Mechanical Degradation of Tungsten Alloys at Extreme Temperatures in Vacuum and Oxidation Atmospheres

Mechanical degradation of tungsten alloys at extreme temperatures in vacuum and oxidation atmospheres.

Modulation of potassium channel function by methionine oxidation and reduction

Oxidation of amino acid residues in proteins can be caused by a variety of oxidizing agents normally produced by cells. The oxidation of methionine in proteins to methionine sulfoxide is implicated in aging as well as in pathological conditions, and it is a reversible reaction mediated by a ubiquitous enzyme, peptide methionine sulfoxide reductase. The reversibility of methionine oxidation suggests that it could act as a cellular regulatory mechanism although no such in vivo activity has been demonstrated. We show here that oxidation of a methionine residue in a voltage-dependent potassium channel modulates its inactivation. When this methionine residue is oxidized to methionine sulfoxide, the inactivation is disrupted, and it is reversed by coexpression with peptide methionine sulfoxide reductase. The results suggest that oxidation and reduction of methionine could play a dynamic role in the cellular signal transduction process in a variety of systems.

Nitration of γ-tocopherol and oxidation of α-tocopherol by copper-zinc superoxide dismutase/H2O2/NO2−: Role of nitrogen dioxide free radical

Copper-zinc superoxide dismutase (Cu,ZnSOD) is the antioxidant enzyme that catalyzes the dismutation of superoxide (O2•−) to O2 and H2O2. In addition, Cu,ZnSOD also exhibits peroxidase activity in the presence of H2O2, leading to self-inactivation and formation of a potent enzyme-bound oxidant. We report in this study that lipid peroxidation of l-α-lecithin liposomes was enhanced greatly during the SOD/H2O2 reaction in the presence of nitrite anion (NO2−) with or without the metal ion chelator, diethylenetriaminepentacetic acid. The presence of NO2− also greatly enhanced α-tocopherol (α-TH) oxidation by SOD/H2O2 in saturated 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine liposomes. The major product identified by HPLC and UV-studies was α-tocopheryl quinone. When 1,2-diauroyl-sn-glycero-3-phosphatidylcholine liposomes containing γ-tocopherol (γ-TH) were incubated with SOD/H2O2/NO2−, the major product identified was 5-NO2-γ-TH. Nitrone spin traps significantly inhibited the formation of α-tocopheryl quinone and 5-NO2-γ-TH. NO2− inhibited H2O2-dependent inactivation of SOD. A proposed mechanism of this protection involves the oxidation of NO2− by an SOD-bound oxidant to the nitrogen dioxide radical (•NO2). In this study, we have shown a new mechanism of nitration catalyzed by the peroxidase activity of SOD. We conclude that NO2− is a suitable probe for investigating the peroxidase activity of familial Amyotrophic Lateral Sclerosis-linked SOD mutants.

Theoretical investigation of the [1,2]-sigmatropic hydrogen migration in the mechanism of oxidation of 2-aminobenzoyl-CoA by 2-aminobenzoyl-CoA monooxygenase/reductase

The flavin hydroperoxide at the active site of the mixed-function oxidase 2-aminobenzoyl-CoA monooxygenase/reductase (Azoarcus evansii) transfers an oxygen to the 5-position of the 2-aminobenzoyl-CoA substrate to provide the alkoxide intermediate II−. Hydrogen migration from C5 to C6 follows this monooxygenation. The nature of the monooxygenation intermediate and plausible competing reactions leading to hydrogen migration have been considered. Ab initio molecular orbital theory has been used to calculate structures and electron distributions in intermediate and transition state structures. Electrostatic potential surface calculations establish that the transition state and product, associated with the C5 to C6 hydrogen transfer, are stabilized by electron distribution to the benzoyl-CoA thioester carbonyl oxygen. This is not so for the transition state and product associated with hydrogen transfer from C5 to C4. The activation energy for the 5,6-shift is 2.5 kcal/mol lower than that for the 5,4-shift. In addition, the product of the hydrogen 5,6-shift is more stable than is the product of the hydrogen 5,4-shift, by ≈6 kcal/mol. These results explain why only the shift of hydrogen from C5 to C6 is observed experimentally. Oxygen transfer and hydrogen migration almost coincide in the gas phase (activation energy of ≈0.6 kcal/mol, equivalent to a single bond vibration). Enzymatic formation of alkoxide II− requires its stabilization; thus, the rate constant for its breakdown would be slower than in the gas phase.

Synthesis of medium-chain-length polyhydroxyalkanoates in Arabidopsis thaliana using intermediates of peroxisomal fatty acid β-oxidation

Polyhydroxyalkanoate (PHA) is a family of polymers composed primarily of R-3-hydroxyalkanoic acids. These polymers have properties of biodegradable thermoplastics and elastomers. Medium-chain-length PHAs (MCL-PHAs) are synthesized in bacteria by using intermediates of the β-oxidation of alkanoic acids. To assess the feasibility of producing MCL-PHAs in plants, Arabidopsis thaliana was transformed with the PhaC1 synthase from Pseudomonas aeruginosa modified for peroxisome targeting by addition of the carboxyl 34 amino acids from the Brassica napus isocitrate lyase. Immunocytochemistry demonstrated that the modified PHA synthase was appropriately targeted to leaf-type peroxisomes in light-grown plants and glyoxysomes in dark-grown plants. Plants expressing the PHA synthase accumulated electron-lucent inclusions in the glyoxysomes and leaf-type peroxisomes, as well as in the vacuole. These inclusions were similar to bacterial PHA inclusions. Analysis of plant extracts by GC and mass spectrometry demonstrated the presence of MCL-PHA in transgenic plants to approximately 4 mg per g of dry weight. The plant PHA contained saturated and unsaturated 3-hydroxyalkanoic acids ranging from six to 16 carbons with 41% of the monomers being 3-hydroxyoctanoic acid and 3-hydroxyoctenoic acid. These results indicate that the β-oxidation of plant fatty acids can generate a broad range of R-3-hydroxyacyl-CoA intermediates that can be used to synthesize MCL-PHAs.