Electron paramagnetic resonance saturation studies of P-700 reaction center chlorophyll in plant photosynthesis

Electron paramagnetic resonance (EPR) power saturation and saturation recovery methods have been used to determine the spin lattice, T₁, and spin-spin, T₂, relaxation times of P-700⁺ reaction-center chlorophyll in Photosystem I of plant chloroplasts for 10 K T 100 K. T₁ was 200 μs at 100 K and increased to 900 μs at 10 K. T₂ was 40 ns at 40 K and increased to 100 ns at 10 K. T₁ for 40 K T 100 K is inversely proportional to temperature, which is evidence of a direct-lattice relaxation process. At T = 20 K, T₁ deviates from the 1/T dependence, indicating a cross relaxation process with an unidentified paramagnetic species. The individual effects of ascorbate and ferricyanide on T₁ of P-700⁺ were examined: T₁ of P-700⁺ was not affected by adding 10 mM ascorbate to digitonin-treated chloroplast fragments (D144 fragments). The P-700⁺ relaxation time in broken chloroplasts treated with 10 mM ferricyanide was 4-times shorter than in the untreated control at 40 K. Ferricyanide appears to be relaxing the P-700⁺ indirectly to the lattice by a cross-relaxation process. The possibility of dipolar-spin broadening of P-700⁺ due to either the iron-sulfur center A or plastocyanin was examined by determining the spin-packet linewidth for P-700⁺ when center A and plastocyanin were in either the reduced or oxidized states. Neither reduced center A nor oxidized plastocyanin was capable of broadening the spin-packet linewidth of the P-700⁺ signal. The absence of diplolar broadening indicates that both center A and plastocyanin are located at a distance at least 3.0 nm from the P-700⁺ reaction center chlorophyll. This evidence supports previous hypotheses that the electron donor and acceptor to P-700 are situated on opposite sides of the chloroplast membrane. It is also shown that the ratio of photo-oxidized P-700 to photoreduced centers A and B at low temperature is 2 : 1 if P-700 is monitored at a nonsaturating microwave power.     

Modulation of ouabain-insensitive Na(+)-ATPase activity in the renal proximal tubule by Mg(2+), MgATP and furosemide

In addition to the (Na⁺+K⁺)ATPase another P-ATPase, the ouabain-insensitive Na⁺-ATPase has been observed in several tissues. In the present paper, the effects of ligands, such as Mg²⁺, MgATP and furosemide on the Na⁺-ATPase and its modulation by pH were studied in the proximal renal tubule of pig. The principal kinetics parameters of the Na⁺-ATPase at pH 7.0 are: (a) K_0.5 for Na⁺=8.9±2.2 mM; (b) K_0.5 for MgATP=1.8±0.4 mM; (c) two sites for free Mg²⁺: one stimulatory (K_0.5=0.20±0.06 mM) and other inhibitory (I_0.5=1.1±0.4 mM); and (d) I_0.5 for furosemide=1.1±0.2 mM. Acidification of the reaction medium to pH 6.2 decreases the apparent affinity for Na⁺ (K_0.5=19.5±0.4) and MgATP (K_0.5=3.4±0.3 mM) but increases the apparent affinity for furosemide (0.18±0.02 mM) and Mg²⁺ (0.05±0.02 mM). Alkalization of the reaction medium to pH 7.8 decreases the apparent affinity for Na⁺ (K_0.5=18.7±1.5 mM) and furosemide (I_0.5=3.04±0.57 mM) but does not change the apparent affinity to MgATP and Mg²⁺. The data presented in this paper indicate that the modulation of the Na⁺-ATPase by pH is the result of different modifications in several steps of its catalytical cycle. Furthermore, they suggest that changes in the concentration of natural ligands such as Mg²⁺ and MgATP complex may play an important role in the Na⁺-ATPase physiological regulatory mechanisms.     

Gill microsomal (Na+,K+)-ATPase from the blue crab Callinectes danae: Interactions at cationic sites

Euryhaline crustaceans tolerate exposure to a wide range of dilute media, using compensatory, ion regulatory mechanisms. However, data on molecular interactions occurring at cationic sites on the crustacean gill (Na⁺,K⁺)-ATPase, a key enzyme in this hyperosmoregulatory process, are unavailable. We report that Na⁺ binding at the activating site leads to cooperative, heterotropic interactions that are insensitive to K⁺. The binding of K⁺ ions to their high affinity sites displaces Na⁺ ions from their sites. The increase in Na⁺ ion concentrations increases heterotropic interactions with the K⁺ ions, with no changes in K_0.5 for K⁺ ion activation at the extracellular sites. Differently from mammalian (Na⁺,K⁺)-ATPases, that from C. danae exhibits additional NH₄⁺ ion binding sites that synergistically activate the enzyme at saturating concentrations of Na⁺ and K⁺ ions. NH₄⁺ binding is cooperative, and heterotropic NH₄⁺ ion interactions are insensitive to Na⁺ ions, but Na⁺ ions displace NH₄⁺ ions from their sites. NH₄⁺ ions also displace Na⁺ ions from their sites. Mg²⁺ ions modulate enzyme stimulation by NH₄⁺ ions, displacing NH₄⁺ ion from its sites. These interactions may modulate NH₄⁺ ion excretion and Na⁺ ion uptake by the gill epithelium in euryhaline crustaceans that confront hyposmotic media.     

Using simple 13C NMR linewidth and relaxation measurements to make detailed chemical shift assignments in triacylglycerols and related compounds

Two simple experiments measuring the ¹³C linewidths ν_1/2 and spin–lattice relaxation times T₁ of each of the signals in the spectrum of trilinolein indicate that the ν_1/2 and T₁ values are consistent with the different degrees of motional freedom expected for the various ¹³C nuclei. However, for each chain, the ν_1/2 and T₁ measurements indicate a small reversal in mobility at C-10 relative to C-9 before motional freedom again steadily increases on each chain starting at C-11. The T₁ experiment allows unambiguous assignments of the C-8 signal and C-14 signal, which differ by only 0.010 ppm. Measurements of ¹³C ν_1/2 and T₁ values on tripalmitin provide secure assignments for the C-5 and C-6 signals, for which conflicting assignments have been reported. The T₁ measurements also show that among the tightly clustered C-8 through C-12 signals, the C-11 signals are the most downfield, while the C-12 signals are the most upfield, again contrary to a previous report. Similar measurements of ¹³C ν_1/2 and T₁ values on other triacylglycerols or related compounds may prove equally useful in making chemical shift assignments and detecting any discontinuities in motional freedom along a chain. The benefits and possible limitations of ultrahigh field NMR for studying triacylglycerols and related compounds are discussed.     

Simulation analysis of intracellular Na+ and Cl- homeostasis during beta 1-adrenergic stimulation of cardiac myocyte

To quantitatively understand intracellular Na⁺ and Cl⁻ homeostasis as well as roles of Na⁺/K⁺ pump and cystic fibrosis transmembrane conductance regulator Cl⁻ channel (I_CFTR) during the β1-adrenergic stimulation in cardiac myocyte, we constructed a computer model of β1-adrenergic signaling and implemented it into an excitation-contraction coupling model of the guinea-pig ventricular cell, which can reproduce membrane excitation, intracellular ion changes (Na⁺, K⁺, Ca²⁺ and Cl⁻), contraction, cell volume, and oxidative phosphorylation. An application of isoproterenol to the model cell resulted in the shortening of action potential duration (APD) after a transient prolongation, the increases in both Ca²⁺ transient and cell shortening, and the decreases in both Cl⁻ concentration and cell volume. These results are consistent with experimental data. Increasing the density of I_CFTR shortened APD and augmented the peak amplitudes of the L-type Ca²⁺ current (I_CaL) and the Ca²⁺ transient during the β1-adrenergic stimulation. This indirect inotropic effect was elucidated by the increase in the driving force of I_CaL via a decrease in plateau potential. Our model reproduced the experimental data demonstrating the decrease in intracellular Na⁺ during the β-adrenergic stimulation at 0 or 0.5 Hz electrical stimulation. The decrease is attributable to the increase in Na⁺ affinity of Na⁺/K⁺ pump by protein kinase A. However it was predicted that Na⁺ increases at higher beating rate because of larger Na⁺ influx through forward Na⁺/Ca²⁺ exchange. It was demonstrated that dynamic changes in Na⁺ and Cl⁻ fluxes remarkably affect the inotropic action of isoproterenol in the ventricular myocytes.     

Kinetics and equilibria of Ga(III)---thiocyanate complex formation. Mechanism of ligand substitution reactions of Ga(III) in aqueous solution

The kinetics and equilibria of complex formation by Ga(III) with NCS⁻ in aqueous solution have been measured over a range of acidities and temperatures, the contributing paths to the reaction resolved, and their rate constants and activation parameters determined. The hydrolysis equilibria required to carry out this resolution of kinetic behaviour have also been measured.

Unlike the other reported complexation reactions of Ga(III) in aqueous solution, the separate reaction pathways can be assigned with no ambiguity. At 25 °C and ionic strength 0.5 M, the observed forward rate constant for the complex formation is described by {k₁ + k₂K_1h/[H⁺] + k₃K_1hK_2h/[H⁺]²} M⁻¹ s⁻¹. For these conditions, the first and second successive hydrolysis constants of Ga(H₂O)₆³⁺ are given by pK_1h = 3.69 ± 0.01 and pK_2h = 3.74 ± 0.04. The rate constants corresponding to the reactions of the species Ga(H₂O)₆³⁺, Ga(H₂O)₅(OH)²⁺ and Ga(H₂O)₄(OH)₂⁺ with NCS⁻ are k₁ = 57 ± 4 M^{−1 −1}, k₂ = (1.08 ± 0.01) × 10⁵ M⁻¹ s⁻¹ and k₃ = 3 × 10⁶ M⁻¹ s⁻¹ respectively. The complexation equilibrium quotient [GaNCS²⁺]/([Ga³⁺][NCS⁻]) has been independently determined by spectrophotometric titration to be 20.8 ± 0.3 M⁻¹ at 25 °C and ionic strength 0.5 M.

These kinetic results lead to an interpretation of the data, and a reinterpretation of other data for aquo-Ga(III) complex formation kinetics from the literature which support the assignment of a dissociative interchange mechanism for these reactions rather than the associative activation mode sometimes proposed.     

Comparison in halide binding ability between the unsaturated clusters [Pd3(μ-dppm)3(CO)] and [PdPtCo(μ-dppm)2(CO)3(CNBu)] (dppm = Ph2PCH2PPh2)

The trinuclear clusters [Pd₃(μ-dppm)₃(CO)]²⁺ and [PtPdCo(μ-dppm)₂(CO)₃(CN^tBu)]⁺ exhibit a large and a small cavity, respectively, formed by the phenyl rings of the bridging diphosphine ligands. Their binding constants (K₁₁) with halide ions (X⁻) were obtained by UV-Vis spectroscopy. The binding ability varies as I⁻ > Br⁻ > Cl⁻, and [Pd₃(μ-dppm)₃(CO)]²⁺ > [ptPdCo(μ-dppm)₂-(CO)₃(CN^tBu)]⁺. The MO diagram for the related cluster [Pd₂Co(μ-dppm)₂(CO)₄]⁺ has been addressed theoretically in order to predict the nature of the lowest energy electronic bands. For this class of compounds, the lowest energy bands are assigned to charge transfers from the Co center to the Pd₂ centers.     

A solid state NMR study of locust bean gum galactomannan and Konjac glucomannan gels

This paper describes a ¹³C solid state NMR study of hydrated powders and gels of locust bean gum galactomannan-LBG and Konjac glucomannan-KGM. Changes in relative spectral intensities, cross-polarization dynamics (T_CH, T_1ρH) and relaxation times (T_1C, T_1H, T_2H) show that hydration (0–90%) of LBG powders increases the 10⁸ Hz frequency molecular motions, probably reflecting the enhanced motion of non-aggregating segments and chain ends. Slower motions (10⁴–10⁵ Hz) are enhanced only slightly at 90% hydration. LBG gel shows higher spatial distinction between aggregated and non-aggregated segments than the hydrated powder and relaxation times indicate higher mobility for galactose-ramified segments, compared to linear mannose segments. While the dynamics of KGM hydration is similar to that of LBG, i.e. mainly affecting fast 10⁸ Hz motions, the gel is significantly more rigid. Both spectra and relaxation times show that glucose residues in KGM gel are particularly hindered, probably due to their preferential involvement in chain aggregation.     

Equilibrium and structure of thallium(III)–ethylenediamine complexes in pyridine solution and in solid

The formation of three [Tl(en)_n]³⁺ complexes (n=1–3) in a pyridine solvent has been established by means of ²⁰⁵Tl and ¹H NMR. Their stepwise stability constants based on concentrations, K_n=[Tl(en)_n ³⁺]/{[Tl(en)_n−1 ³⁺]·[en]}, at 298 K in 0.5 M NaClO₄ ionic medium in pyridine, were calculated from ²⁰⁵Tl NMR integrals: log K₁=7.6±0.7; log K₂=5.2±0.5 and log K₃=2.64±0.05. Linear correlation between both the ²⁰⁵Tl NMR shifts and spin–spin coupling ²⁰⁵Tl–¹H versus the stability constants has been found and discussed. A single crystal with the composition [Tl(en)₃](ClO₄)₃ was synthesized and its structure determined by X-ray diffraction. The Tl³⁺ ion is coordinated by three ethylenediamine ligands via six N-donor atoms in a distorted octahedral fashion.     

磁化微咸水灌溉对欧美杨I-107离子稳态的影响

为探究微咸水磁化处理条件下植株的离子稳态特征,以欧美杨I-107一年生扦插苗为试材,于生长季节分别采用Hoagland营养液和4.0 g·L^-1 NaCl微咸水,经磁化处理后连续灌溉30 d.采用原子吸收分光光度法对叶片和根系中K⁺、Na⁺、Ca²⁺和Mg²⁺含量进行测定,分析离子平衡系数(K)和根-叶之间的离子选择性运输系数(S_Xi,Na).结果表明: 与非盐分胁迫处理相比,盐分胁迫处理根系和叶片中Na⁺和Ca²⁺含量及S_K,Na、S_Mg,Na升高,K⁺和Mg²⁺含量、K⁺/Na⁺及S_Ca,Na降低.与非磁化微咸水灌溉处理相比,磁化微咸水灌溉处理的根系和叶片中Na⁺含量降低、K⁺含量及K⁺/Na⁺提高;根系和叶片中Ca²⁺含量降低、Mg²⁺含量提高;磁化微咸水灌溉处理中K提高,且叶片中K值显著高于根系;S_K,Na和S_Mg,Na较非磁化微咸水灌溉提高,S_Ca,Na较其降低.磁化微咸水灌溉中根系和叶片Na⁺积累量减少,K⁺、Ca²⁺和Mg²⁺含量增加,且维持了较高水平的K⁺/Na⁺,这有利于植株整株水平生理代谢的调控.因此,盐分胁迫下磁化作用可通过调节离子的选择性吸收和运输来维持植株体内的离子平衡.     

13C and 1H solid state NMR investigation of hydration effects on gluten dynamics

The effect of hydration on the molecular dynamics of soft wheat gluten was investigated by solid state NMR. For this purpose, we recorded static and MAS ¹H spectra and SPE, CP, and other selective ¹³C spectra under MAS and dipolar decoupling conditions on samples of dry and H₂O and D₂O hydrated gluten. Measurements of carbon-proton CP times and several relaxation times (proton T₁, T_1ρ and T₂, and carbon T₁) were also performed. The combination of these techniques allowed both site-specific and domain-averaged motional information to be obtained in different characteristic frequency ranges. Domains with different structural and dynamic behaviour were identified and the changes induced by hydration on the dynamics of different domains could be monitored. The proton spin diffusion process was exploited to get information on the degree of mixing among different gluten domains. The results are consistent with the “loop and train” model proposed for hydrated gluten.     

Ins(1,4,5)P3 induces Ca2+ release from brain microsomes loaded either by the Ca2+ ATPase or by the Na+/Ca2+ exchanger.

In this study we investigated the release of Ca²⁺ in brain microsomes after Ca²⁺ loading by the Ca²⁺-ATPase or by the Na⁺/Ca²⁺ exchanger. The results show that in microsomes loaded with Ca²⁺ by the Ca²⁺-ATPase, Ins(1,4,5)P₃ (5 μM) release 21±2% of the total Ca²⁺ accumulated, and that in the microsomes loaded with Ca²⁺ by the Na²⁺/Ca²⁺ exchanger, Ins(1,4,5)P₃ released 28±3% of the total Ca²⁺ accumulated. These results suggest that receptors of Ins(1,4,5)P₃ may be co-localized with the Na²⁺/Ca²⁺ exchanger in the endoplasmic reticulum membrane or that there are Ins(1,4,5)P₃ receptors in the plasma membrane where the Na²⁺/Ca²⁺ exchanger is normally present, or both. We also found that Ins(1,4,5)P₃ inhibited the Ca²⁺-ATPase by 33.7%, but that it had no significant effect on the Na²⁺/Ca²⁺ exchanger.     

Uptake of corticosterone into isolated rat liver cells: Possible involvement of Na/K-ATPase

Isolated rat hepatocytes posses a saturable glucocorticoid uptake system with high affinity (K_d value = 2.8 ± 0.7 × 10⁻⁸ M; 318,000 ± 80,000 binding sites per cell; 317 fmol/mg protein). The initial rates of uptake decrease by about 30–40% if the cells are incubated simultaneously with [³H]corticosterone and either SH-reagents (N-ethylmaleimide and p-chloromercuriphenylsulphonate, 1 mM), metabolic inhibitors (2,4-dinitrophenol, 1 mM; and antimycin, 0.1 mM) or the Na⁺/K⁺-ATPase-inhibitors, ouabain and quercetine. These Na⁺/K⁺-ATPase-blockers exert half-maximal inhibition at 3 × 10⁻⁷ and 3 × 10⁻⁶ M, respectively. A slight increase in K⁺ concentration and a corresponding decrease in Na⁺ in the medium leads to a significant reduction in the initial uptake rate. The uptake system from the rat hepatocytes shows a clear steroid specificity, being different from the intracellular receptor. Corticosterone and progesterone are the strongest competitors, cortisol, 5- and 5β-dihydrocorticosterone, 11-deoxycorticosterone, cortisone and testosterone have an intermediate effect and only weak competition is exerted by dexamethasone and by the mineralocorticoid, aldosterone. Estradiol and estrone sulphate as well as the synthetic glucocorticoid triamcinolone acetonide are unable to inhibit initial corticosterone uptake.     

Stimulation of platelet serotonin and Rb uptake by N-ethylmaleimide

The effects of N-ethylmaleimide (NEM) on mouse platelet serotonin (5-HT) and ⁸⁶Rb⁺ uptake were studied. The 5-HT transport system showed a biphasic response to increasing concentrations of NEM, with low concentrations (25–50 μM) stimulating and high concentrations (200–400 μM) inhibiting 5-HT transport. Fluoxetine, an inhibitor of the platelet 5-HT transporter, blocked NEM-induced stimulation of 5-HT transport. The kinetics of 5-HT uptake indicated that NEM (50 μM) markedly increased the maximal rate of 5-HT transport (V_max control = 28.4±1.4 pmol/10⁸ platelets/4 min vs V_max NEM = 64.5±9.5 pmol/10⁸ platelets/4 min but had no significant effect on the K_m value. Platelet Na⁺ K⁺ ATPase activity was determined by measuring ⁸⁶Rb⁺ uptake. Platelet ⁸⁶Rb⁺ uptake showed a biphasic response to NEM, with low concentrations (25–100 μM) significantly stimulating and high concentrations (400 μM) inhibiting uptake. These changes in platelet ⁸⁶Rb⁺ uptake paralleled the biphasic changes in 5-HT transport. In the presence of fluoxetine, 5-HT transport was markedly inhibited but no change in the ability of NEM to stimulate ⁸⁶Rb⁺ uptake was observed. These data suggest that low concentrations of NEM activate plasma membrane Na⁺ K⁺ ATPase which results in a marked stimulation of platelet 5-HT transport.     

The distance between cytochromes a and a3 in the azide compound of bovine-heart cytochrome oxidase

The electron-spin relaxation rates of the two species of cytochrome a³⁺₃-azide found in the azide compound of bovine-heart cytochrome oxidase were measured by progressive microwave saturation at T = 10 K. It has been shown previously that Cyt a⁺³₃-azide gives rise to two distinct EPR resonances, depending upon the oxidation state of Cyt a. When Cyt a is ferrous, Cyt a³⁺₃-azide has g = 2.88, 2.19 and 1.64; upon oxidation of Cyt a, the a³⁺₃-azide g-values become g = 2.77, 2.18, and 1.74 (Goodman, G. (1984) J. Biol. Chem. 259, 15094–15099). The relaxation effect of Cyt a on Cyt a₃ could be measured as the difference in microwave field saturation parameter H_1/2 between the g = 2.77 and g = 2.88 species. For each signal the spin-lattice relaxation time T₁ was determined from H_1/2 using the transverse relaxation time T₂. The value of T₂ at 10 K was extrapolated from a plot of line-width vs. temperature at higher temperature. The dipolar contribution to T₁ was related to the Cyt a-Cyt a₃ spin-spin distance utilizing available information on the relative orientation of Cyt a₃-azide and Cyt a (Erecinska, M., Wilson, D.F. and Blasie, J.K. (1979) Biochim. Biophys. Acta 545, 352–364). By taking into account the relaxation parameters for both g_x and g_z components of the Cyt a₃-azide g-tensor, the angle between the g_z components of the Cyt a and Cyt a₃g-tensors was determined to be between 0 and 18°, and the Cyt a-Cyt a₃ spin-spin distance was found to be 19 ± 8 Å.     

The sodium cycle. II. Na-coupled oxidative phosphorylation in Vibrio alginolyticus cells

The role of Na⁺ in Vibrio alginolyticus oxidative phosphorylation has been studied. It has been found that the addition of a respiratory substrate, lactate, to bacterial cells exhausted in endogenous pools of substrates and ATP has a strong stimulating effect on oxygen consumption and ATP synthesis. Phosphorylation is found to be sensitive to anaerobiosis as well as to HQNO, an agent inhibiting the Na⁺-motive respiratory chain of V. alginolyticus. Na⁺ loaded cells incubated in a K⁺ or Li⁺ medium fail to synthesize ATP in response to lactate addition. The addition of Na⁺ at a concentration comparable to that inside the cell is shown to abolish the inhibiting effect of the high intracellular Na⁺ level. Neither lactate oxidation nor Δω generation coupled with this oxidation is increased by external Na⁺ in the Na⁺-loaded cells. It is concluded that oxidative ATP synthesis in V. alginolyticus cells is inhibited by the artificially imposed reverse ΔPNa, i.e., [Na⁺]_in > [Na⁺]_out. Oxidative phosphorylation is resistant to a protonophorous uncoupler (0.1 mM CCCP) in the K⁺-loaded cells incubated in a high Na⁺ medium, i.e., when ΔpNa of the proper direction ([Na⁺]_in < [Na⁺]_out) is present. The addition of monensin in the presence of CCCP completely arrests the ATP synthesis. Monensin without CCCP is ineffective. Oxidative phosphorylation in the same cells incubated in a high K⁺ medium (ΔpNa is low) is decreased by CCCP even without monensin. Artificial formation of ΔpNa by adding 0.25 M NaCl to the K⁺-loaded cells (Na⁺ pulse) results in a temporary increase in the ATP level which spontaneously decreases again within a few minutes. Na⁺ pulse-induced ATP synthesis is completely abolished by monensin and is resistant to CCCP, valinomycin and HQNO. 0.05 M NaCl increases the ATP level only slightly. Thus, V. alginolyticus cells at alkaline pH represent the first example of an oxidative phosphorylation system which uses Na⁺ instead of H⁺ as the coupling ion.     

Extracellular Na removal enhances granule secretion in platelets 3-evidence that Na/H exchange is inhibitory to secretion induced by some agonists

The effect of extracellular Na⁺ ([Na⁺]_e) removal on agonist-induced granule secretion in platelets in relation to [ph]_i and [Ca²⁺]_i changes was investigated. Substitution of [Na⁺]_e with choline⁺ of K⁺ resulted in a significant enhancement of 5HT secretion induced by thrombin, collagen, U46619 and the protein kinase C activators, PMA and diC₈. Increases in [Ca²⁺]_i induced by thrombin and U46619 were slightly inhibited or unaffected in these buffers, but [pH]_i increases induced by thrombin, U46619, PMA and diC₈ were abolished and a drop in [pH]_i (0.05–0.1 units below resting) was observed. Although preincubation with potassium acetate produced a big drop in [pH]_i and greatly increased secretion with all the agonists, particularly in the absence of [Na⁺]_e, clear evidence that [pH]_i rises due to Na⁺/H⁺ exchange are inhibitory to secretion was obtained only with thrombin. Thus, (i) NH₄Cl, which restored the increase in [pH]_i in the absence of [Na⁺]_e reduced the potentiated secretory response to thrombin, (ii) no increase in thrombin-induced secretion was observed when Na⁺ was replaced with Li⁺, which allowed a normal increase in [pH]_i and (iii) ethyl isopropyl amiloride (EIPA) abolished the [pH]_i rise and potentiated thrombin-induced secretion. With collagen and U46619, the results suggest that removal of [Na⁺]_e per se rather than inhibition of Na⁺/H⁺ exchange results in enhanced secretion. It is concluded that [Na⁺]_e per se and [pH]_i elevations via Na⁺/H⁺ exchange both have important inhibitory roles in the control of platelet granule secretion.     

钙对盐胁迫下冰叶日中花不同器官离子含量和根部K~+、Na~+吸收的影响

以冰叶日中花(Mesembryanthemum crystallinum L.)实生苗为材料,经NaCl、NaCl+ CaCl_2、NaCl+LaCl_3处理后,利用电感耦合等离子发射光谱仪检测叶、茎、根中Na~+、K~+、Ca~(2+)、Mg~(2+)含量,计算K~+/Na~+、Ca~(2+)/Na~+和Mg~(2+)/Na~+比值,利用非损伤微测技术测定根尖Na~+流和K~+流,研究盐胁迫下钙在维持离子平衡中的作用。结果显示,NaCl处理后,冰叶日中花各器官中Na~+含量增加,K~+、Ca~(2+)、Mg~(2+)含量降低,离子比值降低;CaCl_2处理降低了Na~+含量,提高了K~+、Ca~(2+)、Mg~(2+)含量,离子比值升高,而LaCl_3处理后的结果相反。经NaCl处理24 h后,冰叶日中花根尖Na~+和K~+明显外流,加入CaCl_2后,Na~+外流速度显著增加,K~+外流速度受到抑制,而加入LaCl_3后则降低了Na~+的外流速度,促进了K~+的外流。研究结果表明冰叶日中花受到盐胁迫后,钙参与了促进根部Na~+外排、抑制K~+外流的过程,进而保持各器官中较低的Na~+含量,表明钙在维持和调控离子平衡中起到重要作用。     

The effect of ionic strength on melting of DNA modified by platinum(II) complexes

Thermal denaturation of calf thymus DNA modified by antitumor cis-diamminedichloroplatinum(II) (cis-DDP) and by two related Pt(II) compounds which had been shown to be clinically inefective, viz. trans-diamminedichloroplatinum(II) (trans-DDP) or monodentate diethylenetriaminechloroplatinum(II) chloride {[Pt(dien)Cl)]Cl}, was studied by monitoring changes of absorbance at 260 nm. The melting of DNA platinated to different levels was investigated in neutral media containing varying concentrations of Na⁺. It has been shown that the ionic strength has a strong influence on the character and magnitude of changes in the melting temperature of DNA (T_m) induced by the platination. The modification of DNA by either platinum complex used in this work results in an increase of T_m if DNA melting is measured in media containing low Na⁺ concentrations (ca. 1 mM). This effect is reversed at higher Na⁺ concentrations. The concentration of Na⁺ at which this reversal occurs is, however, markedly lower for DNA modified by cis-DDP than for DNA modified by the other two platinum complexes. These results have been iterpreted to mean that at least three factors affect the thermal stability of DNA modified by the platinum(II) complexes: stabilization effects of the positive charge on the platinum moiety and of interstrand cross-links, and a destabilization effect of conformational distortions in DNA. Thus, in order to compare and interpret the melting behavior of DNA modified by different compounds, a great attention has to be paid to the composition of the medium in which the melting experiments are carried out.     

Characterization of a new leiurotoxin I-like scorpion toxin: PO5 from Androctonus mauretanicus mauretanicus

Three novel peptide inhibitors of the SK_Ca channels were purified to homogeneity from the venom of the scorpion Androctonus mauretanicus mauretanicus using one step of RP-HPLC and competition assays with [¹²⁵I]apamin to rat brain synaptosomes. PO_i, PO₂ and PO₅ have K_0.5 of 100,100 and 0.02 nM, respectively, for the apamin binding site. The sequence of PO₅ was established and compared to that of other scorpion toxins active on K⁺ channels: it contains 31 residues and has a free carboxyl end. It shares sequence similarity with apamin and leiurotoxin I.