Factors affecting the transport of β-amino acids in rat renal brush-border membrane vesicles. The role of external chloride

The effect of a variety of ions and other solutes on the accumulation of the β-amino acid, taurine, was examined in rat renal brush-border membrane vesicles. Initial taurine uptake (15 and 30 s) is sodium-dependent with a typical overshoot. This Na⁺ effect was confirmed by exchange diffusion and gramicidin inhibition of taurine uptake. External K⁺ or Li⁺ do not increase taurine accumulation more than Na⁺-free mannitol, except that the combination of external K⁺ and Na¹ in the presence of nigericin enhances uptake. Of all anions tested, including more permeant (SCN⁻ and NO₃⁻) or less permeant (SO₄²⁻), chloride supported taurine accumulation to a significantly greater degree. Preloading vesicles with choline chloride reduced taurine uptake, suggesting that external Cl⁻ stimulates uptake. Since this choline effect could be related to volume change, due to the slow diffusion of choline into vesicles, brush-border membrane vesicles were pre-incubated with LiCl, LiNO₃ and LiSO₄. Internal LiCl, regardless of the final Na⁺ anion mixture, reduced initial rate (15 and 60 s) and peak (360 s) taurine uptake. Internal LiNO₃ or LiSO₄ with external NaCl resulted in similar or higher values of uptake at 15, 60 and 360 s, indicating a role for external Cl⁻ in taurine uptake in addition to Na⁺ effect. Although uptake by vesicles is greatest at pH 8.0 and inhibited at acidic pH values (pH less than 7.0), an externally directed H⁺ gradient does not influence uptake. Similarly, amiloride, an inhibitor of the Na⁺/H⁺ antiporter, had no influence on taurine accumulation over a wide variety of concentrations or at low Na⁺ concentrations. Taurine uptake is blocked only by other β-amino acids and in a competitive fashion. d-glucose and p-aminohippurate at high concentrations (> 10⁻³ M) reduce taurine uptake, possibly by competing for sodium ions, although gramicidin added in the presence of d-glucose inhibits taurine uptake even further. These studies more clearly define the nature of the renal β-amino acid transport system in brush-border vesicles and indicate a role for external Cl in this uptake system.     

Conformational changes in coupling factor 1 may control the rate of electron flow in spinach chloroplasts.

The relationship between the rate of electron flow, internal H⁺ concentration and the magnitude of the H⁺ concentration gradient (ΔpH) in chloroplasts illuminated at various light intensities has been examined. At an external pH of 7.0, the internal H⁺ concentration is a linear function of the rate of electron flow except at saturating light intensity. In contrast, at pH 8.1, this relationship between electron flow and internal H⁺ concentration holds only at values of ΔpH below about 2.8 – 2.9 units. At higher ΔpH values, the rate of electron flow increases much more dramatically than the internal H⁺ concentration. ATP (0.1 mM) prevents this increase. It is suggested that at pH 8.1 but not at pH 7.0, the conformation of coupling factor 1 is altered at high ΔpH values. Its altered conformation may result in an increased efflux of H⁺ from the chloroplasts. This notion is supported by the effects of ATP on electron flow and ΔpH as well as the effect of external pH and light intensity on the reactivity of coupling factor 1 to N-ethylmaleimide.     

Thermodynamics and coordination characteristics of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 extraction complex

The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)₂·(H₃O⁺·χH₂O)₂·UO₂Cl₄²⁻ (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (K_ex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log K_ex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol⁻¹. The value of the entropy change, ΔS°, was calculated from ΔH° and K_ex and equals −690 J K⁻¹ mol⁻¹, whereas ΔG° = −6.45 kJ mol⁻¹. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K⁻¹ mol⁻¹, ΔH° = −101 kJ mol⁻¹ and ΔG° = −8.37 kJ mol⁻¹), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H₅O₂⁺ ions in it, in contrast to the H₃O⁺ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H₅O₂⁺ ions is held in the crown ether cavity by four hydrogen bonds. The H₅O₂⁺ ion has a central bond. The uranium atom forms UO₂Cl₄²⁻ as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above.     

Primary charge separation in native and plant pheophytin a-modified reaction centers of Chloroflexus aurantiacus: Ultrafast transient absorption measurements at low temperature

Ultrafast transient absorption (TA) spectroscopy was used to study electron transfer (ET) at 100 K in native (as isolated) reaction centers (RCs) of the green filamentous photosynthetic bacterium Chloroflexus (Cfl.) aurantiacus. The rise and decay of the 1028 nm anion absorption band of the monomeric bacteriochlorophyll a molecule at the B_A binding site were monitored as indicators of the formation and decay of the P⁺B_A⁻ state, respectively (P is the primary electron donor, a dimer of bacteriochlorophyll a molecules). Global analysis of the TA data indicated the presence of at least two populations of the P^⁎ excited state, which decay by distinct means, forming the state P⁺H_A⁻ (H_A is a photochemically active bacteriopheophytin a molecule). In one population (~65 %), P^⁎ decays in ~2 ps with the formation of P⁺H_A⁻ via a short-lived P⁺B_A⁻ intermediate in a two-step ET process P^⁎ → P⁺B_A⁻→ P⁺H_A⁻. In another population (~35 %), P^⁎ decays in ~20 ps to form P⁺H_A⁻ via a superexchange mechanism without producing measurable amounts of P⁺B_A⁻. Similar TA measurements performed on chemically modified RCs of Cfl. aurantiacus containing plant pheophytin a at the H_A binding site also showed the presence of two P^⁎ populations (~2 and ~20 ps), with P^⁎ decaying through P⁺B_A⁻ only in the ~2 ps population. At 100 K, the quantum yield of primary charge separation in native RCs is determined to be close to unity. The results are discussed in terms of involving a one-step P^⁎ → P⁺H_A⁻ superexchange process as an alternative highly efficient ET pathway in Cfl. aurantiacus RCs.     

Kinetics and mechanism of the decomposition of μ-amido-μ-hydroxo(tetraamminecobalt(III))(tetraaquacobalt(III)) ion in acidic aqueous solution

The title complex undergoes decomposition in acidic aqueous solution resulting in equimolar concentration of aquapentaamminecobalt(III) and hexa- aquacobalt(II). The kinetic studies over the ranges of 0.048 M ⩽ [H⁺] ⩽ 0.385 M, 25 ⩽ θ_c ⩽ 41.5°C and at I = 0.5 M reveals that the intricate mechanism involves protonation equilibrium of the title complex, followed by a rate determining bridge cleavage. The further follow-up reaction is a fast electron transfer process to form products. The rate expression derived from the mechanism is k_obs = k₁K₁[H⁺]/(1 + K₁[H⁺]) where the values of k, and K, are found to be 8.9 × 10⁻⁴ s⁻¹ and 3.5 M⁻¹ respectively at 25 °C. The results are compared with that obtained for the decomposition reactions of mononuclear aquaammine complexes of cobalt(III).     

Primary electron transfer in Rhodobacter sphaeroides R-26 reaction centers under dehydration conditions

The photoinduced charge separation in Q_B-depleted reaction centers (RCs) from Rhodobacter sphaeroides R-26 in solid air-dried and vacuum-dried (~10⁻² Torr) films, obtained in the presence of detergent n-dodecyl-β-D-maltoside (DM), is characterized using ultrafast transient absorption spectroscopy. It is shown that drying of RC-DM complexes is accompanied by reversible blue shifts of the ground-state absorption bands of the pigment ensemble, which suggest that no dehydration-induced structural destruction of RCs occurs in both types of films. In air-dried films, electron transfer from the excited primary electron donor P^⁎ to the photoactive bacteriopheophytin H_A proceeds in 4.7 ps to form the P⁺H_A⁻ state with essentially 100% yield. P⁺H_A⁻ decays in 260 ps both by electron transfer to the primary quinone Q_A to give the state P⁺Q_A⁻ (87% yield) and by charge recombination to the ground state (13% yield). In vacuum-dried films, P^⁎ decay is characterized by two kinetic components with time constants of 4.1 and 46 ps in a proportion of ~55%/45%, and P⁺H_A⁻ decays about 2-fold slower (462 ps) than in air-dried films. Deactivation of both P^⁎ and P⁺H_A⁻ to the ground state effectively competes with the corresponding forward electron-transfer reactions in vacuum-dried RCs, reducing the yield of P⁺Q_A⁻ to 68%. The results are compared with the data obtained for fully hydrated RCs in solution and are discussed in terms of the presence in the RC complexes of different water molecules, the removal/displacement of which affects spectral properties of pigment cofactors and rates and yields of the electron-transfer reactions.     

Characteristics of light-induced H+ transport in spinach chloroplast at lower temperatures I. Relationship between H+ transport and physical changes of the microenvironment in chloroplast membranes

Light-induced H⁺ transport of spinach chloroplasts was investigatedin the temperature range from 5° to 30°C with a glasselectrode. The rate of H⁺ transport was reduced by lowering the temperature.Addition of 1 µM phenazine methosulfate (PMS) considerablystimulated the H⁺ uptake in chloroplasts. PMS was also effectivein stimulating the H⁺ efflux when the illumination was turnedoff. The latter effect became more marked at lower temperatures.These results indicate that electron transfer reactions in thechloroplast not only drive the forward process of H⁺ gradientformation, but also participate in the backward H⁺ efflux. The Arrhenius plot applied to the first-order rate constantof the H⁺ efflux showed a discontinuity at about 20°C. Nohysteresis was detected with the temperature dependence andits discontinuity in the H⁺ transport. On the other hand, theaddition of PMS abolished the discontinuity and a linear relationshipwas observed in the Arrhenius plot. Probably, temperature-dependentphysical changes in the microenvironment of the chloroplastlamellae are responsible for determining the characteristicsof the H⁺transport. (Received September 11, 1975; )     

Rotation of a single swollen thylakoid vesicle in a rotating electric field. Electrical properties of the photosynthetic membrane and their modification by ionophores,lipophilic ions and pH

Rotation of single swollen thylakoid vesicles (‘blebs’) was induced by means of a rotating electric field of strength 104 V · cm⁻¹, inducing a membrane voltage of 72 mV peak. Within the range of medium conductives described (40–300 μS · cm⁻¹), measurement of the field frequency (2–100 kHz) giving maximum rotation rate is equivalent to measuring the electrical time constant of the bleb membrane. Hence the membrane capacity (specific capacitance) was determined, and the value found at pH 8.1 (0.93 ± 0.07 μF · cm⁻²) is in agreement with values deduced from measurements using other techniques. However, the capacity was also found to decreased with pH: a minimum value of 0.77 ± 0.01 μF · cm⁻² was measured at pH 4.4. The present study was extended to measurements of the effects of the lipid-soluble anion of dipicrylamine on the membrane capacity. At pH 7.2 and dipicrylamine concentration of 1.0 μM, a minimum estimate of the apparent membrane capacity was found to be 2.0 ± 0.2 μF · cm⁻², with 2.6 ± 0.2 μF · cm⁻² being observed at 5.0 μM concentration. In addition, it was found possible to measure the membrane resistivity (specific resistance) in the presence of either gramicidin (1.0 to 10 nM) or valinomycin (1.0 to 10 μM). In the case of gramicidin, it was possible to derive a maximum estimate of the mean channel conductance, and this agrees very well with the values for individual, single channels that may be deduced from artificial bilayer work. Unless the gramicidin channels in blebs are in fact substantially more conductive than in artificial bilayers, this indicates that a high percentage of the added gramicidin forms channels which are open for most of the time. In the case of valinomycin, a much greater amount had to be added to produce the same reduction of membrane resistivity as seen with a given concentration of gramicidin. However, calculations indicate that the majority of this effect is due to the difference in partioning behaviour of the two ionophores.     

Ion pathways in renal brush border membranes

The absorbance change of the weak base dye probe, Acridine orange, was used to monitor alterations of pH gradients across renal brush border membrane vesicles. The presence of Na⁺/H⁺ or Li⁺/H⁺ exchange was demonstrated by diluting Na₂SO₄ or Li₂SO₄ loaded vesicles into Na⁺- or Li⁺-free solutions, which caused dye uptake. About 20% of the uptake was abolished by lipid permeable cations such as valinomycin-K⁺ or tetraphenylphosphonium, indicating perhaps the presence of a finite Na⁺ conductance smaller than electroneutral Na⁺/H⁺ exchange. The protonophore tetrachlorosalicylanilide raised the rate of dye uptake under these conditions, hence the presence of an Na⁺ conductance greater than the H⁺ conductance was suggested. K⁺ gradients also induced changes of pH, at about 10% of the Na⁺ or Li⁺ rate. Partial inhibition (21%) was seen with 0.1 mM amiloride indicating that K⁺ was a low affinity substrate for the Na⁺/H⁺ exchange. Acceleration both by tetrachlorosalicylanilide (2-fold) and valinomycin (4-fold) suggested the presence of 2 classes of vesicles, those with high and those with low K⁺ conductance. The larger magnitude of the valinomycin dependent signal suggested that 75% of the vesicles had a low K⁺ conductance. Inward Cl^? gradients also induced acidification, partially inhibited by the presence of tetraphenylphosphonium, and accelerated by tetrachlorosalicylanilide. Thus both a Cl^? conductance greater than the H⁺ conductance and a Cl^?/OH^? exchange were present. The rate of Na⁺/H⁺ exchange was amiloride sensitive with a pH optimum of 6.5 and an apparent K_m for Na⁺ or Li⁺ of about 10 mM and an E_A of 14.3 kcal per mol. A 61-fold Na₂SO₄ gradient resulted in a pH gradient of 1.64 units which increased to 1.8 with gramicidin. An equivalent NaCl gradient gave a much lower ΔpH even in the presence of gramicidin showing that the H⁺ and Cl^? pathways could alter the effects of the Na⁺/H⁺ exchange.     

Interaction kinetics of liposome-incorporated unsaturated fatty acids with fatty acid-binding protein 3 by surface plasmon resonance

The role of heart-type fatty acid-binding protein (FABP3) in human physiology as an intracellular carrier of fatty acids (FAs) has been well-documented. In this study, we aimed to develop an analytical method to study real-time interaction kinetics between FABP3 immobilized on the sensor surface and unsaturated C18 FAs using surface plasmon resonance (SPR). To establish the conditions for SPR experiments, we used an FABP3-selective inhibitor 4-(2-(1-(4-bromophenyl)-5-phenyl-1H-pyrazol-3-yl)-phenoxy)-butyric acid. The affinity index thus obtained was comparable to that reported previously, further supporting the usefulness of the SPR-based approach for evaluating interactions between FABPs and hydrophobic ligands. A pseudo-first-order affinity of FABP3 to K⁺ petroselinate (C18:1 Δ6 cis), K⁺ elaidate (C18:1 Δ9 trans), and K⁺ oleate (C18:1 Δ9 cis) was characterized by the dissociation constant (K_d) near micromolar ranges, whereas K⁺ linoleate (C18:2 Δ9,12 cis/cis) and K⁺ α-linolenate (C18:3 Δ9,12,15 cis/cis/cis) showed a higher affinity to FABP3 with K_d around 1 × 10⁻⁶ M. Interactions between FAPB3 and C18 FAs incorporated in large unilamellar vesicles consisting of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and FAs (5:1 molar ratio) were also analysed. Control DMPC liposomes without FA showed only marginal binding to FABP3 immobilized on a sensor chip while liposome-incorporated FA revealed significant responses in sensorgrams, demonstrating that the affinity of FAs to FABP3 could be evaluated by using the liposome-incorporated analytes. Significant affinity to FABP3 was observed for monounsaturated fatty acids (K_d in the range of 1 × 10⁻⁷ M). These experiments demonstrated that highly hydrophobic compounds in a liposome-incorporated form could be subjected to SPR experiments for kinetic analysis.     

Synthetic and mechanistic studies of the addition of 2,6-dimethylaniline to tricarbonyl (1–5-η-dienyl) iron(II) complexes (dienyl=C6H7 or C7H9)

Detailed synthetic and mechanistic studies of the addition of 2, 6-dimethylaniline to the organometallic complexes [Fe(CO)₃(1–5-η-dienyl)]BF₄ (1, dienyl=C₆H₇ or C₇H₉) indicates the general rate law k_obs=k_a [2,6-(Me)₂C₆H₃NH₂]+k_b which is consistent with an equilibrium process. The greater reactivity of the C₆H₇ complex and the low ΔH_a3 and large negative ΔS_a3 values are in accordance with direct addition of the amine to the dienyl rings of 1. On the other hand the relatively much higher ΔH_b 3 values are consistent with bond cleavage in dissociation as is the positive ΔS_b 3 value of +220 J K⁻¹ mol⁻¹ determined for the C₆H₇ reaction. The negative ΔS_b3 value of −43 J K⁻¹ mol⁻¹ found for the C₇H₉ reaction suggests the presence of an ordered transition state through which the starting dienyl complex is reformed via some internal S_N2 process.     

Potential difference responses to secretory K+, Cl− and Na+ changes in secreting and resting states of frog stomach

The effects of changes in secretory concentrations of K⁺, Cl⁻ and Na⁺ on transmembrane potential difference (PD) and resistance were compared for secreting fundus and resting fundus of Rana pipiens. In the resting fundus experiments histamine was present, and SCN and omeprazole gave similar results. Increase of K⁺ from 4 to 80 mM, decrease of Cl⁻ from 160 to 16 mM and decrease of Na⁺ from 156 to 15.6 mM gave, respectively, 10 min after the change, in the secreting fundus ΔPD = 7.6, 10.0 and −2.2 mV and in the resting fundus ΔPD = 4.3, 14.4 and 0 mV. With cimetidine and no histamine, increase of K⁺ from 4 to 80 mM gave a ΔPD which decreased to near zero after exposure to cimetidine for at least 30 min. For the same K⁺ change, replacement of cimetidine with SCN or omeprazole and without histamine maintained ΔPD near zero and subsequent addition of histamine with inhibitor present gave a ΔPD of about 12 mV. The change in ΔPD was attributed to histamine increasing the secretory membrane area, which results in an increase in K⁺ conductance. Increase in ΔPD in the resting fundus compared to the secreting fundus for a decrease from 160 to 16 mM Cl⁻ may be due to relatively little Cl⁻ entering the lumina from cells in the resting fundus, which would result in a greater change of the ratio intracellular Cl⁻/luminal Cl⁻ in the resting fundus than in the secreting fundus for the decrease in Cl⁻ studied.     

Contractile and relaxant effects of dimaprit on guinea pig isolated intestinal cells

The effect of the histamine H₂ receptor agonist dimaprit on intestinal contractility was characterized on smooth muscle cells isolated from the longitudinal muscle of the guinea pig ileum. Dimaprit exerted two opposite effects on the contractility of isolated muscle cells: relaxation of cholecystokinin octapeptide (CCK-S)-induced contractions in the range of concentrations 10⁻¹⁷-10⁻¹³ M and contraction at concentrations higher than 10⁻¹³ M. The relaxant effect of dimaprit was totally prevented by the H₂ blocker famotidine (10⁻⁷ M), which, at the same time, enhanced the contractile effect of dimaprit, shifting to the left the concentration-response curve to the agonist. This contraction was not modified by the histamine H₁ receptor antagonists pyrilamine and temelastine, tested both at 10⁻⁷ M. By contrast, atropine 10⁻⁸ M abolished the contractile effect of dimaprit, while leaving unchanged the response to CCK-8. Our results clearly indicate that longitudinal muscle cells of the guinea pig ileum possess inhibitory H₂ receptors, which can be activated by very low concentrations of dimaprit; moreover, they revealed that dimaprit can have non-histaminergic effects, probably due to muscarinic receptor activation; however, concentrations about 10000 times higher than those necessary to activate H₂ receptors, are required.     

Stabilities,solubility, and kinetics and mechanism for formation and hydrolysis of some palladium(II)and platinum(II) iodo complexes in aqueous solution

Complex formation between Pd(II), Pt(II) and iodide has been studied at 25 °C for an aqueous 1.00 M perchloric acid medium. Measurements of the solubility of PdI₂(s) in aqueous mercury(II) perchlorate and of AgI(s) and PdI₂(s) in aqueous solutions of Pd²⁺(aq) and Ag⁺(aq) gave the solubility product of PdI₂(s) as K_s^o=(7±3) × 10⁻³² M³, which is much smaller than previous literature values.The stability constants β₁=[MI(H₂O)₃⁺]/([M(H₂O)₄²⁺][I⁻]) for the two systems were obtained as the ratio between rate constants for the forward and reverse reactions of (i).

The following values of k₁ (s⁻¹ M⁻¹), k₋₁ (s⁻¹) and β₁ (M⁻¹) were obtained at 25 °C: (1.14±0.11) × 10⁶, (0.92±0.18), (12±4) × 10⁵ for MPd, and (7.7±0.4), (8.0±0.7) × 10⁻⁵, (9.6±1.3) × 10⁴ for MPt. Combination with previous literature data gives the following values of log(β₁ (M⁻¹)) to log(β₄ (M⁻⁴)): 6.08, ∼22, 25.8 and 28.3 for MPd, and 4.98, ∼25, ∼28, and ∼30 for MPt. The present results show that the large overall stability constants β₄ observed for the M²⁺I⁻ systems are most likely due to a very large stability of the second complex MI₂(H₂O)₂, which is probably a cis-isomer. A distinct plateau in the formation curve for mean ligand number 2 is obtained both for MPd and Pt. The other iodo complexes are not especially stable compared to those of chloride and bromide.ΔH^≠ (kJ mol⁻¹) and ΔS^≠ (JK⁻¹ mol⁻¹) for the forward reaction of (i), MPd, are (17.3±1.7) and (−71±5), and for the reverse reaction of (i) MPd, (45±3) and (−95±6), respectively. The kinetics are compatible with associative activation (I_a). The contribution from bond-breaking in the formation of the transition state seems to be less important for Pd than for Pt.     

Metagenomic cloning and characterization of Na+/H+ antiporter genes taken from sediments in Chaerhan Salt Lake in China

A metagenomic library containing 8,000 clones was constructed by using genomic DNA obtained from Chaerhan Salt Lake in northwest China. Three Na⁺/H⁺ antiporters, C4-NhaG, C47-NhaG and C49-NhaG that grouped to the NhaG family, were screened and cloned from this metagenome by complementing Escherichia coli strain KNabc (ΔnhaA ΔnhaB ΔchaA) in medium containing 0.2 M NaCl. The three putative Na⁺/H⁺ antiporters were membrane proteins with 10, 11 and 11 transmembrane segments, respectively. They enabled E. coli KNabc to grow in medium containing 0.2–0.6 M Na⁺ or 7–14 mM Li⁺. Everted membrane vesicles prepared from E. coli KNabc cells carrying C49-NhaG exhibited Na⁺/H⁺ and Li⁺/H⁺ antiport activities.     

Chloroplast-generated ROS dominate NaCl- induced K+ efflux in wheat leaf mesophyll

Mesophyll K⁺ retention ability has been recently reported as an important component of salinity stress tolerance in wheat. In order to investigate the role of ROS in regulating NaCl^-induced K⁺ efflux in wheat leaf mesophyll, a series of pharmacological experiments was conducted using MV (methyl viologen, superoxide radical inducer), DPI (an inhibitor of NADPH oxidase), H₂O₂ (to mimic apoplastic ROS), and EGCG ((−)-Epigallocatechin gallate, ROS scavenger). Mesophyll pre-treatment with 10 μM MV resulted in a significantly higher NaCl^-induced K⁺ efflux in leaf mesophyll, while 50 μM EGCG pre-treatment alleviated K⁺ leakage under salt stress. No significant change in NaCl^-induced K⁺ efflux in leaf mesophyll was found in specimens pre-treated by H₂O₂ and DPI, compared with the control. The highest NaCl^-induced H⁺ efflux in leaf mesophyll was also found in samples pre-treated with MV, suggesting a futile cycle between increased H⁺-ATPase activity and ROS-induced K⁺ leak. Overall, it is suggested that, under saline stress, K⁺ efflux from wheat mesophyll is mediated predominantly by non-selective cation channels (NSCC) regulated by ROS produced in chloroplasts, at least in bread wheat.     

Oxidation of (hydroxyethyl)ferrocene by [2-pyridylmethylbis(2-ethylthioethyl)/amine]copper(II)

A kinetic study of the oxidation of (hydroxyethyl)ferrocene (HEF) by [2-pyridylmethylbis(2-ethyl-thioethyl)ainine]copper(II) (Cu(pmas)²⁺) is reported, with the objective of documenting the influence of the two thioether sulfur ligands on the electron transfer rate. Both reactants exhibit a first-order dependence at pH 6, I = 0.1 M(NaNO₃); k(25°C) = 1.3 × 10⁴M⁻¹sec⁻¹, ΔH3 = 10.1 kcal/mole, ΔS3 = −6 eu. The apparent Cu(pmas)^2+/+ self-exchange electron transfer rate constant calculated from this reaction on the basis of relative Marcus theory (4.7 × 10¹M⁻¹ sec⁻¹) agrees well with previous findings on ferrocytochrome c, Fe(CN)₆⁴⁻, and Ru(NH₃)₅py²⁺ oxidations. Spectrophotometric titrations of Cu(pmas)²⁺ and Cu(tmpa)²⁺ (tmpa = tris(2-pyridylmethyl)amine) with azide ion showed that both Cu(pmas)N₃)⁺ (K_f1 = 3.1 × 10³M⁻¹) and Cu(pmas)(N₃)₂ (K_f2 = 3.5 × 10¹M⁻¹) but Cu(tmpa)(N₃)⁺ (K_f = 6.6 × 10²M⁻¹) are formed up to 0.15 M N₃⁻ (25°C, pH 6, I = 0.2 M), suggesting that a thioether sulfur atom is displaced in the uptake of a second N₃⁻ ion by Cu(pmas)(N₃)⁺. The effect of thioether sulfur displacement by azide ion on the HEF-Cu(pmas)²⁺ reaction rate may be understood entirely through the tendency of N₃⁻ to shift the position of the redox equilibrium towards the reactant side, without invoking any special role for the sulfur ligand in promoting electron transfer reactivity.     

Influence of monovalent cations on the Ca2+-ATPase of sarcoplasmic reticulum isolated from rabbit skeletal and dog cardiac muscles. An interpretation of transient-state kinetic data

Transient-state kinetics of phosphorylation and dephosphorylation of the Ca²⁺-ATPase of sarcoplasmic reticulum vesicles from rabbit skeletal and dog cardiac muscles were studied in the presence of varying concentrations of monovalent and divalent cations. Monovalent cations affect the two types of sarcoplasmic reticulum differently. When the rabbit skeletal sarcoplasmic reticulum was Ca²⁺ deficient, preincubation with K⁺ (as compared with preincubation with choline chloride) did not affect initial phosphorylation at various concentrations of Ca²⁺, added with ATP to phosphorylate the enzyme. This is in contrast to preincubation with K⁺ of the Ca²⁺-deficient dog cardiac sarcoplasmic reticulum, which resulted in an increase in the phosphoenzyme level. When Ca²⁺ was bound to the rabbit skeletal sarcoplasmic reticulum, K⁺ inhibited E ~ P formation; but under the same conditions, E ~ P formation of dog cardiac sarcoplasmic reticulum was activated by K⁺ at 12 μM Ca²⁺ and inhibited at 0.33 and 1.3 μM Ca²⁺. Li⁺, Na⁺ and K⁺ also have different effects on E ~ P decomposition of skeletal and cardiac sarcoplasmic reticulum. The latter responded less to these cations than the former. Studies with ADP revealed differences between the two types of sarcoplasmic reticulum. For rabbit skeletal sarcoplasmic reticulum, 40% of the phosphoenzyme formed was ‘ADP sensitive’, and the decay of the remaining E ~ P was enhanced by K⁺ and ADP. Dog cardiac sarcoplasmic reticulum yielded about 40–48% ADP-sensitive E ~ P, but the decomposition rate of the remaining E ~ P was close to the rate measured in the absence of ADP. Thus, these studies showed certain qualitative differences in the transformation and decomposition of phosphoenzymes between skeletal and cardiac muscle which may have bearing on physiological differences between the two muscle types.