Influence of oxygen concentration on redox cycling of alloxan and dialuric acid.

Alloxan, a chemical diabetogen, decays in the absence of reductants into alloxanic acid. In the presence of glutathione, it is reduced via the alloxan radical into dialuric acid, which autoxidizes back to alloxan. During this redox cycling process, reactive oxygen species are formed that destroy beta-cells in islets of Langerhans. Previous experiments were conducted with oxygen concentrations about ten times as high as within cells. The aim of our in vitro study was to evaluate the impact of different oxygen concentrations (0, 25, 250 micromol/l) at a given initial ratio of glutathione and alloxan on this redox cycling. Reduction of alloxan, oxidation of glutathione, and the formation of glutathiol (GSSG) were continuously recorded by HPLC for 90 minutes at 25 degrees C in air, calibration gas, or argon. In the absence of reductants, alloxan irreversibly decomposed into alloxanic acid regardless of oxygen presence. When the reaction system contained glutathione, decomposition was significantly retarded and therefore influenced by oxygen. In argon, decay could not be observed due to its reduction and the absence of oxygen. Increasing oxygen concentration enabled a redox cycling and therefore an ongoing decay. The highest decomposition along with the highest consumption of glutathione occurred at 250 micromol/l oxygen. The lower the oxygen, the more dialuric acid could be detected. After calculation, about 33 redox cycles per hour generates an amount of reactive oxygen species sufficient to damage pancreatic beta cells and induce insulin deficiency.     

The fairytale of the GSSG/GSH redox potential

Background

The term GSSG/GSH redox potential is frequently used to explain redox regulation and other biological processes.

Scope of review

The relevance of the GSSG/GSH redox potential as driving force of biological processes is critically discussed. It is recalled that the concentration ratio of GSSG and GSH reflects little else than a steady state, which overwhelmingly results from fast enzymatic processes utilizing, degrading or regenerating GSH.

Major conclusions

A biological GSSG/GSH redox potential, as calculated by the Nernst equation, is a deduced electrochemical parameter based on direct measurements of GSH and GSSG that are often complicated by poorly substantiated assumptions. It is considered irrelevant to the steering of any biological process. GSH-utilizing enzymes depend on the concentration of GSH, not on [GSH]², as is predicted by the Nernst equation, and are typically not affected by GSSG. Regulatory processes involving oxidants and GSH are considered to make use of mechanistic principles known for thiol peroxidases which catalyze the oxidation of hydroperoxides by GSH by means of an enzyme substitution mechanism involving only bimolecular reaction steps.

General significance

The negligibly small rate constants of related spontaneous reactions as compared with enzyme-catalyzed ones underscore the superiority of kinetic parameters over electrochemical or thermodynamic ones for an in-depth understanding of GSH-dependent biological phenomena. At best, the GSSG/GSH potential might be useful as an analytical tool to disclose disturbances in redox metabolism. This article is part of a Special Issue entitled Cellular Functions of Glutathione.     

The relationship between GSH, GSSG and non-GSH thiol in GSH-deficient erythrocytes from Finnish landrace and Tasmanian merino sheep.

1. Two automated colorimetric methods have been developed for assaying the GSH and total thiol in protein-free extracts of erythrocytes. They employ as chromogens 5,5'-dithiobis-(2-nitrobenzoate) (DTNB) and alloxan. 2. The concentrations of GSH, GSSG and total non-protein thiol have been estimated in high and low GSH erythrocytes from Finnish Landrace and Tasmanian Merino sheep. 3. In both breeds of sheep low GSH cells were found to have low concentrations of total non-protein thiol and GSSG as well as of GSH. 4. Nevertheless high and low GSH cells have similar values for the oxidation-reduction potential of the GSH : GSSG couple.     

即时浸酸显著降低滞育家蚕卵的还原型与氧化型谷胱甘肽比值

即时浸酸在阻止家蚕Bombyx mori卵滞育发动的同时, 显著提高了家蚕卵H₂O₂含量。还原型谷胱甘肽（reduced glutathione, GSH）与氧化型谷胱甘肽（oxidized glutathione, GSSG）的比值是一种氧化胁迫状态的动态指标。为了调查即时浸酸是否造成滞育家蚕卵氧化胁迫, 本研究利用分光光度法分别测定了滞育家蚕卵和5 min即时浸酸滞育家蚕卵中GSH和GSSG含量以及谷胱甘肽转移酶（glutathione-S-transferase, GST）活性。结果表明: 处理后24 h, 即时浸酸处理家蚕卵的总谷胱甘肽（GSH+2GSSG）含量、 GSH含量、 GSSG含量、 GSH/GSSG比值和GST活性分别相当于同期滞育家蚕卵的204%, 78%, 550%, 14%和97%。据此推测, 即时浸酸在阻止滞育发动的同时, 可能通过促进GSH氧化为GSSG, 而显著降低了GSH/GSSG比值, 使家蚕卵处于过氧化状态。     

Relative importance of cellular uptake and reactive oxygen species for the toxicity of alloxan and dialuric acid to insulin-producing cells

The diabetogenic agent alloxan is selectively accumulated in insulin-producing cells through uptake via the GLUT2 glucose transporter in the plasma membrane. In the presence of intracellular thiols, especially glutathione, alloxan generates "reactive oxygen species" (ROS) in a cyclic reaction between this substance and its reduction product, dialuric acid. The cytotoxic action of alloxan is initiated by free radicals formed in this redox reaction. Autoxidation of dialuric acid generates superoxide radicals (O(2)(*-)) and hydrogen peroxide (H(2)O(2)), and finally hydroxyl radicals ((*)OH). Thus, while superoxide dismutase (SOD) only reduced the toxicity, catalase, in particular in the presence of SOD, provided complete protection of insulin-producing cells against the cytotoxic action of alloxan and dialuric acid due to H(2)O(2) destruction and the prevention of hydroxyl radical ((*)OH) formation, indicating that it is the hydroxyl radical ((*)OH) which is the ROS ultimately responsible for cell death. After selective accumulation in pancreatic beta cells, which are weakly protected against oxidative stress, the cytotoxic glucose analogue alloxan destroys these insulin-producing cells and causes a state of insulin-dependent diabetes mellitus through ROS-mediated toxicity in rodents and in other animal species, which express this glucose transporter isoform in their beta cells.     

A redox microenvironment is essential for MAPK-dependent secretion of pro-inflammatory cytokines: modulation by glutathione (GSH/GSSG) biosynthesis and equilibrium in the alveolar epithelium

The characterization of oxidant (glutathione)-dependent regulation of MAPK^p38/RK-mediated TNF-α secretion was undertaken in vitro, and the ramifications of the influence of a redox microenvironment were unraveled. Intermittent exposure of alveolar epithelial cells (FATEII) to LPS (endotoxin) transiently and temporally induced the expression of MAPK^p38/RK. This upregulation was associated with the activation of MAPKAP-K₂, manifested by the specific phosphorylation of the downstream heat-shock protein (Hsp)-27. Selective blockading of the MAPK^p38/RK pathway using the pyridinyl imidazole SB-203580 abrogated the LPS-dependent release of TNF-α. N-acetyl-l-cysteine (NAC), a precursor of glutathione, reduced TNF-α secretion and increased [GSH]. Conversely, l-buthionine-(S,R)-sulfoximine (BSO), an irreversible inhibitor of γ-glutamylcysteine synthetase (γ-GCS), the rate-limiting enzyme in the pathway mediating GSH biosynthesis, augmented the secretion of TNF-α and [GSSG] accumulation. Whereas NAC abrogated the phosphorylation of MAPK^p38/RK, BSO reversibly amplified this effect. Furthermore, intermittent exposure of FATEII cells to the exogenous oxidants X/XO and H₂O₂ upregulated the secretion of pro-inflammatory cytokines IL-1β, IL-6 and TNF-α; this upregulation was correlated with increasing activity of key glutathione-related enzymes, closely involved with maintaining the cyclic GSH/GSSG equilibrium. These results indicate that a redox microenvironment plays a major role in regulating MAPK-dependent production of cytokines in the alveolar epithelium.     

Formation of compound 305 requires the simultaneous generation of both alloxan and GSH radicals.

This in vitro study investigates the conditions under which "compound 305" is formed. Using HPLC, ESR as well as UV spectroscopy, "compound 305" was largely separated and characterized. It has an absorption peak at 314 nm, which changes after reoxygenation to shorter wavelengths within hours. The retention time of "compound 305" amounts to 10.93 +/- 0.042 min. The formation of "compound 305" does not depend on alloxan (ALX) or reduced glutathione (GSH), but most likely on the steady-state concentration of the paramagnetic derivatives of both reactants (ALX* and GS*). The alloxan radical (ALX*) is formed by either a one-electron transfer from e. g. GSH to alloxan or oxidation of dialuric acid. The concentration of the ALX* was determined to be 12 +/- 3.6 micromol/l using the stable ultramarine radical as an ESR standard. ALX* is stable only under anaerobic conditions. It disappears within 2 min in air. Since formation of "compound 305" needs both ALX* as well as GS*, which are also necessary for the generation of reactive oxygen species (ROS), it is assumed that formation of "compound 305" diminishes the toxicity of alloxan.     

Detection and quantitative determination of abscisic acid by immunological assay

Summary Antibodies with specificity towards abscisic acid (ABA) were produced in rabbits. These antibodies were used for assaying ABA by the inhibition of inactivation of modified bacteriophage. For this assay conjugates of ABA with bacteriophage T₄ were prepared and characterized. Such chemically modified bacteriophages were completely inactivated by the specific anti-ABA serum and this inactivation was inhibited by free ABA. The identification and quantitative determination of ABA in plant extracts by this method are demonstrated and the method is compared with a common bioassay.     

Simultaneous determination of two antioxidants, uric and ascorbic acid, in animal tissue by high-performance liquid chromatography.

A rapid and simple method for the simultaneous analysis of uric and ascorbic acid in extracts of animal tissue is described. The method uses reversed-phase ion-pair chromatography with ultraviolet detection. The technique allows efficient separation of both acids while showing high selectivity, recovery, reproducibility, and sample stability. Calculated levels of both substances in mouse liver tissue were 1.00 +/- 0.05 mumol ascorbic acid/g and 130 +/- 5 nmol uric acid/g.     

Hydroxylated metabolites of the antimalarial drug primaquine. Oxidation and redox cycling.

Oxidation and redox cycling of the hydroxylated metabolites of the antimalarial drug primaquine (i.e. 5-hydroxyprimaquine, 5-hydroxydemethylprimaquine, and 5,6-dihydroxy-8-aminoquinoline) were studied. The three metabolites readily oxidized under physiological conditions, forming hydrogen peroxide and the corresponding quinone-imine derivatives as the main products. The latter compounds were characterized by visible, NMR, and infrared spectroscopy. Concomitant formation of drug-derived radicals and hydroxyl radicals was attested by direct and spin-trapping EPR experiments, respectively. The use of the spin stabilization method indicated that the radicals derived from 5-hydroxydemethylprimaquine and 5,6-dihydroxy-8-aminoquinoline are of the o-semiquinone type. Tentative structures are proposed for the radicals based on product identification and computer simulation of the experimental EPR spectra. The quinone-imines obtained from the reduced metabolites did not react at appreciable rates with NADPH but underwent redox cycling upon addition of ferredoxin:NADP+ oxidoreductase, forming hydrogen peroxide and hydroxyl radicals. The effect of antioxidant enzymes on hydroxyl radical yield obtained during oxidation and redox cycling indicates that the main route for hydroxyl radical formation is the metal ion-catalyzed reaction between the drug-derived radicals and hydrogen peroxide. Taken together, the results indicate that hydrogen peroxide is the potential toxic product formed from the primaquine metabolites.     

Simultaneous determination of dopamine, ascorbic acid, and uric acid using carbon ionic liquid electrode

A recently constructed carbon composite electrode using room temperature ionic liquid as pasting binder was employed as a novel electrode for sensitive, simultaneous determination of dopamine (DA), ascorbic acid (AA), and uric acid (UA). The apparent reversibility and kinetics of the electrochemical reaction for DA, AA, and UA found were improved significantly compared to those obtained using a conventional carbon paste electrode. The results show that carbon ionic liquid electrode (CILE) reduces the overpotential of DA, AA, and UA oxidation, without showing any fouling effect due to the deposition of their oxidized products. In the case of DA, the oxidation and reduction peak potentials appear at 210 and 135mV (vs Ag/AgCl, KCl, 3.0M), respectively, and the CILE shows a significantly better reversibility for dopamine. The oxidation peak due to the oxidation of AA occurs at about 60mV. For UA, a sharp oxidation peak at 340mV and a small reduction peak at 250mV are obtained at CILE. Differential pulse voltammetry was used for the simultaneous determination of ternary mixtures of DA, AA, and UA. Relative standard deviation for DA, AA, and UA determinations were less than 3.0% and DA, AA, and UA can be determined in the ranges of 2.0x10(-6)-1.5x10(-3), 5.0x10(-5)-7.4x10(-3), and 2.0x10(-6)-2.2x10(-4)M, respectively. The method was applied to the determination of DA, AA, and UA in human blood serum and urine samples.     

Simultaneous determination of l- and d-lactic acid in plasma by capillary electrophoresis

A novel method for simultaneous determination of d- and l-lactic acids in plasma was presented by capillary electrophoresis with photodiode array detection at 195nm. The separation was performed in an uncoated fused-silica capillary. The parameters influencing the resolution and the migration time of lactic acids were optimized. When 150mM phosphate-Tris buffer (pH 7.0) consisting of 220mM 2-hydroxypropyl-beta-cyclodextrin and 0.2mM tetradecyltrimethylammonium bromide was utilized as the running buffer, highly effective chiral separation of d- and l-lactic acids was achieved at about 42min at an effective voltage of -25kV. The resolution of lactic acid enantiomers was >/=1.25. The limits of detection of d- and l-lactic acids in standard solution without any pretreatment were 80 and 50muM (S/N=3), respectively. Sample pretreatment was preceded by protein-removal procedure with acetonitrile. With a pre-concentration procedure by 10 times, the limits of detection of d- and l-lactic acids were 20 and 15muM (S/N=10), respectively. The satisfactory analytical performance of the proposed method was validated.     

Simultaneous determination of procaine and para-aminobenzoic acid by LC-MS/MS method

A sensitive high performance liquid chromatography tandem mass spectrometry (LC-MS/MS) method has been developed for simultaneous determination of procaine and its metabolite p-aminobenzoic acid (PABA). N-Acetylprocainamide (NAPA) was used as an internal standard for procaine and PABA analysis. This assay method has also been validated in terms of linearity, lower limit of detection, lower limit of quantitation, accuracy and precision as per ICH guidelines. Chromatography was carried out on an XTerra MS C(18) column and mass spectrometric analysis was performed using a Quattro Micro mass spectrometer working with electro-spray ionization (ESI) source in the positive ion mode. Enhanced selectivity was achieved using multiple reaction monitoring (MRM) functions, m/z 237-->100, m/z 138-->120, and m/z 278-->205 for procaine, PABA and NAPA, respectively. Retention times for PABA, procaine and NAPA were 4.0, 4.7 and 5.8min, respectively. Linearity for each calibration curve was observed across a range from 100nM to 5000nM for PABA, and from 10nM to 5000nM for procaine. The intra- and inter-day relative standard deviations (RSD) were <5%.