Showing papers on "Redox published in 1969"

The redox state of free nicotinamide–adenine dinucleotide phosphate in the cytoplasm of rat liver

Abstract: 1. The concentrations of the oxidized and reduced substrates of the ;malic' enzyme (EC 1.1.1.40) and isocitrate dehydrogenase (EC 1.1.1.42) were measured in freeze-clamped rat livers. By assuming that the reactants of these dehydrogenase systems are at equilibrium in the cytoplasm the [free NADP(+)]/[free NADPH] ratio was calculated. The justification of the assumption is discussed. 2. The values of this ratio obtained under different nutritional conditions (well-fed, 48hr.-starved, fed with a low-carbohydrate diet, fed with a high-sucrose diet) were all of the same order of magnitude although characteristic changes occurred on varying the diet. The value of the ratio fell on starvation and on feeding with the low-carbohydrate diet and rose slightly on feeding with the high-sucrose diet. 3. The mean values of the ratio were calculated to be between 0.001 and 0.015, which is about 100000 times lower than the values of the cytoplasmic [free NAD(+)]/[free NADH] ratio. 4. The differences in the redox state of the two nicotinamide-adenine dinucleotide couples can be explained on a simple physicochemical basis. The differences are the result of equilibria that are determined by the equilibrium constants of a number of highly active readily reversible dehydrogenases and transaminases and the concentrations of the substrates and products of these enzymes. 5. The decisive feature is the fact that the NAD and NADP couples share substrates. This sharing provides a link between the redox states of the two couples. 6. The application of the method of calculation to data published by Kraupp, Adler-Kastner, Niessner & Plank (1967), Goldberg, Passonneau & Lowry (1966) and Kauffman, Brown, Passonneau & Lowry (1968) shows that the redox states of the NAD and NADP couples in cardiac-muscle cytoplasm and in mouse-brain cytoplasm are of the same order as those in rat liver. 7. The determination of the equilibrium constant at 38 degrees , pH7.0 and I 0.25 (required for the calculation of the [free NADP(+)]/[free NADPH] ratio), gave a value of 3.44x10(-2)m for the ;malic' enzyme (with CO(2) rather than HCO(3) (-) as the reactant) and a value of 1.98x10(-2)m(-1) for glutathione reductase.

Mechanism of the Electrochemical Reduction of Persulfates and Hydrogen Peroxide

Abstract: Electrochemical studies have shown that the reduction of persulfate and hydrogen peroxide is a two step mechanism, the first step occurs by electron transfer with the conduction band and the second step by hole injection with the valence band. It could be concluded from corresponding measurements performed with a semiconductor electrode that the electrochemical properties of these oxidizing agents have to be described by two instead of one redox (normal) potential. One normal potential is much lower and the other much larger than the theoretical value determined from thermodynamic data. These values are estimated as and .

On the Redox Potentials of Ubiquinone and Cytochrome b in the Respiratory Chain

Abstract: The standard redox potential of ubiquinone in submitochondrial particles is measured by equilibration with the succinatelfumarate system to about 65 mV at pH 7, with the redox ratio, n = 2. The pH dependence is -60 mV/ApH. The lower value measured for bound ubiquinone is discussed in comparison with the value found for isolated ubiquinone (104 to 112 mV, pH 7) determined in ethanol-HC1. In parallel experiments the standard redox potential of cytochrome b was measured to 72.5mV at pH 7. The pH dependence is AE/A pH = 0 at pH 6.8. The relative position of ubiquinone and cytochrome b in the respiratory chain has been under controversy. One basis for understanding the position of the respiratory carriers is their redox potential. Until now the redox potential of isolated ubiquinone was measured in ethanol-1N HCl and extrapolated for pH = 7 [l]. The present study attempts to measure a redox potential of ubiquinone in a submitochondrial respiratory chain preparation utilizing equilibration of ubiquinone with an appropriate substrate couple such as succinate/fumarate. For comparison the redox potential of cytochrome 6 has also been followed by equilibration with the couple succinate/fumarate, a procedure which has been applied for this purpose in earlier studies [2,3]. New measurements of cytochrome b in the present context appear to be necessary since the values reported in the literature vary widely. The reason for these variations may not only reside in experimental inconsistencies, but also in the dependence of the redox potential of cytochrome b on the state of respiratory chain preparation. In this connection it is helpful that apparently both ubiquinone and cytochrome b can be equilibrated with succinate/fumarate, since their redox potentials turn out to be rather close. The present studies were performed on sonic particles from beef heart mitochondria, where it can be assumed that succinate dehydrogenase is accessible to succinate and fumarate without a membrane barrier, in contrast to the situation in intact mitochondria.

Iron-sulphur proteins.

Abstract: These electron transfer agents often have unusually low redox potentials. Eighteen are known from higher plants and bacteria, and are involved in nitrogen fixation as well as photosynthesis.

Electron Transfer and Energy Conservation

Abstract: Publisher Summary This chapter discusses electron transfer and energy conservation. Band theory applies well to metals and many covalent semiconductors and organic crystals. Proteins alone and groups of molecules of the chlorophyll or carotenoid type can act as metallic-type electron conductors. A plausible mechanism for electron transfer involves initial transfer of redox equivalents to or from the solvent or its ion-dissociation products, namely, OH − and H + . When no external energy is supplied, electron transfer goes downhill, but when it is applied, there is higher apparent activation energy than for the forward reaction. Porphyrins and chlorophylls can form complexes with acceptors in solution, and they are flat aromatic molecules peculiarly suitable to charge transfer interactions and reactions with other flat aromatic molecules. Weak acid anions that uncouple act in a highly specific way, suggesting that they act in the membrane and cannot act in the free solution. Cation transport does not require phosphorylation, and it is easy to set up transfer systems involving proton movements that could also transport cations. The structure of biological systems permits the nonequilibrium condition between different parts of space and this condition, which in turn permits energy transformations of a kind, is unknown in models

Effect of Electrode Materials on the Kinetics of Electron Exchange Reactions

Abstract: The electrode reaction Fe(CN)63−/Fe(CN)64− is studied at sodium tungsten bronzes of compositions Na0.50WO3, Na0.70WO3, and Na0.90WO3. The average standard exchange current densities of the order 101·cm mole−1 appear to be slightly dependent on the composition of bronzes. Transfer coefficient is found to be 0.5. On the basis of these results and those available in the literature for several redox reactions at various metals, a theory of heterogeneous electron exchange reactions is discussed. The influence of electrode material on the rate of electrode reaction is shown to depend on the density of states of electrons at the Fermi level of metals. The exchange integral, the other parameter where the influence of material should be contained, appears only slightly dependent either on a particular electrode reaction or on electrode material. Standard exchange current densities for some other redox reactions at platinum are predicted.

The Intra-Mitochondrial Localization of Flavoproteins Previously Assigned to the Respiratory Chain

Abstract: 1 Flavoproteins of intact and fractionated mitochondria have been studied by means of spectrophotometric and fluorimetric techniques 2 The high and low redox potential fluorescent flavoproteins of rat liver mitochondria previously assigned to the respiratory chain are located in the soluble fraction obtained on ultrasonic disintegration 3 In intact mitochondria, the low potential species has a redox potential of −279 mV at pH 72 and 25° measured by fluorescence or absorbance, and undergoes a 2 equivalent reduction After ultrasonic disruption of mitochondria, its redox potential is -294 m V at pH 72 and 25° 4 In submitochondrial particles, low potential flavoprotein reducible by NADH in the presence of rotenone is not fluorescent, appears to have a redox potential more positive than −209 mV and is attributed mainly to cytochrome b5 reductase and cytochrome b5 Mersalyl can be used to selectively inhibit cytochrome b5 reductase, and it has been possible to estimate the concentration of the low redox potential flavoprotein free from cytochrome b5 interference 5 The high redox potential flurescent flavoprotein is recovered in the soluble fraction after ultrasonic disruption of rat liver mitochondria and can be reduced by succinate in the presence of antimycin A inhibited particulate fraction This fluorescent species is also found in rat kidney mitochondria but not in blowfly mitochondria 6 In submitochondrial particles, a non-fluorescent high redox potential species is found which may be flavoprotein or non-heme iron This species has a redox potential of + 30 m V at pH 72 and 25° and undergoes 1 equivalent reduction 7 Flavoprotein fluorescence measurements are subject to significant errors caused by absorption of the exciting light, and conseuently do not always measure flavoproteins free from interference by other respiratory carriers, e q cytochromes and non-heme iron proteins

THE EFFECT OF REDOX POTENTIAL ON P870 FLUORESCENCE IN REACTION CENTERS FROM Rhodopseudomonas spheroides.

Abstract: We have prepared photosynthetic reaction centers from Rhodopseudomonas spheroides and have studied the fluorescence of the photochemical electron donor, P870. The yield of this fluorescence rises at low redox potential, presumably because the photochemical electron acceptor becomes reduced and the photochemical utilization of excitation energy is then prevented. The redox titration curve for this increase in the fluorescence has a shape corresponding to the transfer of one electron. The midpoint potential is -0.05 volt, independent of the pH from 6.5 to 8.8. The amplitude of a light-induced absorbance change at 280 nanometers varies with redox potential and shows, at pH 7.5, a midpoint potential of 0.00 volt. These studies indicate that the primary photochemical electron acceptor is not ubiquinone and is not the substance responsible for the absorbance change at 280 nanometers.

Electron spin resonance studies. Part XIX. Oxidation of organic radicals, and the occurrence of chain processes, during the reactions of some organic compounds with the hydroxyl radical derived from hydrogen peroxide and metal ions

Abstract: Earlier studies by e.s.r. spectroscopy of the radicals formed by the interaction of titanium(III) ions and hydrogen peroxide, in the presence or absence of organic compounds, have been extended to the examination of the reactions of the initially formed radicals. Particular use has been made of flow cells which allow one or more reactants to be added shortly after the others have been mixed and before the solution reaches the spectrometer cavity. The results are consistent with the view that the free hydroxyl radical is the primary oxidising agent but that the one-electron oxidation of the resulting organic radicals by both metal ions and hydrogen peroxide is important in determining the relative concentrations of the various radicals which are observed; in this respect, hydrogen peroxide is of special interest because its reduction by organic radicals leads to the formation of further hydroxyl radicals and thence the perpetuation of a chain reaction. This observation has led to the rationalisation both of the dependence of the observed concentrations of readily oxidised radicals on the concentrations of the redox reactants, and of the increase in concentration of the relatively stable radicals derived from titanium(IV)–peroxide complexes which is effected by the addition of small amounts of certain organic compounds. Evidence is also adduced that the markedly different relative concentrations of radicals which are observed when iron(II) ion is used in place of titanium(III) ion are not the result of there being different types of oxidising radicals in the two systems but reflect the fact that iron(III) ion is both a much stronger oxidising agent than titanium(IV) ion for organic radicals and shows a significant degree of selectivity in this capacity between tervalent carbon atoms in different environments.

Role of sodium 9,10-anthraquinone-2-sulphonate in photo-oxidation reactions

Abstract: The photo-oxidation of sodium 9,10-anthraquinone-2-sulphonate in aqueous and aqueous-alcoholic solutions has been studied by flash, continuous photolysis, and electron spin resonance (e.s.r.) techniques. Whereas in aqueous-alcoholic solutions it is well established that the photo-excited quinone abstracts a hydrogen atom from the alcohol by A*+ RCH2OH → AH·+ RĊHOH it is shown here that in aqueous solution the quinone reacts by A*+ A → A·++ A·– From the flash photolysis and e.s.r. observations a mechanism is proposed for the reaction in neutral aqueous solution which satisfactorily accounts for the products and the kinetics of the reaction as observed by oxygen absorption and by spectral measurements under continuous photolysis conditions.

220 MHz proton magnetic resonance spectrum of NADH.

Abstract: NICOTINAMIDE adenine dinucleotide (NAD+) and reduced nicotinamide adenine dinucleotide (NADH) are important participants in enzymatic redox reactions1,2 (Fig. 1). Deuterium labelling experiments have shown direct and reversible transfer of hydrogen between substrate and position 4 of the nicotinamide ring of NAD+, to give one or the other of the configurations shown below3,4.