake activity produced by NE. This decrease of NE uptake activity was associated with reductions of NE uptake binding sites and NET protein expression by using the radioligand assay and Western blot analysis, but no changes in NET gene expression. In addition, the free-radical scavenger mannitol, and antioxidant enzymes superoxide dismutase and catalase, reduced oxidative stress and attenuated the reductions of NE uptake activity and NET protein produced by NE/Cu. Thus our results support a functional role of oxidative stress in mediating the neuronal NE uptake reducing effect of NE and that this effect of NE on NET is a posttranscriptional event.

Influence of transition and heavy metal ions on ATP-ases activity in rat synaptic plasma membranes

The influence of transition metal (Cu2+, Zn2+, Fe2+ and Co2+) and heavy metal ions (Hg2+, Pb2+ and Cd2+) on the activities of Na+/K+-ATPase and Mg2+-ATPase isolated from rat synaptic plasma membranes (SPM) was investigated. The aim of the study was to elucidate the inhibition of both ATPase activities by exposure to the considered metal ions as a function of their affinity to bind to the ?SH containing ligand L-cysteine, as a model system. The half-maximum inhibitory activities (IC 50) of the enzymes were determined as parameters of rectangular hyperbolas and correlated with the stability constant (K s) of the respectivemetal- ion-L-cysteine complex. The linear Dixon plots indicate equilibrium binding of the investigated ions to both enzymes.

Effect of Cd(2+) and Hg(2+) on the activity of Na(+)/K(+)-ATPase and Mg(2+)-ATPase adsorbed on polystyrene microtiter plates

In the present study a polystyrene microtiter plate was tested as a support material for synaptic plasma membrane (SPM) immobilization by adsorption. The adsorption was carried out by an 18-h incubation at +4 degrees C of SPM with a polystyrene matrix, at pH 7.4. Evaluation of the efficiency of the applied immobilization method revealed that 10% protein fraction of initially applied SPM was bound to the support and that two SPM enzymes, Na(+)/K(+)-ATPase and Mg(2+)-ATPase, retained 70-80% activity after the adsorption. In addition, adsorption stabilizes Na(+)/K(+)-ATPase and Mg(2+)-ATPase, since the activities are substantial 3 weeks after the adsorption. Parallel kinetic analysis showed that adsorption does not alter significantly the kinetic properties of Na(+)/K(+)-ATPase and Mg(2+)-ATPase and their sensitivity to and mechanism of Cd(2+)- or Hg(2+)-induced inhibition. The only exception is the "high affinity" Mg(2+)-ATPase moiety, whose affinity for ATP and sensitivity toward Cd(2+) were increased by the adsorption. The results show that such system may be used as a practical and comfortable model for the in vitro toxicological investigations.