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1.
The divalent cation requirements of NOS activity in bovine retina homogenate supernatant were investigated. Supernatants were assayed under standard conditions (in mM: EDTA 0.45, Ca 2+ 0.25, Mg 2+ 4.0). In order to investigate the enzyme's dependence on divalent cations, the tissue homogenate was depleted of di- and trivalent cations by passing it over a cation-exchange column (Chelex 100). Surprisingly, NOS activity was 50-100% higher in this preparation. However, addition of either EDTA (33 M) or EGTA (1 mM) almost fully inhibited NOS activity, suggesting a requirement for residual divalent metal cation(s). Phenanthroline or iminodiacetic acid at low concentrations had little effect on activity, suggesting no requirement for Fe 2+, Zn 2+ or Cu 2+. Ca 2+ had a moderate stimulatory effect, with an optimum activity around 0.01 mM. Mg 2+ or Mn 2+ had little effect at concentrations < 0.25 mM. However, in the presence of EDTA, Mn 2+ or Ca 2+ markedly stimulated NOS activity with the optimum at 0.1 mM. At high concentrations (> 0.1-0.2 mM), all divalent cations tested (Ba 2+, Zn 2+, Co 2+, Mn 2+, Mg 2+, Ca 2+), as well as La 3+, dose-dependently inhibited NOS activity. We propose that retinal NOS requires low concentrations of naturally occurring divalent metal ions, most probably Ca 2+, for optimal activity and is inhibited by high di- and trivalent metal concentrations, probably by competition with Ca 2+. 相似文献
2.
The aim of the present study was to investigate the effects of environmental pollutants, such as heavy metals and pesticides on ion transport across the skin of the leech ( Hirudo medicinalis). We wanted to examine the suitability of this epithelium as a model system for studies concerning the mechanisms of toxic action caused by environmental pollutants. For this purpose we performed Ussing chamber experiments to test three representative heavy metals and pesticides, respectively, for their effects on current flow across leech dorsal integument. Two representatives of each substance class showed distinct effects on ion transport across this epithelium. The heavy metal ions Pb 2+ and Hg 2+ produced a significant inhibition of amiloride-sensitive Na + transport across leech skin in concentrations below or close to their limiting values in waste water. Therefore, it seems feasible to use leech skin for future investigations of the toxic actions of these heavy metals. The fact that Pb 2+ and Hg 2+ exerted their effects only when applied apically points to a specific action of these divalent cations on ion channels in the apical membrane. However, this inhibition does not seem to be a general feature of divalent cations because Cd 2+ did not influence ion transport across leech skin at all. Since current flow through amiloride-sensitive Na + channels in typical vertebrate tight epithelia is stimulated by numerous divalent cations, the pronounced inhibition of amiloride-sensitive Na + channels in leech skin by Pb 2+ and Hg 2+ might lead to a further differentiation of amiloride-sensitive Na + channels. The two widespread pesticides lindane and promecarb exerted their effects only at comparativ high concentrations. This low sensitivity restricts the usefulness of leech skin as a subject for further analysis of toxicity mechanisms, at least for these two pesticides. 相似文献
3.
Aluminum chloride (AlCl 3), a neurotoxic compound, inhibited ATP diphosphohydrolase activity of synaptosomes obtained from cerebral cortex of adult
rats. The metal ion significantly inhibited ATPase and ADPase activities of the enzyme at all concentrations tested in vitro
(0.01, 0.05, 0.5, 5 and 10 m M) in the presence of 1.5 m M calcium. When tested in the absence of Ca 2+, and with increasing amounts of Al 3+, enzyme activity remained below basal levels, suggesting that the trivalent cation Al 3+ is not a substitute for the divalent cation Ca 2+ in ATP-Ca 2+ and ADP-Ca 2+ complexes. The Al 3+ inhibition was competitive with respect to Ca 2+. The enzyme inhibition was reversed by the addition of deferoxamine (DFO). NaF significantly inhibited ATP diphosphohydrolase
activity, and this inhibition was reversed by the addition of Ca 2+ to the medium. Such inhibition was not potentiated by AlF 4, which is an inhibitor of cation-transport ATPases. 相似文献
4.
The inositol monophosphatase (IMPase) enzyme from the hyperthermophilic archaeon Methanocaldococcus jannaschii requires Mg 2+ for activity and binds three to four ions tightly in the absence of ligands: KD = 0.8 μM for one ion with a KD of 38 μM for the other Mg 2+ ions. However, the enzyme requires 5–10 m M Mg 2+ for optimum catalysis, suggesting substrate alters the metal ion affinity. In crystal structures of this archaeal IMPase with products, one of the three metal ions is coordinated by only one protein contact, Asp38. The importance of this and three other acidic residues in a mobile loop that approaches the active site was probed with mutational studies. Only D38A exhibited an increased kinetic KD for Mg 2+; D26A, E39A, and E41A showed no significant change in the Mg 2+ requirement for optimal activity. D38A also showed an increased Km, but little effect on kcat. This behavior is consistent with this side chain coordinating the third metal ion in the substrate complex, but with sufficient flexibility in the loop such that other acidic residues could position the Mg 2+ in the active site in the absence of Asp38. While lithium ion inhibition of the archaeal IMPase is very poor (IC 50~250 m M), the D38A enzyme has a dramatically enhanced sensitivity to Li + with an IC 50 of 12 m M. These results constitute additional evidence for three metal ion assisted catalysis with substrate and product binding reducing affinity of the third necessary metal ion. They also suggest a specific mode of action for lithium inhibition in the IMPase superfamily. 相似文献
5.
Summary 1. Whole-cell patch clamp experiments were performed on rat dorsal root ganglion (DRG) neurons to investigate the actions of various combinations of Pb 2+, Zn 2+, and Al 3+ on voltage-activated calcium channel currents (VACCCs).2. Each of these metals has been shown to reduce VACCCs.3. We investigated the effects of simultaneous application of two cations in the range of their IC 50 values. For all possible combinations (Pb 2+/Zu 2+, Zn 2+/Al 3+, Al 3+/Pb 2+), independent of the order of application, we found additive actions on VACCCs.4. We observed a 75% (±9%) block of the control current when two cations were applied simultaneously. This observation is consistent with both, an action of two metals at the same site as well as independent actions at different locations of the ion channel.5. The additivity of the effects should be taken into account for questions of public health and the assessment of threshold limits in cases of environmental contamination. 相似文献
6.
猪红细胞膜Ca~(2+)-ATP酶是一种钙调蛋白(CaM)依赖酶,其活力又依赖巯基的完整性。实验应用Ca~(2+)-ATP酶这一模型体系观察到重金属离子,Pb~(2+)、Cd~(2+)和Hg~(2+)都能替代Ca~(2+),激活CaM,从而激活Ca~(2+)-ATP酶;其最大刺激活力分别为85%、80%和30%,半刺激浓度分别为32、27和0.7μmol/L。当三种重金属离子的浓度增加时,则与Ca~(2+)-ATP酶的巯基结合,抑制酶的活力,Pb2~(2+)、Cd~(2+)和Hg~(2+)的半抑制浓度分别为370、440和2μmol/L。抑制作用为渐进性过程,而刺激作用为即时效应。抑制作用可为巯基化物,特别是二巯基化物所逆转。研究结果提示,CaM可能是重金属中毒最初作用的靶分子,而重金属中毒不仅使CaM“开关”失灵,还可能导致细胞内Ca~(2+)的调节全面失控。 相似文献
7.
The structure of the D254,256E double mutant of Arthrobacter xylose isomerase with Al 3+ at both metal-binding sites was determined by the molecular replacement method at a conventional R-factor of 0.179. Binding of the two Al 3+ does not alter the overall structure significantly. However, there are local rearrangements in the octahedral co-ordination sphere of the Al 3+. The inhibitor molecule moves somewhat away from the active site. Furthermore, evidence was revealed for metal ion movement from site 2 1 to site 2 2 upon double mutation. Xylose isomerase requires two divalent metal cations for activation. The catalytic metal ion is translocated 1.8 Å away from its initial position during the catalytic reaction. The fact that both activating and inactivating metals (including Al 3+) were found exclusively at a single location in the double mutant was an indication that the consequently missing shuttle may account for the crippled catalytic efficiency. 相似文献
8.
Summary 1. We examined the actions of mercury (Hg 2+) and zinc (Zn 2+) on voltage-activated calcium channel currents of cultured rat dorsal root ganglion (DRG) neurons, using the whole-cell patch clamp technique.2. Micromolar concentrations of both cations reduced voltage-activated calcium channel currents. Calcium channel currents elicited by voltage jumps from a holding potential of –80 to 0 mV (mainly L- and N-currents) were reduced by Hg 2+ and Zn 2+. The threshold concentration for Hg 2+ effects was 0.1 µ M and that for Zn 2+ was 10 µM. Voltage-activated calcium channel currents were abolished (>80%) with 5 µM Hg 2+ or 200 µM Zn 2+. The peak calcium current was reduced to 50% (IC 50) by 1.1 µM Hg 2+ or 69 µM Zn 2+. While Zn 2+ was much more effective in reducing the T-type calcium channel current—activated by jumping from –80 to –35 mV—Hg 2+ showed some increased effectiveness in reducing this current.3. The effects of both cations occurred rapidly and a steady state was reached within 1–3 min. While the action of Zn 2+ was not dependent on an open channel state, Hg 2+ effects depended partially on channel activation.4. While both metal cations reduced the calcium channel currents over the whole voltage range, some charge screening effects were detected with Hg 2+ and with higher concentrations (>100 µM) of Zn 2+.5. As Zn 2+ in the concentration range used had no influence on resting membrane currents, Hg 2+ caused a clear inward current at concentrations µM.6. In the present study we discuss whether the actions of both metals on voltage-activated calcium channel currents are mediated through the same binding site and how they may be related to their neurotoxic effects. 相似文献
9.
We present the results of molecular dynamics simulations for alkali earth metal cations (Mg 2+, Ca 2+, Sr 2+ and Ba 2+) in an aqueous solution at 25°C using the extended simple point charge water potential with Ewald summation. The ion mobilities (defined by ui = DiziF/ RT) obtained from the simulations are in good accord with the experimental measures. The strong divalent cation–water interactions explain well the static and dynamic properties of the alkali earth metal cations. The classical solvent-berg picture describes the ion mobilities of these cations in water adequately. 相似文献
10.
Herein, we identify the coordination environment of Cu 2+ in the human α1-glycine receptor (GlyR). GlyRs are members of the pentameric ligand-gated ion channel superfamily (pLGIC) that mediate fast signaling at synapses. Metal ions like Zn 2+ and Cu 2+ significantly modulate the activity of pLGICs, and metal ion coordination is essential for proper physiological postsynaptic inhibition by GlyR in vivo. Zn 2+ can either potentiate or inhibit GlyR activity depending on its concentration, while Cu 2+ is inhibitory. To better understand the molecular basis of the inhibitory effect we have used electron spin resonance to directly examine Cu 2+ coordination and stoichiometry. We show that Cu 2+ has one binding site per α1 subunit, and that five Cu 2+ can be coordinated per GlyR. Cu 2+ binds to E192 and H215 in each subunit of GlyR with a 40 μM apparent dissociation constant, consistent with earlier functional measurements. However, the coordination site does not include several residues of the agonist/antagonist binding site that were previously suggested to have roles in Cu 2+ coordination by functional measurements. Intriguingly, the E192/H215 site has been proposed as the potentiating Zn 2+ site. The opposing modulatory actions of these cations at a shared binding site highlight the sensitive allosteric nature of GlyR. 相似文献
11.
Summary The toxicity of chromium and tin on growth, photosynthetic carbon-fixation, oxygen evolution, heterocyst differentiation and nitrogenase activity of Anabaena doliolum and its interaction with bivalent cations has been studied. Some interacting cations, viz. Ca 2+, Mg 2+ and Mn 2+, substantially antagonised the toxic effects of chromium and tin with reference to growth, heterocyst differentiation and nitrogenase activity in the following hierarchal sequence: Ca 2+ > Mg 2+ > Mn 2+. However, the sequence of hierarchy was Mg 2+ > Ca 2+ > Mn 2+ for carbon fixation and Mn 2+ > Mg 2+ > Ca 2+ for photosynthetic oxygen evolution. Synergistically inhibitory patterns were noticed for all the parameters, viz. growth, 14CO 2 uptake, oxygen evolution, heterocyst differentiation and nitrogenase activity of A. doliolum when Ni 2+, Co 2+ and Zn 2+ were combined with the test metals in the growth medium. These cations followed the following sequence of synergistic inhibition: Ni 2+ > Co 2+ > Zn 2+. Among all the interacting cations, Ca 2+, Mg 2+ and Mn 2+ exhibited antagonistic effects which relieved the test cyanobacterium from metal toxicity. In contrast to this, Ni 2+, CO 2+ and Zn 2+ showed synergistic inhibition which potentiating the toxicity of test metals in the N 2-fixing cyanobacterium A. doliolum. It is evident from the present study that bivalent cations, viz. Ca 2+, Mg 2+, Mn 2+, Ni 2+, Co 2+ and Zn 2+, may appreciably regulate the toxicity of heavy metals in N 2-fixing cyanobacteria if present in aquatic media. 相似文献
12.
1. Brief interruption of spinal cord blood flow resulting from transient abdominal aortic occlusion may lead to degeneration of specific spinal cord neurons and to irreversible loss of neurological function. The alteration of nitric oxide/nitric oxide synthase (NO/NOS) pool occurring after ischemic insult may play a protective or destructive role in neuronal survival of affected spinal cord segments.2. In the present study, the spatiotemporal changes of NOS following transient ischemia were evaluated by investigating neuronal NOS immunoreactivity (nNOS-IR), reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry, and calcium-dependent NOS (cNOS) conversion of [ 3H] l-arginine to [ 3H] l-citrulline.3. The greatest levels of these enzymes and activities were detected in the dorsal horn, which appeared to be most resistant to ischemia. In that area, the first significant increase in NADPHd staining and cNOS catalytic activity was found immediately after a 15-min ischemic insult.4. Increases in the ventral horn were observed later (i.e., after a 24-h reperfusion period). While the most intense increase in nNOS-IR was detected in surviving motoneurons of animals with a shorter ischemic insult (13 min), the greatest increase of cNOS catalytic activity and NADPHd staining of the endothelial cells was found after stronger insult (15 min).5. Given that the highest levels of nNOS, NADPHd, and cNOS were found in the ischemia-resistant dorsal horn, and nNOS-IR in surviving motoneurons, it is possible that NO production may play a neuroprotective role in ischemic/reperfusion injury. 相似文献
13.
Summary The divalent metal ion binding site and binding constant of ribonuclease HI from Escherichia coli were investigated by observing chemical shift changes using 1H– 15N heteronuclear NMR. Chemical shift changes were monitored during the titration of the enzyme with salts of the divalent cations. The enzyme was uniformly labeled by 15N, which facilitated the monitoring of the chemical shift change of each cross peak between the backbone amide proton and the amide 15N. The chemical shifts of several amide groups were affected upon the addition of a divalent metal ion: Mg 2+, Ca 2+, or Ba 2+. These amide groups resided close to the active site, consistent with the previous X-ray crystallographic studies. From the titration analysis, a single divalent ion binding site was observed with a weak binding constant (K D=2–4 mM for the current divalent ions). 相似文献
14.
Arginine kinase is closely associated with adaptation to environmental stresses such as high salinity and heavy metal ion levels in marine invertebrates. In this study, the effects of Cd 2+ on the cuttlefish Sepia pharaonis’ arginine kinase (SPAK) were investigated. SPAK was isolated from the muscles of S. pharaonis and upon further purification, showed a single band on SDS-PAGE. Cd 2+ effectively inactivated SPAK, and the double-reciprocal kinetics indicated that Cd 2+ induced non-competitive inhibition of arginine and ATP. Spectrofluorometry results showed that Cd 2+ induced tertiary structure changes in SPAK with the exposure of hydrophobic surfaces that directly induced SPAK aggregation. The addition of osmolytes, glycine, and proline successfully blocked SPAK aggregation and restored the conformation and activity of SPAK. Molecular dynamics simulations involving SPAK and Cd 2+ showed that Cd 2+ partly blocks the entrance of ATP to the active site, and this result is consistent with the experimental results showing Cd 2+-induced inactivation of SPAK. These results demonstrate the effect of Cd 2+ on SPAK enzymatic function and unfolding, including aggregation and the protective effects of osmolytes on SPAK folding. This study provides concrete evidence of the toxicity of Cd 2+ in the context of the metabolic enzyme SPAK, and it illustrates the toxic effects of heavy metals and detoxification mechanisms in cuttlefish. 相似文献
16.
It is well known that the activity of calcineurin (CaN) could be modulated by several transitional metal ions. In the present
work, the effects of a calcium analog, lanthanum ion (La 3+), on the activity of CaN were studied. It was found that La 3+ exerted multiple effects on CaN activity. La 3+ could stimulate CaN in the absence of calmodulin (CaM); whereas at low concentrations of La 3+, there was a slight inhibition of activation of CaN in the presence of CaM. Competitive experiments and limited trypsin proteolysis
confirmed that La 3+ did not act on the catalytic core of CaN, but exerted its effect through direct action on the CaN regulatory domain similar
to Mg 2+. In activity titration and spot blotting studies, La 3+-containing CaM complexes were less effective in stimulating CaN than Ca 2+ or Mn 2+-containing CaM; however, the binding affinity of these metal–CaM complexes to CaN was similar. These effects of La 3+ on CaN activity are unique among metal ions and may provide clues to understand the biological effects of La 3+. 相似文献
17.
内质网应激激活的未折叠蛋白反应(Unfolded protein response,UPR)途径在酿酒酵母和哺乳动物细胞中是非常保守的。内质网(Endoplasmic reticulum,ER)是蛋白质合成、折叠和修饰的细胞器,也是贮存钙的主要场所之一。酵母细胞内质网钙平衡与UPR的作用是相互的;两个MAPK途径——HOG途径和CWI途径都是细胞应答内质网应激压力时生存所必需的;重金属镉离子能够激活UPR途径,它通过激活钙离子通道Cch1/Mid1进入细胞影响钙离子的功能。本文结合最新研究进展对酿酒酵母细胞中的两个MAPK途径、镉离子和钙离子稳态与内质网应激激活的UPR途径之间相互关系进行综述。 相似文献
18.
The degree of inhibition of respiration-dependent vs respiration-independent Ca 2+ binding by rat liver mitochondria by different members of the lanthanide family was used to establish the existence of two different classes of Ca 2+ binding sites. The distinction is based on the differences in cation:site interactions between the two classes of sites and the members of the lanthanide series. Lanthanide inhibition of respiration-dependent Ca 2+ uptake suggests that the binding site is specific for the calcium ion. Those members of the lanthanide family whose ionic radii are nearer that of Ca 2+ are the best inhibitors. The inhibition of respiration-independent Ca 2+ binding is much different, indicating non-specific cation absorption. 相似文献
19.
Fe(II) cations bind with high efficiency and specificity at the high-affinity (HA), Mn-binding site (termed the “blocking effect” since Fe blocks further electron donation to the site) of the oxygen-evolving complex (OEC) in Mn-depleted, photosystem II (PSII) membrane fragments (Semin et al. in Biochemistry 41:5854, 2002). Furthermore, Fe(II) cations can substitute for 1 or 2Mn cations (pH dependent) in Ca-depleted PSII membranes (Semin et al. in Journal of Bioenergetics and Biomembranes 48:227, 2016; Semin et al. in Journal of Photochemistry and Photobiology B 178:192, 2018). In the current study, we examined the effect of Ca2+ cations on the interaction of Fe(II) ions with Mn-depleted [PSII(-Mn)] and Ca-depleted [PSII(-Ca)] photosystem II membranes. We found that Ca2+ cations (about 50 mM) inhibit the light-dependent oxidation of Fe(II) (5 µM) by about 25% in PSII(-Mn) membranes, whereas inhibition of the blocking process is greater at about 40%. Blocking of the HA site by Fe cations also decreases the rate of charge recombination between QA? and YZ?+ from t1/2?=?30 ms to 46 ms. However, Ca2+ does not affect the rate during the blocking process. An Fe(II) cation (20 µM) replaces 1Mn cation in the Mn4CaO5 catalytic cluster of PSII(-Ca) membranes at pH 5.7 but 2 Mn cations at pH 6.5. In the presence of Ca2+ (10 mM) during the substitution process, Fe(II) is not able to extract Mn at pH 5.7 and extracts only 1Mn at pH 6.5 (instead of two without Ca2+). Measurements of fluorescence induction kinetics support these observations. Inhibition of Mn substitution with Fe(II) cations in the OEC only occurs with Ca2+ and Sr2+ cations, which are also able to restore oxygen evolution in PSII(-Ca) samples. Nonactive cations like La3+, Ni2+, Cd2+, and Mg2+ have no influence on the replacement of Mn with Fe. These results show that the location and/or ligand composition of one Mn cation in the Mn4CaO5 cluster is strongly affected by calcium depletion or rebinding and that bound calcium affects the redox potential of the extractable Mn4 cation in the OEC, making it resistant to reduction. 相似文献
20.
The interactions between oligonucleotides and inorganic cations have been measured by capillary zone electrophoresis. With
increasing concentrations of divalent cations (Ca 2+, Mg 2+, Mn 2+ and Ni 2+) in the running buffer, the migration behavior was evaluated by calculation of the binding constants. Besides these fundamental
studies of binding equilibria, different buffer components, tris(hydroxymethyl)aminomethane and 3-( N-morpholino)propanesulfonic acid, have been investigated and their effects on metal ion binding quantified. 相似文献
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