Inhibition of the sarcoplasmic reticulum (Ca2+ + Mg2+-ATPase by Zn(II)

Sarcoplasmic reticulum calcium ATPase interactions with decaniobate, decavanadate, vanadate, tungstate and molybdate

Over the last few decades there has been increasing interest in oxometalate and polyoxometalate applications to medicine and pharmacology. This interest arose, at least in part, due to the properties of these classes of compounds as anti-cancer, anti-diabetic agents, and also for treatment of neurodegenerative diseases, among others. However, our understanding of the mechanism of action would be improved if biological models could be used to clarify potential toxicological effects in main cellular processes. Sarcoplasmic reticulum (SR) vesicles, containing a large amount of Ca(2+)-ATPase, an enzyme that accumulates calcium by active transport using ATP, have been suggested as a useful model to study the effects of oxometalates on calcium homeostasis. In the present article, it is shown that decavanadate, decaniobate, vanadate, tungstate and molybdate, all inhibited SR Ca(2+)-ATPase, with the following IC(50) values: 15, 35, 50, 400 μM and 45 mM, respectively. Decaniobate (Nb(10)), is the strongest P-type enzyme inhibitor, after decavanadate (V(10)). Atomic-absorption spectroscopy (AAS) analysis, indicates that decavanadate binds to the protein with a 1:1 decavanadate:Ca(2+)-ATPase stoichiometry. Furthermore, V(10) binds with similar extension to all the protein conformations, which occur during calcium translocation by active transport, namely E1, E1P, E2 and E2P, as analysed by AAS. In contrast, it was confirmed that the binding of monomeric vanadate (H(2)VO(4)(2-); V(1)) to the calcium pump is favoured only for the E2 and E2P conformations of the ATPase, whereas no significant amount of vanadate is bound to the E1 and E1P conformations. Scatchard plot analysis, confirmed a 1:1 ratio for decavanadate-Ca(2+)-ATPase, with a dissociation constant, k(d) of 1 μM(-1). The interaction of decavanadate V(10)O(28)(6-) (V(10)) with Ca(2+)-ATPase is prevented by the isostructural and isoelectronic decaniobate Nb(10)O(28)(6-) (Nb(10)), whereas no significant effects were detected with ATP or with heparin, a known competitive ATP binding molecule, suggesting that V(10) binds non-competitively, with respect to ATP, to the protein. Finally, it was shown that decaniobate inhibits SR Ca(2+)-ATPase activity in a non competitive type of inhibition, with respect to ATP. Taken together, these data demonstrate that decameric niobate and vanadate species are stronger inhibitors of the SR calcium ATPase than simple monomeric vanadate, tungstate and molybdate oxometalates, thus affecting calcium homeostasis, cell signalling and cell bioenergetics, as well many other cellular processes. The ability of these oxometalates to act either as phosphate analogues, as a transition-state analogue in enzyme-catalysed phosphoryl group transfer processes and as potentially nucleotide-dependent enzymes modulators or inhibitors, suggests that different oxometalates may reveal different mechanistic preferences in these classes of enzymes.

Sarcoplasmic reticulum calcium ATPase is inhibited by organic vanadium coordination compounds: pyridine-2,6-dicarboxylatodioxovanadium(V), BMOV, and an amavadine analogue

The general affinity of the sarcoplasmic reticulum (SR) Ca (2+)-ATPase was examined for three different classes of vanadium coordination complexes including a vanadium(V) compound, pyridine-2,6-dicarboxylatodioxovanadium(V) (PDC-V(V)), and two vanadium(IV) compounds, bis(maltolato)oxovanadium(IV) (BMOV), and an analogue of amavadine, bis( N-hydroxylamidoiminodiacetato)vanadium(IV) (HAIDA-V(IV)). The ability of vanadate to act either as a phosphate analogue or as a transition-state analogue with enzymes' catalysis phosphoryl group transfer suggests that vanadium coordination compounds may reveal mechanistic preferences in these classes of enzymes. Two of these compounds investigated, PDC-V(V) and BMOV, were hydrolytically and oxidatively reactive at neutral pH, and one, HAIDA-V(IV), does not hydrolyze, oxidize, or otherwise decompose to a measurable extent during the enzyme assay. The SR Ca (2+)-ATPase was inhibited by all three of these complexes. The relative order of inhibition was PDC-V(V) > BMOV > vanadate > HAIDA-V(IV), and the IC 50 values were 25, 40, 80, and 325 microM, respectively. Because the observed inhibition is more potent for PDC-V(V) and BMOV than that of oxovanadates, the inhibition cannot be explained by oxovanadate formation during enzyme assays. Furthermore, the hydrolytically and redox stable amavadine analogue HAIDA-V(IV) inhibited the Ca (2+)-ATPase less than oxovanadates. To gauge the importance of the lipid environment, studies of oxidized BMOV in microemulsions were performed and showed that this system remained in the aqueous pool even though PDC-V(V) is able to penetrate lipid interfaces. These findings suggest that the hydrolytic properties of these complexes may be important in the inhibition of the calcium pump. Our results show that two simple coordination complexes with known insulin enhancing effects can invoke a response in calcium homeostasis and the regulation of muscle contraction through the SR Ca (2+)-ATPase.

In vitro assessment of the effects of cadmium and zinc on mammalian nerve fibres

Zinc and cadmium are environmental contaminants that have a wide range of effects on the nervous system, but zinc is also considered to be an important metal in the human body. In this study the effect of CdCl(2) and ZnCl(2), at concentrations of 50,150, 250 and 500 microM, on the nerve fibres of the sciatic nerve of the rat isolated in a three-chamber recording bath were studied. At the same concentrations, CdCl(2) and ZnCl(2) were found to have almost the same inhibitory effect on the compound action potential (CAP) of the nerve fibres. Their concentration-effect curves almost overlap and there was no significant difference in their EC(50) which for CdCl(2) is 250.1+/-18 microM (n=5) and for ZnCl(2) is 282.2+/-25 microM (n=5) correspondingly (P>0.05). The no-observed-effect concentration (NOEC) was estimated to be 50-100 microM for both metals. The identical inhibitory effect of both metals on the sciatic nerve fibres indicates a common mode of action which is related to their potential to generate reactive oxygen species (ROS).

Zn2+ and Cd2+ increase the cyclic GMP level in PC12 cells by inhibition of the cyclic nucleotide phosphodiesterase

In the present study, the influence of the heavy metal ions Cd2+ and Zn2+ on cGMP metabolism in the neurosecretory rat pheochromocytoma (PC12) cell line has been investigated. Cadmium and zinc ions showed a concentration-dependent increase of intracellular cGMP levels as determined by radioimmunoassay: a 20-fold increase in cGMP concentration was found after 15 min of incubation with 20 microM Cd2+, and a 7-fold increase in cGMP was found after incubation with 50 microM Zn2+ (control: 6.05+/-2.1 pmol cGMP/mg protein). To obtain further mechanistic informations, the effects of Cd2+ and Zn2+ on intracellular 3',5'-cyclic nucleotide phosphodiesterase have been studied by a high performance liquid chromatography-based phosphodiesterase-assay. The cellular cGMP hydrolysis was found to be inhibited by these ions with an IC(50) value of 6+/-0.7 microM for Cd2+ and 13+/-2.5microM for Zn2+ . Hence, dose-dependent increase in cellular cGMP content is due to an inhibition of cGMP hydrolysis and not due to an increase in cGMP synthesis. Cd2+ and Zn2+ were taken up by PC12 cells as determined by atomic absorption spectroscopy, all measurements were performed in a subtoxic concentration range. Our data illustrate that zinc and cadmium ions are efficient inhibitors of the cGMP-stimulated cyclic nucleotide PDEII in PC12 cells resulting in elevated cellular cGMP concentrations. Therefore, subtoxic doses of these metals may disturb intracellular cGMP/cAMP-signalling pathways leading to an impaired or altered gene expression.

Structural changes of the sarcoplasmic reticulum Ca(II)-ATPase nucleotide binding domain by pH and La(III)

The Ca(2+)-ATPase from sarcoplasmic reticulum couples the hydrolysis of one molecule of ATP to the transport of two Ca2+ ions in skeletal muscle fibers. Here, we study the accessibility of the fluorescein covalently attached to the Lys515 at the nucleotide binding domain of the ATPase to the small collisional quencher iodide at pH 6 and 8, as well as the effect of ligand binding (La3+, La(3+)-nucleotide, and Ca2+). Our results indicate that bound fluorescein is significantly more accessible at pH 6 than at pH 8, suggesting that pH modulates the structure of the nucleotide binding domain of the ATPase. This notion was further substantiated by the finding that La(3+)-nucleotide only interacted with the catalytic center at acidic pH. Notably, the differential accessibility of the nucleotide binding domain at acidic and basic pH cannot be rationalized in terms of the ATPase E1/E2 conformational equilibrium since a shift of the ATPase toward the E1 (plus Ca2+) or E2 (plus EGTA) did not affect the accessibility of fluorescein-labeled ATPase to the quencher. Taken together, these findings show the presence of structural flexibility in the FITC binding site and suggest a structural modulation of the Ca(2+)-ATPase nucleotide binding domain by pH and La3+ binding through long-range link-age mechanisms.

The binding of ATP and Mg2+ to the calcium adenosinetriphosphatase of sarcoplasmic reticulum follows a random mechanism

The enzyme form of the calcium adenosinetriphosphatase of sarcoplasmic reticulum (CaATPase) that is stable in the presence of calcium, cE.Ca2, has a binding site for the catalytic Mg2+ ion with a dissociation constant of 0.94 +/- 0.15 mM at 25 degrees C, pH 7.0, and 100 mM KCl. This is approximately 10 times smaller than that reported for the free enzyme, E, (8.8 mM) under similar conditions [Punzengruber, C., Prager, R., Kolassa, N., Winkler, F., & Suko, J. (1978) Eur. J. Biochem. 92, 349-359]. This difference shows that the sites for the catalytic and the transported ions interact in the absence of ATP. The addition of ATP and EDTA to enzyme that had been incubated with Ca2+ and Mg2+ resulted in the formation of 61% phosphoenzyme. The addition of unlabeled ATP and Mg2+ to enzyme that had been incubated with 3.5 microM free Ca2+ and labeled ATP gave 39% labeled phosphoenzyme. This shows that the binding of ATP and Mg2+ to cE.Ca2 follows a random mechanism. The rate constants for dissociation of ATP and Mg2+ from cE.Ca2.ATP.Mg are different: kdiss(ATP) = 120 s-1 and kdiss(Mg2+) = 60 s-1. This shows that Mg2+ and ATP can bind and dissociate independently; they do not have to associate or dissociate from cE as a Mg.ATP complex. Calcium-free enzyme binds metal-free ATP at the active site with a dissociation constant of 44 +/- 4 microM, kdiss = 130 +/- 7 s-1, and a calculated association rate constant of 3 x 10(6) M-1 s-1. Calcium-free enzyme that was incubated with [gamma-32P]ATP gave 38% labeled phosphoenzyme when chased with unlabeled ATP, Mg2+, and Ca2+. An increase of the Mg2+ concentration did not increase the amount of E32P formed. This shows that the binding of Mg2+ and ATP to free E also follows a random mechanism. The Mg2+ ion is not buried under ATP, and ATP is not under a Mg2+ ion. Incubation of free E with Mg2+ and ATP causes a conformational change that activates the enzyme for phosphorylation and decreases the rate constant for the dissociation of ATP from kdiss = 120 s-1 to kdiss = 47 s-1.

Modulation by phosphorylation of glycogen phosphorylase-sarcoplasmic reticulum interaction

Glycogen phosphorylase b at concentrations close to those found in skeletal muscle interacts with sarcoplasmic reticulum membranes, but not with liposomes made of lipids extracted from these membranes, and is inhibited upon binding to the membrane. The interaction of glycogen phosphorylase with the sarcoplasmic reticulum membrane is modulated by phosphorylation, for the a form of this enzyme shows a K0.5 of interaction about 10-fold lower than the b form. Upon association to the membrane the fluorescence properties of the coenzyme of glycogen phosphorylase, pyridoxal-5'-phosphate, are strongly altered, for the fluorescence at 535 nm is partially quenched and the fluorescence at 415-420 nm increases. Using fluorescein labeled sarcoplasmic reticulum membranes we have found that the average conformation of the Ca2+ + Mg(2+)-ATPase is also altered on binding of phosphorylase b. In conclusion, the results reported in this paper suggest that glycogen phosphorylase and Ca2+ + Mg(2+)-ATPase directly interact under experimental conditions similar to those found in the sarcoplasm, and that this interaction is modulated by phosphorylation of the phosphorylase.

Distances between functional sites of the Ca2+ + Mg2(+)-ATPase from sarcoplasmic reticulum using Co2+ as a spectroscopic ruler

Cobalt ion inhibits the Ca2+ + Mg2(+)-ATPase activity of sealed sarcoplasmic reticulum vesicles, of solubilized membranes and of the purified enzyme. To use Co2+ appropriately as a spectroscopic ruler to map functional sites of the Ca2+ + Mg2(+)-ATPase, we have carried out studies to obtain the kinetic parameters needed to define the experimental conditions to conduct the fluorimetric studies. 1. The apparent K0.5 values of inhibition of this ATPase are 1.4 mM, 4.8 mM and 9.5 mM total Co2+ at pH 8.0, 7.0 and 6.0, respectively. The inhibition by Co2+ is likely to be due to free Co2+ binding to the enzyme. Millimolar Ca2+ can fully reverse this inhibition, and also reverses the quenching of the fluorescence of fluorescein-labeled sarcoplasmic reticulum membranes due to Co2+ binding to the Ca2+ + Mg2(+)-ATPase. Therefore, we conclude that Co2+ interacts with Ca2+ binding sites. 2. Co2+.ATP can be used as a substrate by this enzyme with Vmax of 2.4 +/- 0.2 mumol ATP hydrolyzed min-1 (mg protein)-1 at 20-22 degrees C and pH 8.0, and with a K0.5 of 0.4-0.5 mM. 3. Co2+ partially quenches, about 10 +/- 2%, the fluorescence of fluorescein-labeled sarcoplasmic reticulum Ca2+ + Mg2(+)-ATPase upon binding to this enzyme at pH 8.0. From the fluorescence data we have estimated an average distance between Co2+ and fluorescein in the ATPase of 1.1-1.8 nm or 1.3-2.1 nm for one or two equidistant Co2+ binding sites, respectively. 4. Co2+.ATP quenches about 20-25% of the fluorescence of fluorescein-labeled Ca2+ + Mg2(+)-ATPase, from which we obtain a distance of 1.1-1.9 nm between Co2+ and fluorescein located at neighbouring catalytic sites.

Modulation of the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by pentobarbital

The dependence of the (Ca2+ + Mg2+)-ATPase activity of sarcoplasmic reticulum vesicles upon the concentration of pentobarbital shows a biphasic pattern. Concentrations of pentobarbital ranging from 2 to 8 mM produce a slight stimulation, approximately 20-30%, of the ATPase activity of sarcoplasmic reticulum vesicles made leaky to Ca2+, whereas pentobarbital concentrations above 10 mM strongly inhibit the activity. The purified ATPase shows a higher sensitivity to pentobarbital, namely 3-4-fold shift towards lower values of the K0.5 value of inhibition by this drug. These effects of pentobarbital are observed over a wide range of ATP concentrations. In addition, this drug shifts the Ca2+ dependence of the (Ca2+ + Mg2+)-ATPase activity towards higher values of free Ca2+ concentrations and increases several-fold the passive permeability to Ca2+ of the sarcoplasmic reticulum membranes. At the concentrations of pentobarbital that inhibit this enzyme in the sarcoplasmic reticulum membrane, pentobarbital does not significantly alter the order parameter of these membranes as monitored with diphenylhexatriene, whereas the temperature of denaturation of the (Ca2+ + Mg2+)-ATPase is decreased by 4-5 C degrees, thus, indicating that the conformation of the ATPase is altered. The effects of pentobarbital on the intensity of the fluorescence of fluorescein-labeled (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum also support the hypothesis of a conformational change in the enzyme induced by millimolar concentrations of this drug. It is concluded that the inhibition of the sarcoplasmic reticulum ATPase by pentobarbital is a consequence of its binding to hydrophobic binding sites in this enzyme.