Low Concentrations Of Hydrogen Peroxide Research Articles

Increase in mitochondrial mass in human fibroblasts under oxidative stress and during replicative cell senescence.

Abnormal proliferation of mitochondria generally occurs in muscle of aged individuals and patients with mitochondrial myopathy. An increase in the mitochondrial DNA (mtDNA) copy number has also been observed in aging human tissues. However, the molecular mechanism underlying the increase in mitochondrial mass and mtDNA is still unclear. In a previous study, we demonstrated that sublethal levels of oxidative stress caused an increase in mitochondrial mass in human lung cells. In this communication, we report our recent findings that the mitochondrial mass in human lung fibroblasts (MRC-5) in a later proliferation stage is significantly increased compared to that in the early stages of proliferation. The extent of the increase in mitochondrial mass in the senescent cells was similar to that in cells in the early stages of proliferation that had been treated with low concentrations (< or = 180 microM) of hydrogen peroxide (H(2)O(2)). Moreover, we found that the rate of reactive oxygen species (ROS) production was higher in cells in the later proliferation stage compared to cells in the early proliferation stages. A similar phenomenon was also observed in cells in the early proliferation stages under low levels of oxidative stress. On the other hand, the mRNA levels of many nuclear DNA-encoded proteins involved in mitochondrial biogenesis, particularly nuclear respiratory factor-1, were found to increase in cells in later proliferation stages and in cells in early proliferation stages that had been treated with 180 microM H(2)O(2). Interestingly, the increase in mitochondrial mass in the cells under oxidative stress could be repressed by treatment with cycloheximide or m-chlorocarbonyl cyanide phenylhydrazone but not by chloramphenicol. Furthermore, the mitochondrial mass of mtDNA-less rho(o) cells was also significantly increased by exposure to low concentrations (e.g. 180 microM) of H(2)O(2). These results suggest that the increase in mitochondrial mass in replicative senescent cells may result from an increase in ROS production, and that it is dependent on both de novo synthesis of nuclear DNA-encoded proteins and their import into mitochondria, dictated by the membrane potential of mitochondria.

Hydrogen Peroxide and Sodium Perborate Bleaching of Pulp from Olive Tree Residues

This paper will consider the influence of the operating conditions used in the hydrogen peroxide bleaching (concentration 1-5 % and process time 30-210 min) and sodium perborate bleaching (sodium perborate concentration 1-5 %, hydrogen peroxide 0-2% and process time 60-180 min) of olive wood trimmings pulp on the yield, kappa index and viscosity of the resulting pulp, and on strength related properties of paper sheets (stretch index and burst index) in order to determine the best bleaching conditions of this pulp. Medium to low hydrogen peroxide concentrations (1-3 %) and a high operation time (210 min) were desired in the bleaching of pulp. A high sodium perborate concentration and hydrogen peroxide concentration (5 % and 2 % respectively) and medium to low operation time (60-120 min) were desired for the sodium perborate bleaching. A comparison of both bleaching agents, under similar or under optimum operating conditions, revealed that sodium perborate bleaching results in lower brightness, a higher kappa index and also higher viscosity than hydrogen peroxide bleaching. Moreover, both provided similar stretch index and burst index values for sodium perborate bleaching with respect to hydrogen peroxide bleaching.

Mechanistic and molecular investigations on stabilization of horseradish peroxidase C.

The enzyme horseradish peroxidase (HRP) shows a decreasing activity when the enzyme's substrate hydrogen peroxide is present with the degree of inactivation being dependent on the incubation time and the hydrogen peroxide concentration. Incubation times of some minutes do not inactivate the enzyme independent of the H2O2 concentration. After several hours, only 50% of the activity is found for a medium H2O2 excess, and a >100-fold excess of H2O2 completely inactivates the enzyme. Polymeric additives, in particular Gafquat, lead to higher residual activities, whereas stabilizers, such as aminopyrine, preserve the full activity. Circular dichroism (CD) measurements reveal that the enzyme structure remains more or less unchanged when hydrogen peroxide is added. Only when a 1000-fold excess of hydrogen peroxide is present are structural changes observed. UV spectra highlight that the heme group in the enzyme is affected by hydrogen peroxide in a first step. Without any prolonged incubation, a decrease of the Soret band to approximately 50% is found for low hydrogen peroxide concentrations (HRP/H2O2 from 1:1 to 1:100). Higher H2O2 concentrations lead to the formation of catalytically inactive HRP forms. Preincubation of Gafquat, which is a copolymer from vinylpyrrolidone and derivatized methyl methacrylate, with hydrogen peroxide shifts the influence of hydrogen peroxide to higher concentrations, the shift being dependent on the Gafquat concentration. This effect is not observed for other polymers, such as dextrans, but it is also found for the stabilizer aminopyrine. Extended incubation times (24 h) of HRP together with H2O2, however, lead to an at least partial recovery of the Soret band for lower H2O2 concentrations (H2O2/HRP from 1:1 to 1:100). When hydrogen peroxide is used in a >100 fold excess, the heme group is irreversibly destroyed, and even the characteristic band of cpd III is not found. Here, the presence of Gafquat only reduces the degree of destruction. Computer modeling of the interaction between the polymers and the enzyme shows no specific binding sites for the functional groups of the vinylpyrrolidone-methacrylate copolymer Gafquat or of DEAE-dextran on the enzyme, whereas for the only activating polymer, polyethylenimine clustering of binding sites is observed.

The mechanism of action of crosslinking additives in polyethylene vulcanisation: an EPR investigation

The nature and kinetics of the free radicals intermediates in the peroxide induced thermal vulcanisation of low density polyethylene and the mechanism of action of two crosslinking co-agents with the structure of alkenyl aromatic hydrocarbons have been elucidated by EPR spectroscopy. At low peroxide concentration, the crosslinking mechanism in the pure low density polyethylene (LDPE) matrix is based, as expected, on the H abstraction from the chains and the coupling of the resulting macroalkyl radicals. The latter reaction takes place in competition with the disproportionation causing the formation of chain unsaturations that accumulate with the reaction time. At relatively large (>2%) peroxide concentrations, such unsaturations reach high enough levels to scavenge all the initiating and macroalkyl radicals giving macroallyl radicals as the dominant intermediates and determining changes in the structure of the crosslinks during the course of the reaction. At the concentration used, the activities of the co-agents 1,3-diisopropenylbenzene and 2,4-diphenyl-4-methyl-1-pentene are sufficient to scavenge all the initiating and macroalkyl radicals by double bond addition; this mechanism, when involving macroalkyl radicals, leads to the attachment of saturated and unsaturated pendants on the LDPE chains respectively. The unsaturated pendants are thought to be responsible for the crosslinking promotion effect by the 1,3 diisopropenylbenzene coagent. The lack of the second unsaturation causes 2,4-diphenyl-4-methyl-1-pentene to act essentially as a mild antiscorch agent.

Oxalate, germins, and higher-plant pathogens.

Earlier surveys (1, B. G. Lane. [1991] FASEB J. 5, 2983-2901; 2, B. G. Lane. [1994] FASEB J. 8, 294-301) helped to uproot entrenched views of plant oxalate as a static substance. It is now recognized that oxalate oxidases (OXOs) found in the "true cereals" (barley, maize, oat, rice, rye, wheat), the so-called germin OXOs (G-OXOs), or simply germins, are involved in cereal defence responses to invasion by fungal pathogens and that they show promise of being valuable agents of plant defence in dicotyledons, where they are not found naturally. G-OXOs have very peculiar properties: (a) their water-soluble oligomeric structures and enzymic activity are stable during SDS-PAGE and nitrocellulose blotting, (b) their undenatured water-soluble forms are refractory to the action of broad-specificity proteases, (c) their water-insoluble forms occur abundantly (approximately 50%) in the extracellular matrix (cell walls) of wheat, and probably in varying amounts in the cell walls of other true cereals. Transfer of the wheat G-OXO coding element to dicotyledons has been found, in all cases so far examined, to result in improved resistance to fungal pathogens. The possible nature of the improved resistance is discussed in relation to (a) generation of microcidal concentrations of hydrogen peroxide when the G-OXOs act on oxalate, (b) elicitation of hypersensitive cell death at lower concentrations of hydrogen peroxide, (c) formation of effective barriers against predator penetration by the hydrogen-peroxide-mediated lignification of cell walls, and (d) destruction of oxalate, which is an inhibitor of the hypersensitive response, a strategy of particular importance in the case of ubiquitous predator organisms such as Sclerotinia sclerotiorum, which secrete high concentrations of oxalate as a toxin.

PE/wax blends: interesting observations

The thermal and mechanical properties of different cross-linked and uncross-linked polyethylene/wax blends were investigated over a period of time. This paper summarizes the results and observations. It was found that (i) both LLDPE and LDPE cross-links in the presence of low dicumyl peroxide concentrations, while wax only grafts onto the polyethylene chains, (ii) polyethylene and wax have relatively good miscibility, (iii) both wax content and cross-linking change the mechanical properties of polyethylene in one way or the other, (iv) the route of sample preparation has a marked influence on the thermal and mechanical properties of the blends, and (v) oxygen-containing groups in oxidised wax apparently do not change the way in which the wax interacts with polyethylene.

The oxidation of iodine to iodate by hydrogen peroxide

The kinetics of the oxidation of iodine by hydrogen peroxide, a part of the Bray–Liebhafsky reaction, has been studied between 298 and 323 K using a spectrophotometric method. The rate of reaction passes through a maximum when the concentration of hydrogen peroxide increases. At low hydrogen peroxide concentration, the order of reaction with respect to [I2] is roughly one, whereas at high hydrogen peroxide concentration the apparent order is higher and there is no simple kinetic law. This reaction can be observed only with sufficient acid and iodate concentrations. However, its rate does not change much with perchloric acid concentration in the range 0.04 to 0.2 mol dm−3 and iodate concentration in the range 0.005 to 0.05 mol dm−3. The maximum first order rate constant is 8 × 10−3 s−1 at 298 K with an activation energy of 80 kJ mol−1. The results are discussed considering a model proposed previously for the Bray–Liebhafsky reaction.

Ets-2 Is Induced by Oxidative Stress and Sensitizes Cells to H2O2-Induced Apoptosis: Implications for Down's Syndrome

An elevated production of hydrogen peroxide mediates the increased rate of apoptosis of cells derived from individuals with Down's syndrome. The mechanism via which this occurs is unknown. Here we show that Ets-2, a transcription factor located on human chromosome 21 and already overexpressed in multiple tissues in Down syndrome (DS, trisomy 21), is induced by low concentrations of hydrogen peroxide. Moreover, cells with an imbalance in the antioxidant enzymes SOD-1/GPX-1, such as occurs in DS through the overexpression of the chromosome 21 gene SOD-1, also results in increased Ets-2 expression. The increase in Ets-2 expression is dependent on mRNA transcription. Importantly, we further demonstrate that 3T3 fibroblasts that overexpress Ets-2 are sensitized to hydrogen peroxide-induced apoptosis. These data implicate Ets-2 in the regulation of oxidant-induced apoptosis and provide a possible rationale for both the (5- to 7-) fold increase in Ets-2 protein level in DS tissues, above the expected gene dosage of 1.5-fold, and the elevated rate of apoptosis in DS cells.

Kinetics and products of the degradation of chitosan by hydrogen peroxide.

Low concentrations of hydrogen peroxide induced random degradation of partially deacetylated chitin and chitosan. Average molecular weight decreased in accordance with first-order kinetics. The degradation rate was much faster than that of the ultrasonic degradation, and it was comparable to that of the enzymatic hydrolysis of chitosan. Chain-end scissions occurred after chitosan was degraded severely and produced significant amounts of oligosaccharides at temperatures > or =80 degrees C. Universal calibration moderated the change in molecular weight more closely than that calculated by the usual calibration using pullulan standards. Trace amounts of transition metal ions and the amino groups in chitosan were critical to the breakdown of the beta-1,4 glycosidic linkages. HPLC results of glucosamine and chito-oligosaccharides could be characterized by correlating the logarithmic values of retention time with the degrees of polymerization. The formation of glucosamine and chito-oligosaccharides depended on the concentration of H(2)O(2), temperature, and the physicochemical property of chitin/chitosan.

Cyclophilin A Binds to Peroxiredoxins and Activates Its Peroxidase Activity

Six distinct peroxiredoxin (Prx) proteins (Prx I-VI) from distinct genes have been identified in mammalian tissues. Prxs are members of a group of peroxidases that have conserved reactive cysteine residue(s) in the active site(s). An immediate physiological electron donor for the peroxidase catalysis for five Prx proteins (Prx I-V) has been identified as thioredoxin (Trx), but that for Prx VI (1-Cys Prx) is still unclear. To identify an immediate electron donor and a binding protein for Prx VI, we performed a Prx VI protein overlay assay. A 20-kDa binding protein was identified by the Prx VI protein overlay assay with flow-through fractions from a High-Q column with rat lung crude extracts. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) and MS-Fit, we identified the 20-kDa Prx VI-binding protein as a cyclophilin A (CyP-A). The binding of recombinant human CyP-A (hCyP-A) to Prx VI was confirmed by using the hCyP-A protein overlay assay and Western immunoblot analysis with hCyP-A-specific antibodies. hCyP-A enhanced the antioxidant activity of Prx VI, as well as the other known mammalian Prx isotypes. hCyP-A supported antioxidant activity of Prx II and Prx VI both against thiol (dithiothreitol)-containing metal-catalyzed oxidation (MCO) systems and ascorbate-containing MCO systems. Prx II was reduced by hCyP-A without help from any other reductant, and the reduction was cyclosporin A-independent. These results strongly suggest that CyP-A not only binds to Prx proteins but also supports its peroxidase activity as an immediate electron donor. In addition, Cys(115) and Cys(161) of hCyP-A were found to be involved in the activation and the reduction of Prx.

Necrotic cell death by hydrogen peroxide in immortal DF-1 chicken embryo fibroblast cells expressing deregulated MnSOD and catalase

The reactive oxygen species are known as endogenous toxic oxidant damaging factors in a variety of cell types, and in response, the antioxidant genes have been implicated in cell proliferation, senescence, immortalization, and tumorigenesis. The expression of manganese superoxide dismutase mRNA was shown to increase in most of the immortal chicken embryo fibroblast (CEF) cells tested, while expression of catalase mRNA appeared to be dramatically decreased in all immortal CEF cells compared to their primary counterparts. The expression of copper–zinc superoxide dismutase mRNA was shown to increase slightly in some immortal CEF cells. The glutathione peroxidase expressed relatively similar levels in both primary and immortal CEF cells. As primary and immortal DF-1 CEF cells were treated with 10–100 μM of hydrogen peroxide (concentrations known to be sublethal in human diploid fibroblasts), immortal DF-1 CEF cells were shown to be more sensitive to hydrogen peroxide, and total cell numbers were dramatically reduced when compared with primary cell counterparts. This increased sensitivity to hydrogen peroxide in immortal DF-1 cells occurred without evident changes in either antioxidant gene expression, mitochondrial membrane potential, cell cycle distribution or chromatin condensation. However, the total number of dead cells without chromatin condensation was dramatically elevated in immortal DF-1 CEFs treated with hydrogen peroxide, indicating that the inhibition of immortal DF-1 cell growth by low concentrations of hydrogen peroxide is due to increased necrotic cell death, but not apoptosis. Taken together, our observation suggests that the balanced antioxidant function might be important for cell proliferation in response to toxic oxidative damage by hydrogen peroxide.

Synergism between Nitric Oxide and Hydrogen Peroxide in the Inhibition of Platelet Function: The Roles of Soluble Guanylyl Cyclase and Vasodilator-Stimulated Phosphoprotein

In previous studies, a strong synergism between low concentrations of hydrogen peroxide and nitric oxide in the inhibition of agonist-induced platelet aggregation has been established and may be due to enhanced formation of cyclic GMP. In this investigation, hydrogen peroxide and NO had no effect on the activity of pure soluble guanylyl cyclase or its activity in platelet lysates and cytosol. H2O2 was found to increase the phosphorylation of vasodilator-stimulated phosphoprotein (VASP), increasing the amount of the 50-kDa form that results from phosphorylation at serine157. This occurs both in the presence and in the absence of low concentrations of NO, even at submicromolar concentrations of the peroxide, which alone was not inhibitory to platelets. These actions of H2O2 were inhibited to a large extent by an inhibitor of cyclic AMP-dependent protein kinase, even though H2O2 did not increase cyclic AMP. This inhibitor reversed the inhibition of platelets induced by combinations of NO and H2O2 at low concentrations. The results suggest that the action on VASP may be one site of action of H2O2 but that this event alone does not lead to inhibition of platelets; another unspecified action of NO is required to complete the events required for inhibition.

Down-regulation of Intrinsic P-glycoprotein Expression in Multicellular Prostate Tumor Spheroids by Reactive Oxygen Species

Intrinsic expression of the multidrug resistance (MDR) transporter P-glycoprotein (Pgp) may be regulated by reactive oxygen species (ROS). A transient expression of Pgp was observed during the growth of multicellular tumor spheroids. Maximum Pgp expression occurred in tumor spheroids with a high percentage of quiescent, Ki-67-negative cells, elevated glutathione levels, increased expression of the cyclin-dependent kinase inhibitors p27Kip1 and p21WAF-1 as well as reduced ROS levels and minor activity of the mitogen-activated kinase (MAPK) members c-Jun amino-terminal kinase (JNK), extracellular signal-regulated kinase ERK1,2, and p38 MAPK. Raising intracellular ROS by depletion of glutathione with buthionine sulfoximine (BSO) or glutamine starvation resulted in down-regulation of Pgp and p27Kip1, whereas ERK1,2 and JNK were activated. Down-regulation of Pgp was furthermore observed with low concentrations of hydrogen peroxide and epidermal growth factor, indicating that ROS may regulate Pgp expression. The down-regulation of Pgp following BSO treatment was abolished by agents interfering with receptor tyrosine kinase signaling pathways, i.e. the protein kinase C inhibitors bisindolylmaleimide I (BIM-1) and Ro-31-8220, the p21ras farnesyl protein transferase inhibitor III, the c-Raf inhibitor ZM 336372 and PD98059, which inhibits ERK1,2 activation. ROS involved as second messengers in receptor tyrosine kinase signaling pathways may act as negative regulators of Pgp expression.

Water Stress Tolerance of Wheat (Triticum aestivum L.): Variations in Hydrogen Peroxide Accumulation and Antioxidant Activity in Tolerant and Susceptible Genotypes

An experiment was conducted on five wheat (Triticum aestivum L.) cultivars, C 306, PBW 175 (tolerant to water stress), DL 153‐2 (moderately tolerant to water stress), HD 2428 and HD 2329 (recommended for irrigated conditions, susceptible to water stress), under pot culture conditions to study the effect of water stress on oxidative injury and antioxidant activity. Water stress significantly decreased relative water content (RWC), ascorbic acid content and membrane stability, and increased hydrogen peroxide and malondialdehyde content, a measure of lipid peroxidation, and activities of antioxidant enzymes in all the genotypes at 7, 17 and 27 days after anthesis (DAA). Water stress tolerant genotypes C 306 and PBW 175, closely followed by DL 153‐2, were superior to HD 2428 and HD 2329 in maintaining high RWC, ascorbic acid content and membrane stability and lower hydrogen peroxide content and lipid peroxidation (malondialdehyde content) under water stress at the three stages. The highest activities of glutathione reductase and catalase under water stress were observed in C 306, PBW 175 and DL 153‐2 and the lowest activities in HD 2428 and HD 2329 at all the stages. Superoxide dismutase activity at all stages under irrigated conditions and at the first and second stages under water stress conditions did not show significant variation among the different genotypes, but at the last stage under water stress the enzyme activity was highest in C 306, closely followed by PBW 175 and DL 153‐2, and lowest in HD 2428 and HD 2329. It is apparent that water stress induces an increase in hydrogen peroxide content and consequently lipid peroxidation and membrane injury (reduced membrane stability). The degree of oxidative stress and antioxidant activity seems to be closely associated with the tolerance/susceptibility of a genotype to water stress.

Mechanism of Acetaminophen Inhibition of Cyclooxygenase Isoforms

Acetaminophen has similar analgesic and antipyretic properties to nonsteroidal antiinflammatory drugs (NSAIDs), which act via inhibition of cyclooxygenase enzymes. However, unlike NSAIDs, acetaminophen is at best weakly antiinflammatory. The mechanism by which acetaminophen exerts its therapeutic action has yet to be fully determined, as under most circumstances, acetaminophen is a very weak cyclooxygenase inhibitor. The potency of acetaminophen against both purified ovine cyclooxygenase-1 (oCOX-1) and human cyclooxygenase-2 (hCOX-2) was increased approximately 30-fold by the presence of glutathione peroxidase and glutathione to give IC50 values of 33 μM and 980 μM, respectively. Acetaminophen was found to be a good reducing agent of both oCOX-1 and hCOX-2. The results are consistent with a mechanism of inhibition of acetaminophen in which it acts to reduce the active oxidized form of COX to the resting form. Inhibition would therefore be more effective under conditions of low peroxide concentration, consistent with the known tissue selectivity of acetaminophen.

Accumulation of Focal Adhesion Protein Hic-5 in the Nucleus by Hydrogen Peroxide.

Reactive oxygen species (ROS) are produced in cultured cells upon stimulation with cytokines and tumor promoters, and participate in signal transduction pathways leading to gene expression. The hic-5 gene was isolated as a hydrogen peroxide-inducible clone by differential screening, and was shown to encode a paxillin-related protein localized in the focal adhesion plaques. In mouse osteoblastic and fibroblastic cells treated with low concentrations of hydrogen peroxide, Hic-5 protein was found to be reversibly localized to the nucleus within 30 min. Hic-5 was also translocated to the nucleus in cells treated with leptomycin B, an inhibitor of nuclear export of proteins. Hic-5 protein with mutations in the nuclear export signal and the LIM-3 domain accumulated in the nucleus. These results indicated that Hic-5 shuttles between the cytoplasm and the nucleus, and may participate both in surface signals and in gene expression.

Contractile response of skeletal muscle to low peroxide concentrations: myofibrillar calcium sensitivity as a likely target for redox-modulation.

Endogenous peroxides and related reactive oxygen species may influence various steps in the contractile process. Single mouse skeletal muscle fibers were used to study the effects of hydrogen peroxide (H2O2) and t-butyl hydroperoxide (t-BOOH) on force and myoplasmic Ca2+ concentration ([Ca2+]i). Both peroxides (1010 to 105 M) decreased tetanic [Ca2+]i and increased force during submaximal tetani. Catalase (1 kU/ml) blocked the effect of H2O2, but not of t-BOOH. The decrease in tetanic [Ca2+]i was constant, while the effect on force was biphasic: A transitory increase was followed by a steady decline to the initial level. Myofibrillar Ca2+ sensitivity remained increased during incubation with either peroxide. Only the highest peroxide concentration (10 mM) increased resting [Ca2+]i and slowed the return of [Ca2+]i to its resting level after a contraction, evidence of impaired sarcoplasmic reticulum Ca2+ re-uptake. The peroxides increased maximal force production and the rate of force redevelopment, and decreased maximum shortening velocity. N-ethylmaleimide (25 mM, thiol-alkylating agent) prevented the response to 1 mM H2O2. These results show that myofibrillar Ca2+ sensitivity and cross-bridge kinetics are influenced by H2O2 and t-BOOH concentrations that approach those found physiologically, and these findings indicate a role for endogenous oxidants in the regulation of skeletal muscle function.

A Redox Signaling Mechanism for Density-dependent Inhibition of Cell Growth

Reactive oxygen species (ROS) have recently drawn significant attention as putative mitogenic mediators downstream of activated growth factor receptors and oncogenic Ras; however, the possibility that a redox-related mechanism also operates in the negative control of cell proliferation by inhibitory signals has not been investigated thus far. Here we show that the arrest of growth induced by cell confluence ("contact inhibition") is due, at least in part, to a decrease in the steady-state levels of intracellular ROS and the consequent impairment of mitogenic redox signaling. In confluent fibroblast cultures, the decrease in the concentration of oxygen species was associated with diminished activity of the small GTPase Rac-1, a signal transducer directly involved in the ligand-dependent generation of oxygen-derived molecules, and was effectively mimicked by exposure of sparse cultures to dithiothreitol (DTT) and inhibitors of enzymes (phospholipase A2 and lipoxygenase) acting in the arachidonic acid cascade downstream of growth factor receptors and Rac-1. Sparse fibroblasts treated with nontoxic amounts of DTT underwent growth arrest, whereas a low concentration of hydrogen peroxide significantly increased thymidine incorporation in confluent cultures, demonstrating a causal link between redox changes and growth control by cell density. Removal of oxygen species from sparse cultures was accompanied by a drastic decrease of protein tyrosine phosphorylation after epidermal growth factor stimulation, which, at a biochemical level, reproduced the signaling hallmarks of contact inhibition. Moreover, the cytosolic tyrosine phosphatase SHP-2 was identified as a putative target for redox signaling by cell density because the enzyme itself and the associated substrates appear markedly dephosphorylated in both confluent and reductant-treated cells after exposure to epidermal growth factor, and SHP-2 enzymatic activity is strongly activated by DTT in vitro. Taken together, these data support a model in which impaired generation of ROS and increased protein tyrosine phosphatase activity impede mitogenic signaling in contact-inhibited cells.

Effects of Hydrogen Peroxide on Intergranular Stress Corrosion Cracking of Stainless Steel in High Temperature Water, (III)

The stress corrosion cracking (SCC) of structural materials used in boiling water reactors has been studied at relatively low hydrogen peroxide (H2O2) concentrations, around lOppb, which was assumed to be representative of the corrosion environment formed in hydrogen water chemistry (HWC). The 1/4T compact tension specimen was used for measurement of crack growth rates (CGRs) of sensitized type 304 stainless steel in high temperature and high purity water. Crack length was monitored by a reversing direct current potential drop method. Since H2O2 is easily decomposed thermally, a polytetrafluoroethylene-lined autoclave was used to minimize its decomposition on the autoclave surface. The CGR in the H2O2 environment differed from that in the O2 environment even though the electrochemical corrosion potential (ECP) for both conditions was the same. The data implied that the ECP could not be used as a common environmental deterministic parameter for SCC behavior at higher potentials for different oxidant conditions. The corrosion current density was found to play an important role as an environmental index for SCC, which was given as just the current density at the ECP at a specific oxidant concentration. The CGRs were found to be written as CGR = (3.8±0.6)xl0-3 icor +(l-5±1.6) x 10-8mm/s using the calculated corrosion current density icorbelow 10-4 A-cm-2.

Horseradish peroxidase mediated free radical polymerization of methyl methacrylate.

This paper reports the free radical polymerization of methyl methacrylate (MMA) catalyzed by horseradish peroxidase (HRP). A novel method was developed whereby MMA polymerization can be carried out at ambient temperatures in the presence of low concentrations of hydrogen peroxide and 2,4-pentanedione in a mixture of water and a water-miscible solvent. Polymers of MMA formed were highly stereoregular with predominantly syndiotactic sequences (syn-dyad fractions from 0.82 to 0.87). Analyses of the chloroform-soluble fraction of syndio-PMMA products by GPC showed that they have number-average molecular weights, Mn, that range from 7500 to 75,000. By using 25% v/v of the cosolvents dioxane, tetrahydrofuran, acetone, and dimethylformamide, 85, 45, 7 and 2% product yields, respectively, resulted after 24 h. Increasing the proportion of dioxane to water from 1:3 to 1:1 and 3:1 resulted in a decrease in polymer yield from 45 to 38 and 7%, respectively. Increase in the enzyme concentration from 70 to 80 and 90 mg/mL resulted in increased reaction kinetics. By adjustment of the molar ratio of 2,4-pentanedione to hydrogen peroxide between 1.30:1.0 and 1.45:1.0, the product yields and Mn values were increased. On the basis of the catalytic properties of HRP and studies herein, we believe that the keto-enoxy radicals from 2,4-pentanedione are the first radical species generated. Then, initiation may take place through this radical or by the radical transfer to another molecule.