Effect of variable glutathione peroxidase activity on H2O2-related cytotoxicity in cultured aortic endothelial cells

Primary cultures of porcine aortic endothelial cells were used to assess the effects of O2 intermediates produced by 10-40 mU/ml xanthine oxidase (XO; +2 mM hypoxanthine) or 25-100 mU/ml glucose oxidase (GO; +5 mM glucose). A 60-min incubation in the presence of the enzyme systems resulted in a dose-dependent toxic effect with evidence of cytolysis (increased LDH release) and cell loss (decrease in DNA and protein content), when these indexes were measured 24 hr after completion of the enzyme reaction. Decreased [3H]thymidine incorporation into DNA was the most sensitive index of cell dysfunction for both enzyme systems. The effects of various scavengers and enzymes indicated that H2O2 was the main O2 intermediate involved in the cytotoxicity resulting from the XO-hypoxanthine reaction. Increased glutathione peroxidase activity associated with the addition of 2 X 10(-7) M selenomethionine to culture medium had a partial protective effect which could be related to an increased rate of H2O2 degradation. Evidence for increased DNA synthesis after injury was found in cells previously exposed to XO-hypoxanthine, the degree of increase in [3H]thymidine incorporation being dependent on the intensity of the initial cytotoxicity. Cultured endothelial cells provide a useful tool to evaluate the role of O2 intermediates in endothelial cell injury, to test the effects of protective agents, and to study the repair process.     

Increased glucose metabolism by enzyme-loaded erythrocytes in vitro and in vivo normalization of hyperglycemia in diabetic mice.

The main metabolic properties of human red blood cells (RBC) overloaded with glucose catabolizing enzymes such as hexokinase and glucose oxidase were evaluated. Human erythrocytes loaded with human hexokinase metabolized 3.1 +/- 0.2 mumol/h/ml RBC of glucose, an amount double that consumed by normal and unloaded cells (1.46 +/- 0.16 mumol/h/ml RBC), while glucose oxidase-loaded erythrocytes consumed up to 5.5 +/- 0.5 mumol/h/ml RBC of glucose but with a time-dependent increase in methemoglobin formation due to the H2O2 produced in the glucose oxidase reaction. This methemoglobin production was greatly reduced while glucose consumption was increased (8.1 +/- 0.4 mumol/h/ml RBC) by coentrapment of hexokinase and glucose oxidase. Similar results were obtained in mouse red blood cells, although the role of hexokinase was less pronounced due to a higher basal level of this enzyme. When administered to diabetic mice the hexokinase/glucose oxidase-overloaded erythrocytes had a circulating half-life of 5 days and were able to regulate blood glucose at near physiological levels. A single intraperitoneal administration of 500 microliters of enzyme-loaded cells maintained a near-normal blood glucose concentration for 7 +/- 1 days, while repeated administrations at 10-day intervals were effective in the regulation of blood glucose levels for several weeks. These results suggest that enzyme-loaded erythrocytes can behave as circulating bioreactors and can provide a new way to reduce abnormally elevated blood glucose.     

Endogenous defenses against the cytotoxicity of hydrogen peroxide in cultured rat hepatocytes

The catalase activity of cultured rat hepatocytes was inhibited by 90% pretreatment with 20 mM aminotriazole without effect on the activities of glutathione peroxidase or glutathione reductase, or on the viability of the cells over the subsequent 24 h. Glutathione reductase was inhibited by 85% by pretreatment with 300 microM 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) without effect on glutathione peroxidase, catalase, or on viability. Both pretreatments sensitized the hepatocytes to the cytotoxicity of H2O2 generated either by glucose oxidase (0.05-0.5 units/ml) or by the autoxidation of the one-electron-reduced state of menadione (50-250 microM). Aminotriazole pretreatment had no effect on the GSH content of the hepatocytes. BCNU reduced GSH levels by 50%. Depletion of GSH levels to less than 20% of control by treatment with diethyl maleate, however, did not sensitize the cells to either glucose oxidase or menadione, indicating that the effect of BCNU is related to inhibition of the GSH-GSSG redox cycle rather than to the depletion of GSH. With glucose oxidase, most of the cell killing in hepatocytes pretreated with either aminotriazole or BCNU occurred between 1 and 3 h. The antioxidant diphenylphenylenediamine (DPPD) had no effect on viability at 3 h. Catalase added to the culture medium 1 h after the addition of glucose oxidase prevented the cell killing measured at 3 h. The sulfhydryl reagents dithiothreitol (200 microM), N-acetyl-L-cysteine (4 mM), and alpha-mercaptopropionyl-L-glycine (2.5 mM) prevented the cell killing with exogenous H2O2 in hepatocytes sensitized by the inhibition of catalase or glutathione reductase. With menadione, there was no killing of nonpretreated hepatocytes at 1 h, and DPPD did not prevent the cell death after 3 h. Aminotriazole pretreatment enhanced the cell killing at 3 h but not at 1 h, and DPPD was not protective. Catalase added to the medium at 1 h inhibited the cell death measured at 3 h. In contrast, menadione killed hepatocytes pretreated with BCNU within 1 h. DPPD prevented cell death at 1 h, and there was evidence of lipid peroxidation in the accumulation of malondialdehyde in the culture medium. Catalase added with menadione did not prevent the cell killing at 1 h but did prevent it at 3 h. These data indicate that catalase and the GSH-GSSG cycle are active in the defense of hepatocytes against the toxicity of H2O2.(ABSTRACT TRUNCATED AT 400 WORDS)     

Mechanisms of the killing of cultured hepatocytes by hydrogen peroxide

Mechanisms of H2O2-induced cell injury were explored in primary cultures of rat hepatocytes. Cells prepared from male rats and cultured for 1 day prior to treatment were killed by H2O2 either added directly to the medium at 0.25-2 mM or generated in situ by glucose oxidase (0.25-2 U/ml) or xanthine oxidase (20-120 mM/ml) and 2 mM xanthine. Catalase protected the cells in each case. Lipid peroxidation as measured by the accumulation of malondialdehyde (MDA) preceded the cell death due to H2O2 added directly to the cultures or generated in the medium. The antioxidants N,N'-diphenyl-p-phenylenediamine (DPPD) and promethazine prevented the accumulation of MDA in both cases and protected the cells treated with H2O2 directly. DPPD and promethazine did not react directly with H2O2. Other antioxidants including butylated hydroxytoluene, vitamin E, and N-propylgallate had varied protective activity against the addition of H2O2 in proportion to their ability to reduce MDA accumulation. In glucose oxidase-treated cultures, DPPD and promethazine prevented the cell killing during the first hour but failed to protect between 1 and 3 h despite prevention of lipid peroxidation. The cell killing between 1 and 3 h in the presence of DPPD was prevented by catalase indicating its dependence upon continued generation of H2O2. Further addition of H2O2 in the presence of DPPD also increased the number of dead cells without lipid peroxidation. The data are consistent with at least two mechanisms of hepatocyte killing by H2O2. The first pathway is prevented by the antioxidants DPPD and promethazine and is very likely related to the peroxidation of membrane phospholipids. The second is independent of lipid peroxidation yet dependent upon the continued presence of H2O2.     

Autocrine regulation of glucose metabolism in pancreatic islets

We evaluated the possible autocrine modulatory effect of insulin on glucose metabolism and glucose-induced insulin secretion in islets isolated from normal hamsters. We measured 14CO2 and 3H2O production from d-[U-14C]glucose and d-[5-3H]glucose, respectively, in islets incubated with 0.6, 3.3, 8.3, and 16.7 mM glucose alone or with 5 or 15 mU/ml insulin, anti-insulin guinea pig serum (1:500), 25 microM nifedipine, or 150 nM wortmannin. Insulin release was measured (radioimmunoassay) in islets incubated with 3.3 or 16.7 mM glucose with or without 75, 150, and 300 nM wortmannin. Insulin significantly enhanced 14CO2 and 3H2O production with 3.3 mM glucose but not with 0.6, 8.3, or 16.7 mM glucose. Addition of anti-insulin serum to the medium with 8.3 and 16.7 mM glucose decreased 14CO2 and 3H2O production significantly. A similar decrease was obtained in islets incubated with 8.3 and 16.7 mM glucose and wortmannin or nifedipine. This latter effect was reversed by adding 15 mU/ml insulin to the medium. Glucose metabolism was almost abolished when islets were incubated in a Ca2+-deprived medium, but this effect was not reversed by insulin. No changes were found in 14CO2 and 3H2O production by islets incubated with 3.3 mM glucose and anti-insulin serum, wortmannin, or nifedipine in the media. Addition of wortmannin significantly decreased insulin release induced by 16.7 mM glucose in a dose-dependent manner. Our results suggest that insulin exerts a physiological autocrine stimulatory effect on glucose metabolism in intact islets as well as on glucose-induced insulin release. Such an effect, however, depends on the glucose concentration in the incubation medium.     

Evidence for the presence of glucose cycling in pancreatic islets of the ob/ob mouse

Pancreatic islets from ob/ob mice incubated with 3H2O and 5.5 mM glucose formed 3H-labeled glucose, 74 picoatoms incorporated/islet/h. Sixty-three percent of the 3H was bound to carbon 2 of the glucose. The amount of glucose-6-P dephosphorylated to glucose, determined from this incorporation, was 48 pmol/islet/h. Glucose utilization, measured by the formation of 3H2O from [5-3H]glucose, was 72 pmol/islet/h. The amount of glucose dephosphorylated was then about 40% of that phosphorylated. Thus, glucose-6-P is dephosphorylated to glucose to a significant extent by intact islets in vitro and presumably by the beta cells of the islets. The extent of this glucose cycling, i.e. glucose----glucose-6-P----glucose, may play a role in determining the extent of glucose-induced insulin secretion.     

Influence of exogenous iron and ascorbate on H2O2-induced glutathione oxidation in red cells

The effective fall in cytosolic reduced glutathione levels in intact red cells exposed to exogenous oxidant stress in the form of Fe2+, H2O2 and ascorbate was caused by H2O2 alone. Relatively high concentrations of Fe2+ had no contributory effect on the oxidizing capacity of H2O2. Ascorbate, at physiological levels, showed no protection whereas glucose was totally protective. Since glucose, via hexose monophosphate shunt, is the only source of reducing equivalent in red cells, the NADPH/NADP+ redox role in the diminution of intracellular reduced glutathione.     

Hypoxia increases glutathione redox cycle and protects rat lungs against oxidants

Preexposure to hypoxia increased survival and lung reduced glutathione-to-oxidized glutathione ratios (GSH/GSSG) and decreased pleural effusions in rats subsequently exposed to continuous hyperoxia. In addition, lungs from hypoxia-preexposed rats developed less acute edematous injury (decreased lung weight gains and lung lavage albumin concentrations) than lungs from normoxia-preexposed rats when isolated and perfused with hydrogen peroxide (H2O2) generated by xanthine oxidase (XO) or glucose oxidase (GO). In contrast, when perfused with elastase or exposed to a hydrostatic left atrial pressure challenge, lungs isolated from hypoxia-preexposed rats developed the same acute edematous injury as lungs from normoxia-preexposed rats. The mechanism by which hypoxia preexposure conferred protection against H2O2 appeared to depend on hexose monophosphate shunt (HMPS)-dependent increases in lung glutathione redox cycle activity. First, before perfusion with GO, lungs from hypoxia-preexposed rats had increased glutathione peroxidase and glucose 6-phosphate dehydrogenase (but not catalase or glutathione reductase) activities compared with lungs from normoxia-preexposed rats. Second, after perfusion with GO, lungs from hypoxia-preexposed rats had increased H2O2 reducing equivalents, as reflected by increased GSH/GSSG and NADPH/NADPH+, compared with lungs from normoxia-preexposed rats. Third, pretreatment of rats with an HMPS inhibitor, (6-aminonicotinamide) or a glutathione reductase inhibitor, [1,3-bis(2-chloroethyl)-1-nitrosourea] prevented hypoxia-conferred protection against H2O2-mediated acute edematous injury in isolated lungs. These findings suggest that increased detoxification of H2O2 by glutathione redox cycle and HMPS-dependent mechanisms contributes to tolerance to hyperoxia and resistance to H2O2 of lungs from hypoxia-preexposed rats.     

Effects of reactive oxygen species on prostacyclin production in perinatal rat lung cells

A differentiation-arrested primary cell culture model was used to examine the role of reactive oxygen species in the control of prostacyclin (PGI2) production in the perinatal rat lung. Coincubation of the lung cells with arachidonic acid (AA) and xanthine (X, 0.25 mM) plus xanthine oxidase (XO, 10 mU/ml) or with AA and glucose (25 mM) plus glucose oxidase (25 mU/ml) augmented the AA-induced PGI2 output. Superoxide dismutase (10 U/ml) did not alter the X + XO effect, whereas catalase (10 U/ml) eliminated both X + XO and glucose plus glucose oxidase effects. H2O2 (1-200 microM) showed a dose-related biphasic augmentation with peak stimulation at 20 microM. Catalase again blocked this effect, but dimethylthiourea, a hydroxyl radical scavenger, did not. A 20-min pretreatment of the cells with X + XO, glucose plus glucose oxidase, or H2O2, however, diminished the capacity of the cells to convert exogenous AA to PGI2. This pretreatment effect was also blocked by catalase. The responses were similar in lung cells obtained from day 20 rat fetuses (term = 22 days) and 1-day-old newborn rats. Lactate dehydrogenase release was not detected during treatment periods but increased significantly after exposure to reactive oxygen species.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of reactive oxygen metabolites on erythropoietin production in renal carcinoma cells

The present studies were undertaken to determine the effects of reactive oxygen metabolites on erythropoietin (Ep) biosynthesis in Ep-producing renal carcinoma (RC) cells using a sensitive radioimmunoassay for Ep. Xanthine (10-5M) and increasing concentrations of xanthine oxidase (8 x 10(-7) to 5 x 10(-4) units/ml) produced a significant dose-related increase in Ep production at a concentration of greater than or equal to 4 x 10(-6) units/ml, whereas xanthine alone had no effect. Catalase, a scavenger of hydrogen peroxide (H2O2), in concentrations of 50 to 500 micrograms/ml produced a significant inhibition of the increase in Ep production induced by xanthine-xanthine oxidase; while no effect was seen on basal levels of Ep production and the growth of RC cells. Glucose oxidase (greater than or equal to 0.032 mU/ml), a direct H2O2 generator, and exogenous H2O2 (greater than or equal to 4 x 10(-6)M) added to the incubation mixture, caused a significant enhancement of Ep production in a dose-dependent manner. Xanthine-xanthine oxidase, glucose oxidase, and H2O2 in the above concentrations did not produce significant cytotoxicity (51Cr release or trypan blue dye exclusion). The present data suggests that H2O2, a reactive oxygen metabolite may play a significant role in Ep production.     

Redox labelled avidin for enzyme sensor architectures

Conjugates of avidin with ferrocene and with microperoxidase 8 have been used as electrochemically active molecular building blocks. Assemblies of the conjugates with biotinylated glucose oxidase or lactate oxidase on gold electrodes were tested as enzyme sensors for glucose and lactate. The electrochemical detection is based either on ferrocene-mediated oxidation of the substrate in oxygen-free solution, or on microperoxidase-catalysed reduction of H2O2 which is enzymatically produced from the substrate and molecular oxygen. Glucose and lactate were detectable with both detection principles in concentrations down to 1 or 0.1 mM, respectively. The molecular architecture concept allows quick adaptation of the sensors to other analytes, and it provides a platform for arrays of sensors with different selectivity.     

The relationship between a novel NAD(P)H oxidase activity of ovoperoxidase and the CN- -resistant respiratory burst that follows fertilization of sea urchin eggs

The extracellular protein coat of the sea urchin egg is cross-linked after fertilization via dityrosyl linkages made by an exocytosed ovoperoxidase. The source of oxidant for this reaction is unknown, but eggs produce H2O2 in amounts equivalent to the cyanide-insensitive O2 uptake "respiratory burst" that follows fertilization. Several possible H2O2-forming oxidase activities, including glucose, xanthine, fatty acyl, and fatty-acyl CoA oxidases, were absent from the egg cortex. However, an NAD(P)H-O2 oxidoreductase activity was found in the egg cortex and was completely accounted for by ovoperoxidase. Homogeneous ovoperoxidase exhibits two types of NAD(P)H oxidase activity. One of these activities is similar to that of horseradish peroxidase and lactoperoxidase; it is dependent on Mn2+ ions and catalytic amounts of phenols, such as 2,4-dichlorophenol and N-acetyltyrosinamide, and is greater than 95% inhibited by 0.1 mM cyanide. A second, novel oxidase activity utilizes Ca2+ and an unidentified, heat-stable, Mr less than 1000 factor that can be extracted by ethanol from egg homogenates. This NADH oxidase activity is only 40% inhibited by 0.1 mM cyanide and is maximally stimulated by 10 mM Ca2+. It has an apparent Km for NADH of 50 microM. The stoichiometry of NADH:O2 consumption is 1.6:1, but approaches 2:1 in the presence of 20 micrograms/ml superoxide dismutase or 200 micrograms/ml catalase. This indicates that complete reduction of O2 to water occurs and that the reaction does not produce H2O2 stoichiometrically. However, nearly complete inhibition of the reaction by higher catalase concentrations suggests that H2O2 is an intermediate. The properties of this novel oxidase activity suggest that it may play such a role in vivo.     

The effect of selenium-deficiency on rat fat-cell glucose oxidation.

When rats are fed a selenium-deficient diet, the glutathione peroxidase activity of epididymal fat-cells decreases to 5-9% of that of control rats fed the same diet supplemented with 0.5 p.p.m. of selenium as sodium selenite. [1-14C]Glucose oxidation in fat-cells from rats fed a selenium-deficient diet is unresponsive to the action of t-butyl hydroperoxide, which stimulates 14CO2 formation from [1-14C]glucose 4-fold in control rats. Insulin enhances [1-14C]glucose oxidation and incorporation into lipids in fat-cells from both groups of rats; however, the response elicited is reduced in fat-cells prepared from selenium-deficient animals. The 'C-1/C-6 ratio' (ratio of glucose C-1 to glucose C-6 oxidized) is enhanced by insulin to a similar degree in fat-cells from both groups of animals. The stimulatory action of Zn2+ and dithiothreitol on [1-14C]glucose oxidation observed in fat-cells from selenium-supplemented rats is greatly reduced in fat-cells from selenium-deficient rats. [1-14C]Glucose oxidation in fat-cells from both groups of animals is highly sensitive to the stimulatory action of adenosine. It is concluded that the enhanced formation and glutathione-linked destruction of H2O2 plays, at the most, only a minor role in the stimulation of the flux of glucose through the pentose phosphate pathway elicited by insulin, although elimination of glutathione peroxidase activity may influence the action of insulin on glucose oxidation. Production and subsequent destruction of H2O2 may play an important role in the stimulatory action of Zn2+ and dithiothreitol on fat-cell [1-14C]glucose oxidation.     

Effects of menadione and hydrogen peroxide on glutathione status in growing Escherichia coli

Menadione (MD) and H2O2 caused distinct effects on glutathione status in growing Escherichia coli. Treatment of E. coli AB1157 with 1-25 mM H2O2 did not result in an appreciable decrease in intracellular total glutathione (reduced glutathione [GSH] + oxidized glutathione [GSSG]). Only when cells were treated with 25 mM H2O2 an increase in GSSG and a decrease in the GSH:GSSG ratio were observed. In cells deficient in catalase HPI, such effect was observed even at 10 mM H2O2. The exposure of E. coli AB1157 to MD caused a dose-dependent decrease in intracellular total glutathione, an increase in GSSG, and a decrease in the ratio of GSH:GSSG. In E. coli deficient in cytosolic superoxide dismutase activity, a decrease in total glutathione after incubation with 0.2 mM MD was not accompanied by an increase in GSSGin, and the ratio of GSHin:GSSGin was three times higher than in the wild-type cells. The changes in the redox status of extracellular glutathione under the action of both oxidants were similar. Although the catalase activity increased several times after exposure to both oxidants, there were little or no changes in the activity of enzymes related to glutathione metabolism. A possible role of changes in redox status of glutathione under oxidative stress is discussed.     

Inhibitory and enhancing effects of NO on H(2)O(2) toxicity: dependence on the concentrations of NO and H(2)O(2)

Nitric oxide (NO) has been shown to both enhance hydrogen peroxide (H(2)O(2)) toxicity and protect cells against H(2)O(2) toxicity. In order to resolve this apparent contradiction, we here studied the effects of NO on H(2)O(2) toxicity in cultured liver endothelial cells over a wide range of NO and H(2)O(2) concentrations. NO was generated by spermine NONOate (SpNO, 0.001-1 mM), H(2)O(2) was generated continuously by glucose/glucose oxidase (GOD, 20-300 U/l), or added as a bolus (200 microM). SpNO concentrations between 0.01 and 0.1 mM provided protection against H(2)O(2)-induced cell death. SpNO concentrations >0.1 mM were injurious with low H(2)O(2) concentrations, but protective at high H(2)O(2) concentrations. Protection appeared to be mainly due to inhibition of lipid peroxidation, for which SpNO concentrations as low as 0.01 mM were sufficient. SpNO in high concentration (1 mM) consistently raised H(2)O(2) steady-state levels in line with inhibition of H(2)O(2) degradation. Thus, the overall effect of NO on H(2)O(2) toxicity can be switched within the same cellular model, with protection being predominant at low NO and high H(2)O(2) levels and enhancement being predominant with high NO and low H(2)O(2) levels.     

Specific biomarkers for stochastic division patterns and starvation-induced quiescence under limited glucose levels in fission yeast

Glucose as a source of energy is centrally important to our understanding of life. We investigated the cell division-quiescence behavior of the fission yeast Schizosaccharomyces pombe under a wide range of glucose concentrations (0-111 mM). The mode of S. pombe cell division under a microfluidic perfusion system was surprisingly normal under highly diluted glucose concentrations (5.6 mM, 1/20 of the standard medium, within human blood sugar levels). Division became stochastic, accompanied by a curious division-timing inheritance, in 2.2-4.4 mM glucose. A critical transition from division to quiescence occurred within a narrow range of concentrations (2.2-1.7 mM). Under starvation (1.1 mM) conditions, cells were mostly quiescent and only a small population of cells divided. Under fasting (0 mM) conditions, division was immediately arrested with a short chronological lifespan (16 h). When cells were first glucose starved prior to fasting, they possessed a substantially extended lifespan (～14 days). We employed a quantitative metabolomic approach for S. pombe cell extracts, and identified specific metabolites (e.g. biotin, trehalose, ergothioneine, S-adenosyl methionine and CDP-choline), which increased or decreased at different glucose concentrations, whereas nucleotide triphosphates, such as ATP, maintained high concentrations even under starvation. Under starvation, the level of S-adenosyl methionine increased sharply, accompanied by an increase in methylated amino acids and nucleotides. Under fasting, cells rapidly lost antioxidant and energy compounds, such as glutathione and ATP, but, in fasting cells after starvation, these and other metabolites ensuring longevity remained abundant. Glucose-starved cells became resistant to 40 mM H(2)O(2) as a result of the accumulation of antioxidant compounds.     

Purification and properties of the membranal thiol oxidase from porcine kidney

A thiol oxidase was purified from porcine kidney cortex by chromatography of detergent-solubilized plasma membranes on cysteinylsuccinamidopropyl-glass beads, hydroxyapatite, and Sephacryl S-200. The oxidase was purified 2600-fold; 28% recovery of activity was obtained. With glutathione as substrate, the apparent Km was 0.73 mM and the V max was a 4.4 U/mg protein. The reaction catalyzed was 2 RSH + O2----RSSR + H2O2, and superoxide production was not detected during the reaction. Other low molecular weight thiols, including cysteine, dithiothreitol, N-acetylcysteine, and cysteamine, were substrates for the oxidase; 2-mercaptoethanol, reductively denatured ribonuclease A, and chymotrypsinogen A were not substrates. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed one band corresponding to 70 kDa; gel filtration on a Sephacryl column produced a single elution of activity with a protein corresponding to 120 kDa, indicating that the functional form is a dimer. On a high-pressure gel permeation column the protein eluted at 70 kDa under dilute conditions but at greater than 200 kDa at high concentrations, indicating that the protein also aggregates into larger multimers. Activity was inhibited by copper chelators, L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (acivicin), H2O2, and N-ethylmaleimide, suggesting the presence of copper and a sulfhydryl group at the active site. Following treatment with metal chelators, enzyme activity was reconstituted with CuSO4, but not with FeSO4. The purified enzyme contained 1 mol copper per subunit which was undetectable by electron paramagnetic resonance, suggesting that the copper is in a binuclear complex.     

Oxidant stress and skeletal muscle glucose transport: roles of insulin signaling and p38 MAPK

Oxidative stress can impact the regulation of glucose transport activity in a variety of cell lines. In the present study, we assessed the direct effects of an oxidant stress on the glucose transport system in intact mammalian skeletal muscle preparations. Type IIb (epitrochlearis) and type I (soleus) muscles from insulin-sensitive lean Zucker rats were incubated in 8 mM glucose for 2 h in the absence or presence of 100 mU/ml glucose oxidase to produce the oxidant hydrogen peroxide (H(2)O(2)) (60-90 microM). Glucose transport, glycogen synthase activity, and metabolic signaling factors were then assessed. H(2)O(2) significantly (p < 0.05) activated basal glucose transport and glycogen synthase activities and increased insulin receptor tyrosine phosphorylation, insulin receptor substrate-1 associated with the p85 subunit of phosphatidylinositol-3' kinase (PI3-kinase), and Ser(473) phosphorylation of Akt in both muscle types. This induction of glucose transport by the oxidant stress was prevented by the PI3-kinase inhibitor wortmannin. The oxidant stress also significantly increased phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) and 5'-AMP-dependent protein kinase. Interestingly, selective inhibition of p38 MAPK using A304000 substantially reduced the activation of glucose transport induced by the oxidant stress. These results support a direct role for oxidative stress in the activation of the glucose transport system in mammalian skeletal muscle and indicate that this process involves engagement of and possible interactions between the PI3-kinase-dependent signaling pathway and activation of p38 MAPK.     

Involvement of hydrogen peroxide and nitric oxide in salt resistance in the calluses from Populus euphratica

Nitric oxide (NO) and hydrogen peroxide (H2O2) function as signalling molecules in plants under abiotic and biotic stresses. Calluses from Populus euphratica, which show salt tolerance, were used to study the interaction of NO and H2O2 in plant adaptation to salt resistance. The nitric oxide synthase (NOS) activity was identified in the calluses, and this activity was induced under 150 mM NaCl treatment. Under 150 mM NaCl treatment, the sodium (Na) percentage decreased, but the potassium (K) percentage and the K/Na ratio increased in P. euphratica calluses. Application of glucose/glucose oxidase (G/GO, a H2O2 donor) and sodium nitroprusside (SNP, a NO donor) revealed that both H2O2 and NO resulted in increased K/Na ratio in a concentration-dependent manner. Diphenylene iodonium (DPI, an NADPH oxidase inhibitor) counteracted H2O2 and NO effect by increasing the Na percentage, decreasing the K percentage and K/Na ratio. NG-monomethyl-L-Arg monoacetate (NMMA, an NO synthase inhibitor) and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxyde (PTIO, a specific NO scavenger) only reversed NO effect, but did not block H2O2 effect. The increased activity of plasma membrane (PM) H+ -ATPase caused by salt stress was reversed by treatment with DPI and NMMA. Exogenous H2O2 increased the activity of PM H+ -ATPase, but the effect could not be diminished by NMMA and PTIO. The NO-induced increase of PM H+ -ATPase can be reversed by NMMA and PTIO, but not by DPI. Western blot analysis demonstrated that NO and H2O2 stimulated the expression of PM H+ -ATPase in P. euphratica calluses. These results indicate that NO and H2O2 served as intermediate molecules in inducing salt resistance in the calluses from P. euphratica under slat stress by increasing the K/Na ratio, which was dependent on the increased PM H+ -ATPase activity.     

Activation of soluble guanylate cyclase from rat lung by incubation or by hydrogen peroxide.

A 37,000 X g supernatant fraction prepared from fat lung homogenate demonstrated a 2- to 3-fold increase in guanylate cyclase activity after incubation at 30 degrees for 30 min (preincubation). Treatment of the supernatant fraction with Triton X-100 increased activity to approximately the same extent as preincubation, but would not increase the activity after preincubation. By chromatography on Sepharose 2B, before and after preincubation, it was demonstrated that the increase in activity was only associated with the soluble guanylate cyclase, and not the particulate enzyme. Activation by preincubation required O2. It was completely inhibited by thiols such as 2-mercaptoethanol, and by bovine serum albumin, KCN, and sodium diethyldithiocarbamate. These inhibitors suggested a copper requirement for activation, and this was confirmed by demonstrating that 20 to 60 muM CuCl2 could relieve the inhibition by 0.1 mM sodium diethyldithiocarbamate. 2-Mercaptoethanol inhibition could also be reversed by removal of the thiol on a Sephadex G-25 column, however, this treatment partially activated the enzyme. Addition of 2-mercaptoethanol to a preincubated preparation would not reverse the activation. H2O2 was found to activate guanylate cyclase, either by its generation in the lung supernatant with glucose oxidase and glucose, or by its addition to a preparation in which the catalase was inhibited with KCN. KCN or bovine serum albumin was able to partially inhibit activation by glucose oxidase plus glucose, however, larger amounts of glucose oxidase could overcome that inhibition, indicating a catalytic role for Cu2+ at low H2O2 concentrations. No direct evidence for H2O2 formation during preincubation could be found, however, indirect evidence was obtained by the spectrophotometric detection of choleglobin formation from hemoglobin present in the lung supernatant fluid. The H2O2 is believed to result from the reaction of oxyhemoglobin with ascorbate.