Oxidation-reduction properties of several low potential iron-sulfur proteins and of methylviologen.

Apparent oxidation-reduction potentials at pH 7.0 and 25 degrees C were determined using the H2-hydrogenase system with ferredoxins from the following sources: Clostridium pasteurianum, -403 mV; C tartarovorum, -424 mV; C. acidi-urici, -434 mV; Peptococcus aerogenes, -427 mV; Chromatium D, -482 mV (pH 8.0); B. polymyxa, Fd I, -377 mV, and Fd II, -422 mV; and spinach, -428 mV. The pH dependence of these values was variable, ranging from -2 to -24 mV/pH unit increase for different ferredoxins. Over the range of buffer concentrations between 0.05 and 0.2 M, the potentials did not vary significantly. The number of electrons transferred during reduction (as determined by integrations of EPR spectra and by dithionite titration) is 2 for the first five proteins, while potentiometric data for all the cases fit a Nernst equation for which n = 1. The E degrees' value for the redox indicator methylviologen at pH 7.4 was found to be -460 mV, according to both the H2-hydrogenase system and cyclic voltammetry, significantly different from the value previously reported at higher pH's. Additionally, the presence of C. pasteuranum ferredoxin appears to shift the E degrees value of methylviologen to even more negative values. An analysis of sources of error inherent with potential determinations with H2 and hydrogenase is presented. The electronic and EPR spectra of P. aerogenes ferredoxin, for which the x-ray structure has been published, are given here. It appears that the determination of potentials of ferredoxin and other low-potential porteins with the H2-hydrogenase system affords certain experimental advantages over alternative methods currently employed with these and similar substances.     

Oxidation-reduction potentials of ferredoxin-NADP+ reductase and flavodoxin from Anabaena PCC 7119 and their electrostatic and covalent complexes.

The oxidation-reduction potentials of ferredoxin-NADP+ reductase and flavodoxin from the cyanobacterium Anabaena PCC 7119 were determined by potentiometry. The potentials at pH 7 for the oxidized flavodoxin/flavodoxin semiquinone couple (E2) and the flavodoxin semiquinone/hydroquinone couple (E1) were -212 mV and -436 mV, respectively. E1 was independent of pH above about pH 7, but changed by approximately -60 mV/pH below about pH 6, suggesting that the fully reduced protein has a redox-linked pKa at about 6.1, similar to those of certain other flavodoxins. E2 varied by -50 mV/pH in the range pH 5-8. The redox potential for the two-electron reduction of ferredoxin-NADP+ reductase was -344 mV at pH 7 (delta Em = -30 mV/pH). In the 1:1 electrostatic complex of the two proteins titrated at pH 7, E2 was shifted by +8 mV and E1 was shifted by -25 mV; the shift in potential for the reductase was +4 mV. The potentials again shifted following treatment of the electrostatic complex with a carbodiimide, to covalently link the two proteins. By comparison with the separate proteins at pH 7, E2 for flavodoxin shifted by -21 mV and E1 shifted by +20 mV; the reductase potential shifted by +2 mV. The potentials of the proteins in the electrostatic and covalent complexes showed similar pH dependencies to those of the individual proteins. Qualitatively similar changes occurred when ferredoxin-NADP+ reductase from Anabaena variabilis was complexed with flavodoxin from Azotobacter vinelandii. The shifts in redox potential for the complexes were used with previously determined values for the dissociation constant (Kd) of the electrostatic complex of the two oxidised proteins, in order to estimate Kd values for the interaction of the different redox forms of the proteins. The calculations showed that the electrostatic complexes, formed when the proteins differ in their redox states, are stronger than those formed when both proteins are fully oxidized or fully reduced.     

The heterogeneity of arginases in rat tissues.

1. The mid-point reduction potentials of the various groups in xanthine oxidase from bovine milk were determined by potentiometric titration with dithionite in the presence of dye mediators, removing samples for quantification of the reduced species by e.p.r. (electron-paramagnetic-resonance) spectroscopy. The values obtained for the functional enzyme in pyrophosphate buffer, pH8.2, are: Fe/S centre I, -343 +/- 15mV; Fe/S II, -303 +/- 15mV; FAD/FADH-; -351 +/- 20mV; FADH/FADH2, -236 +/-mV; Mo(VI)/Mo(V) (Rapid), -355 +/- 20mV; Mo(V) (Rapid)/Mo(IV), -355 +/- 20mV. 2. Behaviour of the functional enzyme is essentially ideal in Tris but less so in pyrophosphate. In Tris, the potential for Mo(VI)/Mo(V) (Rapid) is lowered relative to that in pyrophosphate, but the potential for Fe/S II is raised. The influence of buffer on the potentials was investigated by partial-reduction experiments with six other buffers. 3. Conversion of the enzyme with cyanide into the non-functional form, which gives the Slow molybdenum signal, or alkylation of FAD, has little effect on the mid-point potentials of the other centres. The potentials associated with the Slow signal are: Mo(VI)/Mo(V) (Slow), -440 +/- 25mV; Mo(V) (Slow)/Mo(IV), -480 +/- 25 mV. This signal exhibits very sluggish equilibration with the mediator system. 4. The deviations from ideal behaviour are discussed in terms of possible binding of buffer ions or anti-co-operative interactions amongst the redox centres.     

Determination of reduction potential of an engineered Cu<Subscript>A</Subscript> azurin by cyclic voltammetry and spectrochemical titrations

The reduction potentials of an engineered Cu_A azurin in its native and thermally denatured states have been determined using cyclic voltammetry and spectrochemical titrations. Using a 4,4-dipyridyl disulfide modified gold electrode, the reduction potentials of native and thermally denatured Cu_A azurin are the same within the experimental error (422±5 and 425±5 mV vs. NHE, respectively, in 50 mM ammonium acetate buffer, pH 5.1, 300 mM NaCl, 25 °C), indicating that the potential is that of a nonnative state. In contrast, using a didodecyldimethylammonium bromide (DDAB) film-pyrolytic graphite edge (PGE) electrode, the reduction potentials of native and thermally denatured Cu_A azurin have been determined to be 271±7 mV (50 mM ammonium acetate buffer, pH 5.1, 4 °C) and 420±1 mV (50 mM ammonium acetate buffer, pH 5.1, 25 °C), respectively. Spectroscopic redox titration using [Ru(NH₃)₅Py]²⁺ resulted in a reduction potential (254±4 mV) (50 mM ammonium acetate buffer, pH 5.1, 4 °C) similar to the value obtained using the DDAB film-PGE electrochemical method. Complete reoxidation of [Ru(NH₃)₅Py]²⁺-reduced Cu_A azurin is also consistent with the conclusion that this spectrochemical titration method using [Ru(NH₃)₅Py]²⁺ measures the reduction potential of native Cu_A azurin.Abbreviations CcO cytochrome c oxidase - N₂OR nitrous oxide reductase - ET electron transfer - CV cyclic voltammetry - NHE normal hydrogen electrode - DDAB didodecyldimethylammonium bromide - PGE pyrolytic graphite edge     

An antimony-silver/silver chloride electrode system for measuring pH in insect gut contents

An electrode system consisting of a cast antimony rod with a silver chloride-coated wire as a reference electrode has been developed to measure pH in insect gut contents. The electrode works well with buffer volumes down to 0.1 μl. Response to pH is linear between pH 4 and 11 and equates to a change of 52.3 mV per pH unit at 18–20°C. Electrical resistance is low (0.25 MΩ), so the electrode can be used with low-impedance meters and does not require shielding from induced currents. Its useful range lies between redox potentials −330 mV (pH 7) and +297 mV to 350 mV, corresponding to a pe + pH range of 1.4 to about 10.8. This covers reported gut pH values for most insects so far examined. Consequently the electrode is suitable for measuring pH of gut contents from many insects that are too small to be analysed with current commercial electrodes.     

Redox potentials of flavocytochromes c from the phototrophic bacteria, Chromatium vinosum and Chlorobium thiosulfatophilum.

The redox potentials of flavocytochromes c (FC) from Chromatium vinosum and Chlorobium thiosulfatophilum have been studied as a function of pH. Chlorobium FC has a single heme which has a redox potential of +98 mV at pH 7 (N = 1) that is independent of pH between 6 and 8. The average two-electron redox potential of the flavin extrapolated to pH 7 is +28 mV and decreases 35 mV/pH between pH 6 and 7. The anionic form of the flavin semiquinone is stabilized above pH 6. The redox potential of Chromatium FC is markedly lower than for Chlorobium. The two hemes in Chromatium FC appear to have a redox potential of 15 mV at pH 7 (N = 1), although they reside in very different structural environments. The hemes of Chromatium FC have a pH-dependent redox potential, which can be fit in the simplest case by a single ionization with pK = 7.05. The flavin in Chromatium FC has an average two-electron redox potential of -26 mV at pH 7 and decreases 30 mV/pH between pH 6 and 8. As with Chlorobium, the anionic form of the flavin semiquinone of Chromatium FC is stabilized above pH 6. The unusually high redox potential of the flavin, a stabilized anion radical, and sulfite binding to the flavin in both Chlorobium and Chromatium FCs are characteristics shared by the flavoprotein oxidases. By analogy with glycolate oxidase and lactate dehydrogenase for which there are three-dimensional structures, the properties of the FCs are likely to be due to a positively charged amino acid side chain in the vicinity of the N1 nitrogen of the flavin.     

The two cytochromes c in the facultative methylotroph Pseudomonas AM1

It was previously suggested that there is only one soluble cytochrome c in Pseudomonas AM1, having a molecular weight of 20000, a redox midpoint potential of about +260mV and a low isoelectric pint [Anthony (1975) Biochem. J. 146, 289–298; Widdowson & Anthony (1975) Biochem. J. 152, 349–356]. A more thorough examination of the soluble fraction of methanol-grown Pseudomonas AM1 has now revealed the presence of two different cytochromes c. These were both purified to homogeneity by acid treatment, ion-exchange chromatography, gel filtration, chromatography on hydroxyapatite and preparative isoelectric focusing. Molecular weights were determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis; midpoint redox potentials were determined directly by using platinum and calomel electrodes; isoelectric points were estimated by electrophoresis and by the behaviour of the two cytochromes on ion-exchange celluloses. The more abundant cytochrome c_H (λ_max. 550.5nm) had a low molecular weight (11000), a midpoint potential of about +294mV and a high isoelectric point, not being adsorbed on DEAE-cellulose in 20mm-Tris/HCl buffer, pH8.0. The less abundant cytochrome c_L (λ_max. 549nm) was about 30% of the total; it had a high molecular weight (20900), a midpoint potential of about +256mV and a low isoelectric point, binding strongly to DEAE-cellulose in 20mm-Tris/HCl buffer, pH8.0. The pH-dependence of the midpoint redox potentials of the two cytochromes c were very similar. There were four ionizations affecting the redox potentials in the pH range studied (pH4.0–9.5), two in the oxidized form (pK values about 3.5 and 5.5) and two in the reduced form (pK values about 4.5 and 6.5), suggesting that the ionizing groups involved may be the two propionate side chains of the haem. Neither of the cytochromes c was present in mutant PCT76, which was unable to oxidize or grow on C₁ compounds, although still able to grow well on multicarbon compounds such as succinate. Whether or not these two cytochromes c have separate physiological functions is not yet certain.     

Redox and flavin-binding properties of recombinant flavodoxin from Desulfovibrio vulgaris (Hildenborough).

Flavodoxin from Desulfovibrio vulgaris (Hildenborough) has been expressed at a high level (3-4% soluble protein) in Escherichia coli by subcloning a minimal insert carrying the gene behind the tac promoter of plasmid pDK6. The recombinant protein was readily isolated and its properties were shown to be identical to those of the wild-type protein obtained directly from D. vulgaris, with the exception that the recombinant protein lacks the N-terminal methionine residue. Detailed measurements of the redox potentials of this flavodoxin are reported for the first time. The redox potential, E2, for the couple oxidized flavodoxin/flavodoxin semiquinone at pH 7.0 is -143 mV (25 degrees C), while the value for the flavodoxin semiquinone/flavodoxin hydroquinone couple (E1) at the same pH is -440 mV. The effects of pH on the observed potentials were examined; E2 varies linearly with pH (slope = -59 mV), while E1 is independent of pH at high pH values, but below pH 7.5 the potential becomes less negative with decreasing pH, indicating a redox-linked protonation of the flavodoxin hydroquinone. D. vulgaris apoflavodoxin binds FMN very tightly, with a value of 0.24 nM for the dissociation constant (Kd) at pH 7.0 and 25 degrees C, similar to that observed with other flavodoxins. In addition, the apoflavodoxin readily binds riboflavin (Kd = 0.72 microM; 50 mM sodium phosphate, pH 7.0, 5 mM EDTA at 25 degrees C) and the complex is spectroscopically very similar to that formed with FMN. The redox potentials for the riboflavin complex were determined at pH 6.5 (E1 = -262 mV, E2 = -193 mV; 25 degrees C) and are discussed in the light of earlier proposals that charge/charge interactions between different parts of the flavin hydroquinone play a crucial role in determining E1 in flavodoxin.     

Haptoglobin alters oxygenation and oxidation of hemoglobin and decreases propagation of peroxide-induced oxidative reactions

We compared oxygenation and anaerobic oxidation reactions of a purified complex of human hemoglobin (Hb) and haptoglobin (Hb-Hp) to those of uncomplexed Hb. Under equilibrium conditions, Hb-Hp exhibited active-site heterogeneity and noncooperative, high-affinity O(2) binding (n(1/2)=0.88, P(1/2)=0.33mm Hg in inorganic phosphate buffer at pH 7 and 25°C). Rapid-reaction kinetics also exhibited active-site heterogeneity, with a slower process of O(2) dissociation and a faster process of CO binding relative to uncomplexed Hb. Deoxygenated Hb-Hp had significantly reduced absorption at the λ(max) of 430nm relative to uncomplexed Hb, as occurs for isolated Hb subunits that lack T-state stabilization. Under comparable experimental conditions, the redox potential (E(1/2)) of Hb-Hp was found to be +54mV, showing that it is much more easily oxidized than uncomplexed Hb (E(1/2)=+125mV). The Nernst plots for Hb-Hp oxidation showed no cooperativity and slopes less than unity indicated active-site heterogeneity. The redox potential of Hb-Hp was unchanged by pH over the range of 6.4-8.3. Exposure of Hb-Hp to excess hydrogen peroxide (H(2)O(2)) produced ferryl heme, which was found to be more kinetically inert in the Hb-Hp complex than in uncomplexed Hb. The negative shift in the redox potential of Hb-Hp and its stabilized ferryl state may be central elements in the protection against Hb-induced oxidative damage afforded by formation of the Hb-Hp complex.     

Half reduction potentials and oxygen affinity of the cytochromes of Pseudomonas carboxydovorans

Abstract The midpoint redox potentials (E'₀) of the cytochromes of Pseudomonas carboxydovorans have been studied by means of coupled spectrum deconvolution and potentiometric analysis. Membranes of cells grown on different substrates (CO; H₂+ CO₂; or pyruvate) contained cytochromes with similar absorption peaks and redox potentials. The cytochromes of the CO-sensitive main electron pathway of the respiratory chain revealed redox potentials in the same range as mitochondrial cytochromes (cytochrome b -555, about −20 mV; cytochrome c and cytochrome a , about +220 mV). For the cytochromes of the CO-insensitive alternative electron pathway, which allows uninhibited growth and respiration in the presence of high concentrations of CO, redox potentials of approx. +50 mV (cytochrome b -558) and −11 to −215 mV (cytochrome b -561) were determined. Cytochrome [ib-561], earlier proposed as the alternative terminal oxidase o in this organism, was shown to possess the lowest half reduction potential of all the cytochromes present in the cells. Measurements of the apparent K _m value for oxygen revealed a low affinity of cytochrome a ( K _m/ 5 υ M O₂) and a very high affinity of the CO-insensitive oxidase ( K _m < 0.5 μ M O₂). The high affinity to oxygen might be responsible for the CO-insensitivity of this unusual cytochrome o .     

Blue and red shifts of bacteriochlorophyll absorption band around 880 nm in Rhodospirillum rubrum

The redox potential dependence of the light-induced absorption changes of bacteriochlorophyll in chromatophores and subchromatophore pigment-protein complexes from Rhodospirillum rubrum has been examined. The highest values of the absorption changes due to the bleaching of P-870 and the blue shift of P-800 in chromatophores and subchromatophore complexes are observed in the 360–410 mV redox potential range. At potentials below 300 mV (pH 7.0), the 880 nm band of bacteriochlorophyll shifts to shorter wavelengths in subchromatophore complexes and to longer wavelengths in chromatophores.

The data on redox titration show that the red and blue shifts of 880-nm bacteriochlorophyll band represent the action of a non-identified component (C₃₄₀) which has an oxidation-reduction midpoint potential close to 340 mV (n = 1) at pH 6.0–7.6. The E_m of this component varies by 60 mV/pH unit between pH 7.6 and 9.2.

The results suggest that the red shift is due to the transmembrane, and the blue shift to the local intramembrane electrical field. The generation of both the transmembrane and local electrical fields is apparently governed by redox transitions of the component C₃₄₀.     

The effect of sediment redox potential and soil acidity on nitrogen uptake,anaerobic root respiration,and growth of rice (Oryza sativa)

Summary Oryza sativa Loisel cultivar Mars., a common lowland rice variety was grown under controlled soil redox conditions (Eh) and acidity (pH). The effect of two variables (Eh and pH) on growth, anaerobic root respiration, and uptake of added labelled nitrogen, was investigated. Plant growth, estimated by dry weight showed significantly higher growth under reducing sediment redox potentials (−200 mV and 0 mV) and at a soil pH of 6.5 Using the activity of the inducible enzyme alcohol dehydrogenase (ADH) as an indicator of anaerobic root respiration, a decrease in redox potential resulted in an increase in root ADH. However, growth paralled increases in anaerobic root respiration suggesting nitrogen transformation in the soil to be a primary parameter governing growth. Labelled nitrogen uptake which was greater under anaerobic conditions apparently led to greater growth of lowland rice in the highly reduced or anaerobic soil treatments.     

Redox potentials of chlorophylls in the photosystem II reaction center

Water oxidation generating atmospheric oxygen occurs in photosystem II (PSII), a large protein-pigment complex located in the thylakoid membrane. The recent crystal structures at 3.2 and 3.5 A resolutions provide novel details on amino acid side chains, especially in the D1/D2 subunits. We calculated the redox potentials for one-electron oxidation of the chlorophyll a (Chla) molecules in PSII, considering the protein environment in atomic detail. The calculated redox potentials for the dimer Chla (P(D1/D2)) and accessory Chla (Chl(D1/D2)) were 1.11-1.30 V relative to the normal hydrogen electrode at pH 7, which is high enough for water oxidation. The D1/D2 proteins and their cofactors contribute approximately 390 mV to the enormous upshift of 470 mV compared to the redox potential of monomeric Chla in dimethylformamide. The other subunits are responsible for the remaining 80 mV. The high redox potentials of the two accessory Chla Chl(D1/D2) suggests that they also participate in the charge separation process.     

Redox potentials in hydro-organic media at normal and subzero temperatures. Ferro-ferricyanide and cytochrome c as models.

Redox potentials of ferro-ferricyanide and cytochrome c were measured in water/ethylene glycol and water/dimethylsulfoxide (volume ratio from 100/0 to 50/50) between 25 and -25 degrees C. For both systems, the midpoint potential decreases in the presence of organic solvents and increases by cooling. The magnitude of these variations is larger in dimethylsulfoxide than in ethylene glycol; moreover in the same solvent mixture it is larger with ferro-ferricyanide than with cytochrome c, so that the difference between the redox potentials of these two systems can be strongly affected and even reversed. While in pure water (cacodylate buffer pH 7.0, NaCl 0.1 M) they are respectively +388 and +265 mV, in 50% dimethylsulfoxide at 25 degrees C they decrease to +112 and +208 mV. Reduction of cytochrome c by ferro-ferricyanide, in this mixture, is then expected and was indeed observed. On the other hand, as (deltaE/deltaT)T, (E being the redox potential) is higher for ferro-ferricyanide than for cytochrome c, the oxidative power of the former for the latter is expected to increase as temperature decreases. This effect was observed in 50% ethylene glycol at -16 degrees C. Organic solvents and large temperature variations appear then as powerful perturbants of redox reactions. Their effects should be taken into account in studies of redox reactions carried out in cooled hydro-organic media.     

Entropies of Redox Reactions between Proteins and Mediators: The Temperature Dependence of Reversible Electrode Potentials in Aqueous Buffers

The temperature dependencies of the reversible electrode potentials for a number of charge transfer reactions of redox mediators were used to evaluate the corresponding charge transfer entropies in Tris–HCl (pH 8) buffer. The redox mediator thermodynamic data, along with reaction enthalpy data for mediator redox protein electron transfer, were used to evaluate the charge transfer entropy for the cytochrome c redox couple [(cytc)_ox/(cytc)_red] in Tris–HCl (pH 8) buffer and were found to be equal to −16 cal/°K mol. Reversible electrode potentials at 298°K for the redox mediator half-reactions were observed to vary from −528 to +657 mV (vs NHE). Charge transfer entropies were observed to depend upon the structure of the redox mediators and to vary from −13.8 to −29.7 cal/°K mol for a closely related series of organic dications (viologens) and a value of −43.6 cal/°K mol was observed for the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻couple under the same conditions. A procedure for determining charge transfer entropies of protein redox couples which cannot be studied by direct electrochemical methods is outlined. The factors contributing to the magnitude of the charge transfer entropies are discussed.     

Oxidation-reduction thermodynamics of the acceptor quinone complex in whole-membrane fragments from Chloroflexus aurantiacus

Oxidation-reduction thermodynamic equilibria involving the quinone-acceptor complex have been examined in whole-membrane fragments from Chloroflexus aurantiacus. The primary quinone acceptor was titrated by monitoring the amount of cytochrome c554 photooxidized by a flash of light as a function of the redox potential. In contrast to previous data obtained in purified plasma membranes, in which the primary quinone acceptor exhibited a midpoint potential equal to -50 mV at pH 8.2, in whole-membrane fragments it titrated at -210 mV (pH 8.0), with a pH dependence of -60 mV/pH up to a pK value of 9.3. o-Phenanthroline, an inhibitor of electron transfer from the primary to the secondary quinone acceptor, shifted the Em/pH curve of the primary acceptor to higher redox potentials. The midpoint potential of the secondary quinone acceptor and its dependence on pH has been determined by comparing the kinetics of the charge recombination processes within the reaction center complex in the presence and in the absence of o-phenanthroline. It is concluded that both the primary and the secondary quinone acceptors interact with a proton, with pK values of 9.3 and of approximately 10.2 respectively. At physiological pH the electron appears to be stabilized on the secondary with respect to the primary quinone acceptor by approximately 60 meV.     

Kinetic properties of hydrogenase isolated from Desulfovibrio vulgaris (Hildenborough)

Hydrogenase of Desulfovibrio vulgaris shows nonlinear kinetics in hydrogen production with both the natural electron carrier, cytochrome c3, and the artificial donor, methyl viologen semiquinone. Increasing concentrations of salt progressively inhibit the hydrogen production, as do increasing amounts of dimethylsulfoxide (Me2SO). Hydrogen consumption activity does not change up to 30% (v/v) of Me2SO. Preincubation in Me2SO up to 55% (v/v) does not affect the hydrogen uptake or production. The production activity of the enzyme shows an optimum around pH 6. When plotted as a function of redox potential the activity can be fitted to a Nernst equation with n = 1. Midpoint potentials calculated at various values follow approximately the hydrogen electrode to pH 6. Thereafter, there is a shift of about 40 mV to higher redox potentials.     

Influence of submerged macrophytes,temperature, and nutrient loading on the development of redox potential around the sediment–water interface in lakes

Redox potential is a significant factor in aquatic systems to regulate the availability of nutrients and some metals. To assess the driving variables regulating redox potential, background parameters (dissolved oxygen, pH, temperature, chlorophyll-a, soluble reactive and total phosphorus content of water, coverage and height of submerged macrophytes) and redox potential profiles around the sediment–water interface (SWI) were measured in simulated shallow lake ecosystems. There were two nutrient regimes (enriched and non-enriched) and three temperature scenarios (unheated; +3.5°C; +5°C) installed in the experimental setups, which were constructed to study the effects of global climate change. Temperature did not have any detectable effect on redox potentials, and we presume that nutrient addition had only indirect positive effects through triggering phytoplankton dominance which causes macrophyte absence. When submerged macrophytes were present in high density (80–100% coverage), redox potentials at the SWI varied between 60–215 mV and the mean redox potential was 133 ± 34 mV (mean ± 1 SD). In contrast to this, when phytoplankton dominance was coupled to low macrophyte density (0–20% coverage), the range of redox potentials at the SWI was 160–290 mV and the mean redox potential was 218 ± 34 mV. The results revealed the primary importance of submersed macrophytes; macrophyte coverage determined alone the redox potential of the sediment–water interface by 81%. This study suggests that possible positive effects of macrophytes on redox potential can be suppressed by their negative effects in case of 80–100% coverage and total inhabitation of the water column.     

Ionic movements across the chorion in newly shed salmon eggs (Salmo salar L.)

Summary Sodium and chloride exchange and trans-chorion potentials were investigated in newly shed eggs from Atlantic salmon. Exposure of eggs to pH 3.5 caused the non-labile sodium fraction at pH 7.0 to become labile and lost from the egg. Chloride fluxes appear unaffected by the pH of the external medium. Trans-chorion potential was inside negative (about –100 mV) in dilute media (10^–5 mol·l^–1 NaCl or KCl) and immediately decreased as external cation concentration was increased, by about 46 mV/decade change in cation concentration, reaching about 0 mV in 10^–2 mol·l^–1 cation. Return to dilute cation solutions resulted in a slow increase in potential (repolarisation) and the time course of these potential changes was paralleled by the rate of sodium efflux, although chloride efflux was very rapid. After exposure to acid conditions repolarisation of the egg on return to dilute cation concentrations was preceeded by a phase when the pvp became inside positive. The results are discussed in terms of chorion structure, anionic charge on the perivitelline molecules and unstirred layers within the chorion.Abbreviations pvp perivitelline potential     

Growth and reducing capacity of Listeria monocytogenes under different initial redox potential

Aims:  To determine the reducing capacity of Listeria monocytogenes and to highlight the effect of redox potential on its growth parameters.
Methods and Results:  The reducing capacity of L. monocytogenes was monitored in Brain Heart Infusion Broth media at different initial redox potential (Eh) and pH at 37°C. The effect of Eh obtained by gas flushing (air, N₂ and N₂-H₂) or by adding potassium ferricyanide and dithiotreitol in concentration from 1 to 10 mmol l⁻¹on L. monocytogenes growth parameters at pH 6·0, 7·0 and 8·0 was investigated. A total change of 539 mV (±44 mV) from an initial redox value of +330 ± 8 mV to a more negative potential in redox curves was observed resulting from L. monocytogenes growth at pH 7·0 at 37°C. A significant influence of pH and redox potential on L. monocytogenes lag phase of growth was shown ( P  < 0·05).
Conclusions:  Listeria monocytogenes exhibited longer lag phase in reducing conditions and at pH 6·0. The method used to modify the redox potential was shown to have no effect on growth parameters at pH 7·0.
Significance and Impact of the Study:  The provided information on the extending lag time and the possible delayed growth of this major pathogen in reducing conditions might be useful for its control in foods.