Identification of an essential sulfhydryl group in the ouabain binding site of (Na,K)-ATPase

Cytotoxicity and cellular mechanisms of toxicity of CuO NPs in mussel cells in vitro and comparative sensitivity with human cells

There is a need to assess human and ecosystem health effects of copper oxide nanoparticles (CuO NPs), extensively used in many industrial products. Here, we aimed to determine the cytotoxicity and cellular mechanisms involved in the toxicity of CuO NPs in mussel cells (hemocytes and gill cells) in parallel with exposures to ionic Cu and bulk CuO, and to compare the sensitivity of mussel primary cells with a well-established human cell line (pulmonary TT1 cells). At similar doses, CuO NPs promoted dose-dependent cytotoxicity and increased reactive oxygen species (ROS) production in mussel and human cells. In mussel cells, ionic Cu was more toxic than CuO NPs and the latter more than bulk CuO. Ionic Cu and CuO NPs increased catalase and acid phosphatase activities in both mussel cells and decreased gill cells Na-K-ATPase activity. All Cu forms produced DNA damage in hemocytes, whereas in gill cells only ionic Cu and CuO NPs were genotoxic. Induction of the MXR transport activity was found in gill cells exposed to all forms of Cu and in hemocytes exposed to ionic Cu and CuO NPs. Phagocytosis increased only in hemocytes exposed to CuO NPs, indicating a nanoparticle-specific immunostimulatory effect. In conclusion, toxicity of CuO NPs is driven by ROS in human and mussel cells. Mussel cells respond to CuO NP exposure by triggering an array of defensive mechanisms.

A smartphone-readable barcode assay for the detection and quantitation of pesticide residues

A qualitative assay for pesticide (methyl parathion) residues has been designed as a smartphone-readable barcode.

Halenaquinol, a natural cardioactive pentacyclic hydroquinone, interacts with sulfhydryls on rat brain Na(+),K(+)-ATPase

Halenaquinol inhibited the partial reactions of ATP hydrolysis by rat brain cortex Na(+),K(+)-ATPase, such as [3H]ATP binding to the enzyme, Na(+)-dependent front-door phosphorylation from [gamma-(33)P]ATP, and also Na(+)- and K(+)-dependent E(1)<-->E(2) conformational transitions of the enzyme. Halenaquinol abolished the positive cooperativity between the Na(+)- and K(+)-binding sites on the enzyme. ATP and sulfhydryl-containing reagents (cysteine and dithiothreitol) protected the Na(+),K(+)-ATPase against inhibition. Halenaquinol can react with additional vital groups in the enzyme after blockage of certain sulfhydryl groups with 5,5'-dithio-bis-nitrobenzoic acid. Halenaquinol inhibited [3H]ouabain binding to Na(+),K(+)-ATPase under phosphorylating and non-phosphorylating conditions. Binding of fluorescein 5'-isothiocyanate to Na(+),K(+)-ATPase and intensity of fluorescence of enzyme tryptophanyl residues were decreased by halenaquinol. We suggest that interaction of halenaquinol with the essential sulfhydryls in/or near the ATP-binding site of Na(+),K(+)-ATPase resulted in a change of protein conformation and subsequent alteration of overall and partial enzymatic reactions.

Inhibition of Na+,K+-ATPase in Penaeus indicus postlarvae by lead

The plasma membrane/mitochondrial fractions of Penaeus indicus postlarvae contain Mg2+-dependent ATPase, Na+,K+-stimulated ATPase, Na+-stimulated ATPase and K+-stimulated ATPase. The Na+,K+-activated, Mg2+-dependent ATPase was investigated further in relation to different pH and temperature conditions, and at various concentrations of protein, ouabain, ATP and ions in the incubation medium. In vitro and in vivo effects of lead were studied on the enzyme activity. In vitro lead inhibited the enzyme activity in a concentration-dependent manner with an IC50 value of 204.4 microM. In correlation with in vitro studies, in vivo investigations (both concentration and time dependent) of lead also indicated a gradual inhibition in enzyme activity. A maximum decrease of 85.3% was observed at LC50 (7.2 ppm) of lead for concentration-dependent experiments. In time-dependent studies, the decrease was maximal (81.7%) at 30 days of sublethal exposure (1.44 ppm). In addition, the substrate- and ion-dependent kinetics of Na+,K+-ATPase was studied in relation to in vitro exposure of lead; these studies suggest a non-competitive type of inhibition.

Decreased plasma membrane thiol concentration is associated with increased osmotic fragility of erythrocytes in zinc-deficient rats

Zinc deficiency leads to pathological signs that are related to impaired function of plasma membrane proteins. The purpose of this study was to assess the effect of dietary zinc status on the sulfhydryl (SH) content of erythrocyte plasma membranes and erythrocyte function. Three experiments were performed. In the first, immature male rats were fed for 21 d either a low-zinc (<1.0 mg/kg) diet free choice (-ZnAL), an adequate-zinc (100 mg/kg) diet free choice (+ZnAL), or the adequate-zinc diet limited to the intake of -ZnAL pair-mates (+ZnPF). Tail blood was sampled to measure osmotic fragility and SH concentration of erythrocyte membrane proteins. The zinc-deficient rats were then repleted for 2 d and erythrocytes assayed for fragility and SH content. In the second experiment blood was sampled at 3-d intervals to determine the time course of change in fragility and SH concentration. In the third experiment the SH concentration of erythrocyte band 3 protein and the binding of zinc to isolated plasma membranes were measured. SH concentration decreased from approximately 75 nmol/mg protein to 68 nmol/mg protein during 21 d of depletion and returned to control level within 2 d of repletion. There was an inverse relationship between osmotic fragility and SH concentration of erythrocyte membrane proteins. Maximal decrease in SH occurred within 6 d of consuming the low-zinc diet. The SH content of band 3 protein isolated from deficient rats was also significantly lower than that of pair-fed controls (45 vs. 51 nmol/mg protein). The zinc-binding affinity of plasma membrane proteins tended to be decreased by zinc deficiency. In summary, low-zinc status lowers the plasma membrane SH concentration, and the decreased reducing potential is inversely related to osmotic fragility, and presumably, with impaired volume recovery of erythrocytes.

Extensive random mutagenesis analysis of the Na+/K+-ATPase alpha subunit identifies known and previously unidentified amino acid residues that alter ouabain sensitivity--implications for ouabain binding

Random mutagenesis with ouabain selection has been used to comprehensively scan the extracellular and transmembrane domains of the alpha1 subunit of the sheep Na+/K+-ATPase for amino acid residues that alter ouabain sensitivity. The four random mutant libraries used in this study include all of the transmembrane and extracellular regions of the molecule as well as 75% of the cytoplasmic domains. Through an extensive number of HeLa cell transfections of these libraries and subsequent ouabain selection, 24 ouabain-resistant clones have been identified. All previously described amino acids that confer ouabain resistance were identified, confirming the completeness of this random mutagenesis screen. The amino acid substitutions that confer the greatest ouabain resistance, such as Gln111-->Arg, Asp121-->Gly, Asp121-->Glu, Asn122-->Asp, and Thr797-->Ala were identified more than once in this study. This extensive survey of the extracellular and transmembrane regions of the Na+/K+-ATPase molecule has identified two new regions of the molecule that affect ouabain sensitivity: the H4 and the H10 transmembrane regions. The new substitutions identified in this study are Leu330-->Gln, Ala331-->Gly, Thr338-->Ala, and Thr338-->Asn in the H4 transmembrane domain and Phe982-->Ser in the H10 transmembrane domain. These substitutions confer modest increases in the concentration of cardiac glycoside needed to produce 50% inhibition of activity (IC50 values), 3.1-7.9-fold difference. The results of this extensive screening of the Na+/K+-ATPase alpha1 subunit to identify amino acids residues that are important in ouabain sensitivity further supports our hypothesis that the H1-H2 and H4-H8 regions represent the major binding sites for the cardiac glycoside class of drugs.

Inhibition of Na+/K(+)-ATPase by phenoxyl radicals of etoposide (VP-16): role of sulfhydryls oxidation

In the present work, we studied the effects of phenoxyl radicals, generated by tyrosinase-catalyzed oxidation of a phenolic antitumor drug, Etoposide (VP-16), on a purified dog kidney Na+/K(+)-ATPase by characterizing interactions of VP-16 phenoxyl radicals with the enzyme's SH-groups by ESR and correlating the loss of the enzymatic activity with the oxidation of its SH-groups, and oxidation of VP-16. VP-16/tyrosinase caused inhibition of Na+/K(+)-ATPase which was dependent on the incubation time and concentration of tyrosinase. The inhibition of Na+/K(+)-ATPase was accompanied by a decrease of DTNB (5,5'-dithiobis-(2-nitrobenzoic acid)-titratable SH-groups. In the presence of Na+/K(+)-ATPase, a typical ESR signal of the VP-16 phenoxyl radical could be observed only following a lag period the duration of which was proportional to the concentration of the Na+/K(+)-ATPase added. Our HPLC measurements demonstrated that Na+/K(+)-ATPase protected VP-16 against tyrosinase-catalyzed oxidation. Combined these results suggest that redox-cycling of VP-16/VP-16 phenoxyl radical by SH-groups of Na+/K(+)-ATPase occurred. Ascorbate which is known to reduce the VP-16 phenoxyl radicals, protected the enzyme against inactivation, prevented oxidation of the enzyme's SH-groups. Reduction of VP-16 phenoxyl radicals by ascorbate was directly observed by the semidehydroascorbyl radical signal in the ESR spectra. VP-16 phenoxyl radical-induced oxidation of sulfhydryls and inhibition of the Na+/K(+)-ATPase may be responsible for at least some of its clinical side effects (e.g., cardiotoxicity) which can be prevented by ascorbate.

Na,K-ATPase characterized in artificial membranes. 1. Predominant conformations and ion-fluxes associated with active and inhibited states

The Na,K-ATPase (NKA) system is the receptor for the cardioactive steroids of plant or animal origin. It is not yet known whether passive ion fluxes traverse the inactivated receptor and thereby contribute to the hormonal, pharmacological or toxic actions of these compounds. To look for putative passive ion-fluxes across the ouabain-NKA complex, we incorporated it into the artificial membrane of liposomes. Since this synthetic membrane is virtually impermeable to Na and K ions, the hypothetical ion-fluxes mediated by the NKA can be determined. E2-forms and E2-ouabain-forms of purified NKA were incorporated, in parallel, into separate liposome preparations and the permeability of the resulting E2-liposomes and E2-ouabain-liposomes to K, Na and Ca ions was compared. The E2-liposomes expressed a typical K-permeability which was not observed in the E2-ouabain-liposomes; the latter showed a slightly higher Na-permeability and a similar Ca-permeability as compared to the former. Thus, ouabain does not induce leaks for K or Ca ions in the NKA molecule.

Analysis of thiol-topography in Na,K-ATPase using labelling with different maleimide nitroxide derivatives

Spin-label EPR spectroscopy of shark rectal gland Na,K-ATPase modified at cysteine residues with a variety of maleimide-nitroxide derivatives is used to characterize the different classes of sulphydryl groups. The spin-labelled derivatives vary with respect to charge and lipophilicity, and the chemical reactivity towards modification and inactivation of the Na,K-ATPase is dependent on these properties. Ascorbate is used to reduce the spin-labels in situ, and the kinetics of reduction of the protein-bound spin-labels are found also to depend on the nature of the maleimide-nitroxide derivative. The Na,K-ATPase is labelled either at Class I groups (with retention of enzymatic activity) or at Class II groups (where the enzymatic activity is lost). Although Class I groups are labelled more readily than are Class II groups they are only slightly more susceptible to reduction by ascorbate than the Class II groups, indicating no major difference in environment. The spectral difference observed between immobilized and mobile spin-labels with both Class I and Class II groups labelling is not reflected in widely different reduction kinetics for these two spectral components. Solubilization of the enzyme in an active form does not change the protein structure in terms of increased accessibility of the SH-groups to reduction by ascorbate. The results are discussed in terms of the location of the different SH-groups and the origins of the differences in mobility evident in the EPR spectra of the spin-labelled SH-groups.

Conventional and saturation transfer EPR spectroscopy of Na+/K(+)-ATPase modified with different maleimide-nitroxide derivatives

The membranous Na+/K(+)-ATPase from Squalus acanthias has been covalently modified on either Class I or Class II sulphydryl groups using derivatives of 3-(maleimidomethyl)-1-oxyl-2,2,5,5-tetramethylpyrrolidine with substituents of different charge and hydrophobicity attached at the remaining unsubstituted position of the pyrrolidine ring. The substituent groups used were a methyl and a hexyl ester, and di- and tri-methylammonium ethyl esters, as well as the parent underivatized compound. Additionally, another series of maleimide-nitroxides differing (by zero to seven intervening atoms) in the length of the linking group between the maleimide and the pyrrolidine moieties was used. The sites of attachment have been characterized in terms of the rotational mobility and environmental polarity by using conventional and saturation transfer EPR spectroscopy of these spin-labelled reagents. This provides a further sub-classification of the primary Class I and Class II SH-groups on the alpha-subunit of the enzyme, which differ both in their reactivity and influence on the Na+/K(+)-ATPase activity.

Ouabain binding, ATP hydrolysis, and Na+,K(+)-pump activity during chemical modification of brain and muscle Na+,K(+)-ATPase

The effects of 16 group-specific, amino acid-modifying agents were tested on ouabain binding, catalytical activity of membrane-bound (rat brain microsomal), sodium dodecyl sulfate-treated Na+,K(+)-ATPase, and Na+,K(+)-pump activity in intact muscle cells. With few exceptions, the potency of various tryptophan, tyrosine, histidine, amino, and carboxy group-oriented drugs to suppress ouabain binding and Na+,K(+)-ATPase activity correlated with inhibition of the Na+,K(+)-pump electrogenic effect. ATP hydrolysis was more sensitive to inhibition elicited by chemical modification than ouabain binding (membrane-bound or isolated enzyme) and than Na+,K(+)-pump activity. The efficiency of various drugs belonging to the same "specificity" group differed markedly. Tyrosine-oriented tetranitromethane was the only reagent that interfered directly with the cardiac receptor binding site as its inhibition of ouabain binding was completely protected by ouabagenin preincubation. The inhibition elicited by all other reagents was not, or only partially, protected by ouabagenin. It is surprising that agents like diethyl pyrocarbonate (histidine groups) or butanedione (arginine groups), whose action should be oriented to amino acids not involved in the putative ouabain binding site (represented by the -Glu-Tyr-Thr-Trp-Leu-Glu- sequence), are equally effective as agents acting on amino acids present directly in the ouabain binding site. These results support the proposal of long-distance regulation of Na+,K(+)-ATPase active sites.

Effects of ouabain on the rotational dynamics of renal Na,K-ATPase studied by saturation-transfer EPR

The interaction of a nitroxide spin-labeled derivative of ouabain with sheep kidney Na,K-ATPase and the motional behavior of the ouabain spin label-Na,K-ATPase complex have been studied by means of electron paramagnetic resonance (EPR) and saturation-transfer EPR (ST-EPR). Spin-labeled ouabain binds with high affinity to the Na,K-ATPase with concurrent inhibition of ATPase activity. Enzyme preparations retain 0.61 +/- 0.1 mol of bound ouabain spin label per mole of ATP-dependent phosphorylation sites, even after repeated centrifugation and resuspension of the purified ATPase-containing membrane fragments. The conventional EPR spectrum of the ouabain spin label bound to the ATPase consists almost entirely (greater than 99%) of a broad resonance at 0 degrees C, characteristic of a tightly bound spin label which is strongly immobilized by the protein backbone. Saturation-transfer EPR measurements of the spin-labeled ATPase preparations yield effective correlation times for the bound labels significantly longer than 100 microseconds at 0 degrees C. Since the conventional EPR measurements of the ouabain spin-labeled Na,K-ATPase indicated the label was strongly immobilized, these rotational correlation times most likely represent the motion of the protein itself rather than the independent motion of mobile spin probes relative to a slower moving protein. Additional ST-EPR measurements of ouabain spin-labeled Na,K-ATPase (a) cross-linked with glutaraldehyde and (b) crystallized in two-dimensional arrays indicated that the observed rotational correlation times predominantly represented the motion of large Na,K-ATPase-containing membrane fragments, as opposed to the motion of individual monomeric or dimeric polypeptides within the membrane fragment. The results suggest that the binding of spin-labeled ouabain to the ATPase induces the protein to form large aggregates, implying that cardiac glycoside induced enzyme aggregation may play a role in the mechanism of action of the cardiac glycosides in inhibiting the Na,K-ATPase.

Localization and characterization of binding sites with high affinity for [3H]ouabain in cerebral cortex of rabbit brain using quantitative autoradiography

[3H]Ouabain binding was studied in sections of rabbit somatosensory cortex by quantitative autoradiography and in rabbit brain microsomal membranes using a conventional filtration assay. KD values of 8-12 nM for specific high-affinity binding of [3H]ouabain were found by both methods. High-affinity binding was not uniformly distributed in somatosensory cortex and was localized predominantly to laminae 1, 3, and 4. [3H]Ouabain binding in tissue sections was stimulated by the ligands Mg2+/Pi or Mg2+/ATP/Na+ and was inhibited by K+ (IC50 = 0.7-0.9 mM), N-ethylmaleimide, 5,5'-dithiobis(2-nitrobenzoic acid), and erythrosin B. We conclude that [3H]ouabain is reversibly and specifically bound with high affinity in rabbit brain tissue sections under conditions that favor phosphorylation of Na+,K+-ATPase. Quantitative autoradiography is a powerful tool for assessing the affinity and number of specific ouabain binding sites in brain tissue.

Ouabain-binding site of (Na+ + K+)-ATPase in right-side-out vesicles has not an externally accessible SH group

The fluorescing sulfhydryl reagent N-(7-dimethylamino-4-methylcoumarinyl)maleimide (DACM) inactivates purified (Na+ + K+)-ATPase at 20 microM. This inactivation results in a decrease of the ouabain-binding capacity of the enzyme. Treatment of (Na+ + K+)-ATPase, embedded in right-side-out-oriented vesicles, by DACM does not affect ouabain binding to the enzyme. Incorporation of DACM into the alpha subunit of (Na+ + K+)-ATPase embedded in right-side-out vesicles is also not affected by the presence or absence of 100 microM ouabain. It is therefore concluded that a sulfhydryl group does not reside within the ouabain-binding site of (Na+ + K+)-ATPase.

The effects of p-chloromercuriphenylsulfonic acid on Trypanosoma cruzi infection of mammalian host cells in vitro

Treatment of Trypanosoma cruzi blood trypomastigotes with p-chloromercuriphenylsulfonic acid (PCMS) increased the association of the parasite with either mouse resident peritoneal macrophages or rat heart myoblasts in vitro. The effect was evidenced by elevation of both the percentage of host cells with parasites and the number of flagellates per 100 cells. The effect of PCMS appeared to be largely on the process of parasite penetration rather than surface binding as it was seen at 37 degrees C but not at 4 degrees C. A short pretreatment time, 5 min, was sufficient to elicit the enhancement, suggesting that the primary effect of PCMS might be at the parasite's cell surface. The PCMS effect was reversible as the parasite returned to normal levels of association with the host cells in less than 4 h after removal of excess PCMS. That sulfhydryl groups were involved in the PCMS effect was indicated by the abilities of excess cysteine and glutathione to block it. These results suggest a role for free sulfhydryl groups on the parasite surface in the process of host cell invasion.