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PubMed1983

Divalent cation binding to wheat germ calmodulin.

The Ca2+ binding to plant (wheat germ) calmodulin was measured in 0.1 M NaCl by a flow-dialysis method. The four macroscopic binding constants best fitted to the data were 0.20, 0.25, 0.025, and 0.0024 microM-1. The cysteine residue of this calmodulin is located at the 27th position from the NH2-terminal (Yazawa, M. et al. (1982) Abstr. 33th Conf. Protein Structure pp. 9-12, Osaka). According to the quantitative analysis of the reaction of 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) with Cys 27, the calmodulin which binds 3 Ca2+ showed the minimum reactivity with DTNB. This suggests that the site for the third Ca2+ binding is located close to Cys 27. Cys 27 was spin-labeled with N-(2,2,6,6-tetramethyl-4-piperidine-1-oxyl)maleimide, and its ESR spectrum was measured in the presence of Mn2+ and/or Ca2+. The rotational relaxation time of the label (1.2 ns) was increased by about one-tenth with 1 to 2 mol of bound Ca2+, but was unchanged with Mn2+. On the other hand, Mn2+ induced a remarkable quenching of the spectrum. From the decrease in the peak heights of the ESR spectrum, the distance between the label and the first bound Mn2+ was estimated to be 0.8 nm. It is concluded that the first Mn2+ binds to a domain near the NH2-terminal. The difference UV absorption spectrum induced by Mn2+ was similar to that induced by Ca2+. However, the amount of Mn2+ needed to saturate the difference spectrum was 1 mol more than the amount of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites↗
PubMed1984

Modulation of phospholipase A2 activity associated with human sperm membranes by divalent cations and calcium antagonists.

Phospholipase A2 activity in sonicates and acid extracts of ejaculated, washed human sperm was measured using [1-14C] oleate-labeled autoclaved E. coli and 1-[1-14C] stearoyl-2-acyl-3-sn- glycerophosphorylethanolamine as substrates. Phospholipase A was optimally active at pH 7.5, was calcium-dependent, and exclusively catalyzed the release of fatty acid from the 2-position of phospholipids. The activity was membrane-associated, and was solubilized by extraction with 0.18 N H2SO4. Acid extracts of human sperm had the highest specific activity (1709 nmols /h per mg), followed by mouse, rabbit and bull, which were 105, 36 and 1.7 nmols /h per mg, respectively. para-bromophenacyl bromide inhibited human sperm phospholipase A2 activity, but mepacrine was without effect. In the presence of 1.0 mM added CaCl2, phospholipase A2 activity was inhibited by Zn2+ and Mn2+; whereas Cu2+, Cd2+, Mg2+, or Sr2+ had no effect. Zn2+ stimulated activity at low concentrations (10(-6) to 10(-8) M), and inhibited activity in a dose-dependent manner at concentrations of 10(-5) M. The extent of stimulation by low concentrations of Zn2+ was dependent on Ca2+ concentration; at 10(-7) M, Zn2+ activity was stimulated 160% with 0.5 mM CaCl2, and only 120% with 1.0 mM CaCl2. At low concentrations (10(-5) to 10(-7) M), methoxyverapamil (D600) and trifluoperazine stimulated human sperm phospholipase A2 activity, and trifluoperazine but not D600 produced almost complete inhibition between 10(-5) and 10(-4) M of the drug. The significance of human sperm phospholipase A2 activity and its modulation by Ca2+, Zn2+ and Mn2+ in the sperm acrosome reaction is discussed.

Acrosome↗
PubMed1994

Kainic acid stimulates uptake of divalent cations in postnatal cochlear neurons.

Excitatory amino acids were tested for their ability to induce cobalt uptake in cochlear explants of spiral ganglion neurons. Explants were taken from postnatal day 5 hamsters. Compared with NMDA and quisqualate, kainic acid (KA) stimulated the greatest amount of Co2+ uptake within spiral ganglion neurons as indicated by precipitation of the Co2+ accumulation and relative optical density measurements. Both L-glutamate and KA stimulation were blocked by non-NMDA receptor antagonists kynurenate and DNQX. Nifedipine, a dihydropyridine-sensitive calcium channel blocker, also partially inhibited Co2+ uptake. These data suggest that a mixed population of NMDA and non-NMDA receptors are present. Additionally non-NMDA receptors may have a role in cochlear synaptic maturation.

Animals↗
PubMed1999

Divalent cation selectivity of the subtypes of low voltage-activated Ca2+ channels in thalamic neurons.

LVA Ca2+ current in isolated associative neurons from the laterodorsal thalamic nucleus of 14- to 17-day-old rats was dissected into two, 'fast' and 'slow', components based on the difference in the kinetics of inactivation. The selectivity of the channel responsible for the fast LVA current for Ca2+, Sr2+ and Ba2+ (I(Ca):I(Sr):I(Ba) = 1.0:1.23:0.94) as well as the shifts of the I-V produced by these ions were found to be almost identical to those observed for LVA channels in other preparations. The channel responsible for the slow LVA current showed selectivity more characteristic of HVA Ca2+ channels (I(Ca):I(Sr):I(Ba) = 1.0:2.5:3.4), although the ability of Ca2+, Sr2+ and Ba2+ to shift its voltage dependence remained the same as for the fast channel.

Animals↗
PubMed1991

Effects of some inorganic divalent cations and protein kinase C inhibitors on endothelium-dependent relaxation in rat isolated aorta and mesenteric arteries.

The effects of nitrendipine and several metal ions possessing Ca2+ antagonistic activity were examined on acetylcholine (ACh) and histamine-induced endothelium-dependent relaxations in norepinephrine (NE)-precontracted rat aortic rings and perfused mesenteric arteries. Nitrendipine (1 nM) profoundly attenuated ACh and histamine-induced relaxations of perfused mesenteric arteries but was ineffective against either agonist in aorta. The transition metal ions Co2+, Mn2+, and Ni2+, but not the nontransition ions (Cd2+, Sn2+, and Zn2+), markedly inhibited ACh and histamine relaxations in the aorta, whereas all metal ions antagonized KCl contractions. At the highest concentration devoid of effect on arterial perfusion pressure, none of the transition metal ions altered endothelium-dependent relaxations in the mesenteric arteries. Endothelium-independent relaxations induced by sodium nitroprusside (SNP) were attenuated by Mn2+ but not by Co2+ or Ni2+. Calmidazolium or W-7 inhibited ACh- and histamine-induced relaxations in both aorta and mesenteric arteries, whereas staurosporine and H-7 were ineffective against aortic relaxations; in mesenteric arteries, staurosporine but not H-7 attenuated both endothelium-dependent and -independent relaxations. We conclude (a) that the transition metal ions most likely inhibit endothelium-derived relaxing factor (EDRF) (NO) release in the aorta through endothelial receptor-operated Ca2+ channels; (b) that the effects of nitrendipine (shared by nifedipine) in mesenteric arteries result from an interaction with a site that may have structural similarities with, but is distinct from, the L-type Ca2+ channel; and (c) that the inhibitory effects of the calmodulin antagonists may reflect an action on endothelial NO synthase.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1975

Voltage-dependent changes in the permeability of nerve membranes to calcium and other divalent cations.

Transmitter release from depolarized nerve terminals seems to be preceded by a rise in the intracellular concentration of ionized calcium. In squid giant axons, depolarization promotes calcium entry by two routes: one that is blocked by tetrodotoxin and one that is insensitive to tetrodotoxin. The TTX-sensitive route seems to be the sodium channel of the action potential; but the TTX-insensitive route seems to be quite distinct from the sodium and potassium channels of the action potential. It is blocked by Mg-2+, Mn-2+ and Co-2+ ions and by the organic calcium antagonist D-600 and has many features in common with the mechanism that couples excitation to secretion.

Action Potentials↗
PubMed1992

Nucleotides and divalent cations as effectors and modulators of exocytosis in permeabilized rat mast cells.

The idea that the universal trigger to exocytosis (the terminal step in the secretory process) is an elevation of the cytosol concentration of Ca2+, and that it is dependent on ATP, is no longer tenable. Working with streptolysin-O-permeabilized mast cells (and other myeloid cells) we have shown that non-hydrolysable analogues of GTP can stimulate exocytosis after depletion of Ca2+ (i.e. at concentrations below 10(-9) M) and ATP. Such Ca2+- and ATP-independent exocytosis is strongly dependent on the presence of Mg2+, and the requirement for Mg2+ declines as the concentration of Ca2+ is brought up to 10(-7) M. We argue that Ca2+ serves to regulate the binding of guanine nucleotides to GE, a GTP-binding protein that regulates exocytosis through its interaction with CE, a calcium-binding protein which serves as an intracellular pseudo-receptor. The onset of exocytosis, following provision of Ca2+ and guanine nucleotides to the permeabilized cells, is preceded by delays which are sensitive to the order of provision of the two effectors (i.e. Ca2+ and guanine nucleotides), the presence or absence of Mg2+, and the identity of the activating guanine nucleotide. In view of the similarity of these features with the activation kinetics of adenylyl cyclase, we argue that GE behaves as a member of the heterotrimeric class of signal transducing G-proteins such as GS.

Animals↗
PubMed1985

Solution conformations of valinomycin-divalent cation complexes.

The solution conformations of complexes of valinomycin with magnesium and strontium were investigated by circular dichroism, nuclear magnetic resonance and infrared techniques. The results were compared with our earlier results on lithium, calcium, manganese and barium complexes. All these cations, except lithium, form 2:1 ion sandwich and 1:1 carrier-cation complexes with valinomycin. The 1:1 complex has a conformation different from that of the valinomycin-potassium complex. Lithium forms only the 1:1 complex. Strontium and barium form a large number of 1:2 complexes with open conformations rapidly interconverting in solution in addition to the above 2:1 and 1:1 complexes. These observations are rationalized taking into account the ionic radii and coordination numbers of the cations and the conformational restraints of valinomycin molecules. It is suggested that cations with co-ordination numbers of about six (magnesium and calcium) form the 2:1 and 1:1 complexes whereas those with higher co-ordination numbers (strontium and barium) form 1:2 complexes also.

Cations, Divalent↗
PubMed1989

Effect of divalent cations on structure-function relationships of the antitumor protein alpha-sarcin.

alpha-Sarcin binds one Zn(II) cation per protein molecule, with a Kd value of 0.9 mM, determined by equilibrium dialysis experiments. Ca(II), Mg(II), and Mn(II) do not bind to alpha-sarcin. Cd(II) and Co(II) also behave as Zn(II). The binding produces local modifications on the protein conformation affecting the microenvironment of tryptophan residues. The three cations modify the fluorescence emission of the protein. The near-u.v. circular dichroism spectrum of the protein is also altered. The binding of Zn(II) and related cations does not modify the secondary structure of the protein. The ribonucleolytic activity of alpha-sarcin is inhibited upon Zn(II) binding, but no alteration of the ability of the protein to aggregate phospholipid vesicles has been observed.

Antineoplastic Agents↗