Spontaneous oxidation of methionine: effect on the quantification of plasma methionine levels

IHF and HU belong to a family of proteins that introduce sharp bends into DNA and act as accessory factors in a variety of cellular processes in prokaryotes. In addition to the crystal structure of IHF bound to DNA, the past year has seen a number of advances in the understanding of the interactions of these proteins with DNA in solution.     

Altered metabolism of the methionine methyl group in the leukocytes of patients with schizophrenia

Previous work has indicated that abnormal methylation processes may be associated with schizophrenia. In this study, leukocytes from patients with schizophrenia were incubated with methyl-14C-L-methionine and the evolved 14CO2 measured. With increasing concentration of methionine, the evolved 14CO2 was lower in the patients than in normal control subjects. The incorporation of 14C into protein was the same in both groups, and when carboxyl-14C-L-methionine was used the evolved 14CO2 was the same in both groups, thus excluding the possibility that altered incorporation into protein or oxidation of the methionine molecule as a whole were responsible. The observed differences in methionine-methyl metabolism suggest that an abnormality in transmethylation processes or in oxidation of the methyl group to CO2 is associated with schizophrenia. That this occurs in a peripheral tissue indicates that the abnormality is not restricted to the central nervous system.     

Evidence for S-adenosylmethionine independent catabolism of methionine in the rat

Experiments conducted with rats in vivo comparing the metabolism of methionine and S-methyl-L-cysteine and in vitro comparing methionine, S-methyl-L-cysteine and S-adenosyl-L-methionine indicate that a substantial portion of the oxidative metabolism of the methionine methyl group occurs by pathways that are independent of S-adenosylmethionine formation. Inclusion of 1.2% or 2.4% of S-methyl-L-cysteine in a diet containing 3% of L-methionine depressed the conversion of the methionine methyl and carboxyl carbons to CO2 by 39% and 28%, and 52% and 33%, respectively, for the two levels of S-methyl-L-cysteine. Inclusion of 1.65% of methionine in a diet containing 2.4% of S-methyl-L-cysteine did not affect the conversion of the methyl group of S-methylcysteine to CO2, but 3% of methionine depressed the conversion of the S-methylcysteine methyl group to CO2 to 87% of control values. Greater inhibitions were seen when these substrates were compared in a liver homogenate. In a rat liver homogenate system optimized for the conversion of the methyl group of methionine to CO2, the rate of conversion of the methyl group of S-adenosyl-L-methionine to CO2 was less than 1% of that of methionine even when the concentration of S-adenosylmethionine was saturating. Addition of saturating levels of unlabeled S-adenosymethionine to the homogenate system did not effect the rate of conversion of the methionine methyl carbon to CO2. Although S-adenosylmethionine-dependent metabolism of methionine, leading to incorporation of the methyl carbon into sarcosine and serine, could be demonstrated in liver homogenates, essentially all of the CO2 produced from the methionine methyl group was derived by a pathway or pathways independent of S-adenosylmethionine formation. Formaldehyde and formate have been tentatively identified as intermediates in catabolism of the methionine methyl group by this (these) pathway(s).     

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.     

Antioxidants for prevention of methionine oxidation in recombinant monoclonal antibody HER2

Recombinant humanized monoclonal antibody HER2, rhuMAb HER2, in liquid formulations undergoes oxidation when exposed to intense light and elevated temperatures (30 & 40 degrees C). Met-255 in the heavy chain of the Fc region of the antibody is the primary site of oxidation. Met-431 of the Fc fragment can also be oxidized under extreme conditions. The amount of oxidation was determined by cleaving the Fab and Fc fragments by papain digestion, and the oxidized Fc fragment was detected by hydrophobic interaction chromatography. Oxidation of rhuMAb HER2 was also formulation dependent. The presence of NaCl in the rhuMAb HER2 formulation caused an increase in oxidation at higher temperatures after contact with stainless steel containers or stainless steel components in the filling process. The corrosion of stainless steel by chloride ions at the low pH of the formulation buffer generated iron ions that catalyzed methionine oxidation in rhuMAb HER2. Temperature-induced oxidation of rhuMAb HER2 occurred by the formation of free radicals, and light-induced oxidation of rhuMAb HER2 occurred via single oxygen pathway. Antioxidants, such as methionine, sodium thiosulfate, catalase, or platinum, prevented Met oxidation in rhuMAb HER2, presumably as free radicals or oxygen scavengers. The minimum effective levels (molar ratios of protein to antioxidant) required to inhibit temperature-induced oxidation were 1:5 and 1:25 for methionine and thiosulfate, respectively. A thiosulfate adduct of rhuMAb HER2 was observed by cation-exchange chromatography. These studies demonstrate that stoichiometric amounts of methionine and thiosulfate are sufficient to eliminate temperature-induced oxidation of rhuMAb HER2 caused by free radicals that were generated by the presence of metal ion and peroxide impurities in the formulation.     

Kinetics of oxidation of carbon in liquid iron-carbon-silicon-manganese-sulfur alloys by carbon dioxide in nitrogen

The oxidation of carbon with the simultaneous oxidation of silicon, manganese, and iron of liquid alloys by carbon dioxide in nitrogen and the absorption of oxygen by the alloys from the gas were studied using 1-g liquid iron droplets levitated in a stream of the gas at 1575 °C to 1715 °C. Oxidation of carbon was favored over oxidation of silicon and manganese when cast iron (3.35 pct C, 2.0 pct Si, 0.36 pct Mn, and 0.05 pct S) reacted with CO₂/N₂ gas at 1635 °C. An increase in the flow rate of CO₂/N₂ gas increased the decarburization rate of cast iron. The rate of carbon oxidation by this gas mixture was found to be independent of temperature and alloying element concentrations (in the range of silicon = 0 to 2.0 pct manganese = 0 to 0.36 pct and sulfur = 0 to 0.5 pct) within the temperature range of the present study. Based on the results of a kinetic analysis, diffusion of CO₂ in the boundary layer of the gas phase was found to be the rate-limiting step for the reactions during the earlier period of the reaction when the contents of carbon, silicon, and manganese are higher. However, the limiting step changed to diffusion of the elements in the metal phase during the middle period of the reaction and then to the diffusion of CO in the gas phase during the later period of the reaction when the content of the elements in the metal were relatively low. For the simultaneous oxidation reactions of several elements in the metal, however, the diffusion of CO₂ in the gas phase is the primary limiting step of the reaction rate for the oxidation of carbon during the later period of reaction. Formerly Visiting Assistant Research Scientist, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109     

炭/炭复合材料抗氧化涂层厚度的光学测量

选用A,B,C,D4种炭/炭复合材料试样进行抗氧化涂层厚度的光学测量研究。其中:试样A,B孔隙度相同,均为15.85％,对其涂覆粒径分别为1μm和10μm的B_4C涂层;试样C,D孔隙度分别为13.52％和16.78％,对其均涂覆粒径为10μm的B_4C涂层。然后通过金相光学测量其涂层厚度,在抗氧化涂层厚度的计算过程中,对试样组织的内部未渗入抗氧化涂层的孔隙进行图像处理。结果表明:当试样孔隙度相同时,涂层材料粒径越小,其涂层越厚;当抗氧化涂层材料粒径相同时,试样孔隙度越大,涂层越厚。     

Role of a silicate phase in the reduction of iron and chromium and their oxidation with carbide formation during the manufacture of carbon ferrochrome

The reactions of reduction of chromium and iron from chromospinelide and the reactions of carbide formation from the reduced metals are separated in space in experiments performed on ore grains with an artificially applied silicate shell. It is found that the silicate layer that isolates spinelide fro direct contact with carbon takes part in the reactions of both reduction and carbide formation. Free carbon extracts oxygen anions from the layer at the contact surface with the formation of CO, and the forming anion vacancies transfer “excess” electrons to the iron and chromium cations in the spinelide lattice and reduce them. Free and carbide-fixed carbon extracts iron and chromium cations from the silicate layer, and carbides form on the surface. The cation vacancies and electron holes (high-charge cations) that form in the silicate phase under these conditions are involved in the oxidation of the metal reduced in spinelide and cause its dissolution in the silicate phase and the precipitation of lower carbides on the surface of the silicate phase. The structure that is characterized of carbon ferrochrome forms on the surface of the silicate phase. Carbide formation is slower than reduction because of higher energy consumed for the formation of high-charge cations and the transfer of cations from the spinelide volume to the outer surface of the silicate phase. In the absence of a silicate layer, a carbide shell blocks the contact of carbon with oxides, which leads to the stop of reduction and, then, carbide formation. In the presence of a silicate (slag) shell around a spinelide grain, the following two concentration galvanic cells operate in parallel: an oxygen (reduction) cell and a metal (oxidation) cell. The parallel operation of the two galvanic cells with a common electrolyte (silicate phase) results in a decrease in the electric potentials between spinelide inside the silicate phase and carbon and carbides on its surface, and each of the processes is significantly facilitated and accelerated. In other words, the production of carbon ferrochrome is accelerated.     

Mechanism of the anodic oxidation of carbon by oxide melts

The kinetics of anodic oxidation of carbon-containing materials in borosilicate melts is studied. A mechanism of the anodic process that takes into account the surface diffusion of adsorbed oxygen atoms is proposed. The kinetic characteristics of the process for various systems and experimental conditions are determined.     

Copper-cerium oxides supported on carbon nanomaterial for preferential oxidation of carbon monoxide

The CuxO-Ce O2/Fe@CNSs, CuxO-Ce O2/MWCNTs-Co and CuxO-Ce O2/MWCNTs-Ni catalysts were prepared by the impregnation method and characterized by transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, H2-temperature programmed reduction and N2 adsorption-desorption techniques. It was found that the Fe nanoparticles were encapsulated into the multi-layered carbon nanospheres(CNSs). However, the multi-wall carbon nanotubes(MWCNTS) were generated on the Co/Al2O3 and Ni/Al2O3 precursor. The addition of carbon nanomaterial as supports could improve structural properties and low-temperature activity of the Cu O-Ce O2 catalyst, and save the used amount of metal catalysts in the temperature range with high selectivity for CO oxidation. The copper-cerium oxides supported on carbon nanomaterial had good resistence to H2 O and CO2.     

Bainite formation in low carbon Cr-Ni steels

A low carbon Cr-Ni steel has been used to investigate the formation of upper bainite. Experimental results indicate that the start temperatures of the three morphologies of upper bainite in this steel,i.e., carbide-free bainite, bainite with carbide between and within ferrite laths, are about 600°, 500δ, and 425 °C, respectively; the habit plane of bainitic ferrite in this steel is close to (1 7 11)_α, which is 13.3 deg away from (0 ll)_α; and the orientation relationship between cementite and ferrite is consistent with Bagaryatskii’s. By means of the superelement approach, a thermodynamic treatment which applies to Fe-C alloys is extended into that suitable for low alloy steels, and calculation shows that the driving force for bainite formation at B_S temperatures is insufficient to compensate for shear strain energy. Formerly Graduate Student, Department of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China.     

Graphite formation in high-purity cold-rolled carbon steels

Low-carbon (0.06 pet C) and high-carbon (0.50 pet C) Al-killed (AK) steels were prepared by melting high-purity electrolytic iron. They were hot-rolled, cold-rolled, and then batch-annealed for the investigation of microstructure and mechanical properties. It was found that graphitization of cementite took place in steels of extremely low phosphorus and sulfur contents. Graphite was thought to originate in voids formed by cracking of cementite during the cold rolling. It was speculated that in steels of commercial purity, phosphorus and sulfur segregated to the void surface and, thus, suppressed graphitization during the annealing. It was also found that the high-carbon steels became as soft and ductile as low-carbon steels by graphitization.     

Void formation during oxidation of the ODS alloy MA 6000

The present work deals with investigations on the subsurface void formation in the oxide dispersion strengthened (ODS) alloy MA 6000. The effect that results from oxidation has been analyzed for stress-free and stressed samples exposed to creep deformation up to a maximum exposure time of about 11,000 hours at 1050 ‡C. Additional tests in the stress-free condition have been performed at 1150 ‡C for up to 1300 hours. The depth of voiding increases with time following a parabolic rate law, and in the long-term range, it reaches about 2 mm at 1050 ‡C. For the stressed state during long-term exposure, the penetration was slightly deeper than for the stress-free state, and large voids were elongated perpendicular to the stress axis. For long exposure times at 1050 ‡C, the area of voids was about 1 to 2 pct of the total void-affected zone, and the maximum void diameter could reach about 25.0 Μm. A void-free zone, the width of which increased with time, formed beneath the scale. Among the mechanisms considered to explain the void formation, vacancy injection resulting from outward diffusion of mainly Cr seems the most consistent. J.L. GONZáLEZ-CARRASCO, formerly Postdoctoral Fellow, Commission of the European Communities, Institute for Advanced Materials, Joint Research Centro     

Simultaneous oxidation and sigma-phase formation in a stainless steel

Oxidation of 347 austenitic stainless steel and the simultaneous formation of σ phase have been investigated, in the temperature range 650 °C to 816 °C. Oxidation rates for both 0.09-mm foil and 2.29-mm sheet were found to obey parabolic kinetics in the early stages of growth. However, significant departures from parabolic growth leading to accelerated oxidation occurred at 816 °C, after about 2500 hours in the thin foil and 10,000 hours in the thicker sheet material. Sigma-phase precipitation occurred throughout the temperature range, with a pronounced precipitate-free zone (PFZ) near the metal surface. The PFZ (which occurred as a result of Cr depletion) increased in width as a function of time and temperature. This observation was taken as evidence that the surface oxidation reaction was not limited by the Cr depletion associated with the formation of σ phase. Mathematical modeling of the Cr content at the metal/oxide interface together with an experimental observation of a “breakaway” condition of accelerated oxidation (at high temperature) was used to predict the oxidation-limited lifetime for the 0.09-mm foil at lower temperatures. Calculated Cr-depletion profiles, which corresponded closely with experimental data, resulted in depletion profile widths that corresponded closely with those for the σ-phase PFZs.     

A collision model for fume formation in metal oxidation

During the oxidation of copper at high temperatures, two simultaneous mechanisms are observed in the hot stage environmental scanning electron microscope: the nucleation and growth of solid cuprous oxide crystals and the formation of a solid fume composed of very small copper and copper oxide particles. A collision model for fume formation in metal oxidation is presented. By calculating the collision frequency between evaporated copper atoms and oxygen molecules in the gas phase, the amount of fume formed can be estimated. Although the effect of Van der Waals forces between particles has not been quantified, the temperature difference between the two gases was taken into account and yielded a multiplying factor of 4 in the collision frequency for copper oxidation at 950 °C. Unoxidized copper particles were formed from unsuccessful elastic collisions between Cu(v) and O₂. These copper particles are composed of interpenetrating icosohedra and possess a high crystalline perfection. Formerly Graduate Student, Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210.     

Role of carbon and alloying elements in the formation of bainitic ferrite

One approach to the prediction of the carbon content of austenite, remaining after the precipitation of bainitic ferrite, is based on the assumption that bainitic ferrite during growth inherits the carbon content of the parent austenite. An alternative approach is based on the assumption that bainitic ferrite grows with a low carbon content and there is no major difference between Widmanstätten ferrite and bainitic ferrite. The two approaches are now compared using information from alloyed steels with considerable amounts of Si, where the formation of cementite is retarded. The former approach does not account for the effect of Mn and fails severely at low alloy contents. The latter approach seems more promising but is not without difficulties. In particular, in order to explain the effects of Cr and Mo, it seems necessary to introduce a kinetic effect, presumably caused by solute drag.