Mechanisms of Oxidation with Oxygen

Several topics are dealt with in discussing the reactions of molecular oxygen, but a common goal is pursued in each: to try to understand the reactions in terms of the fundamental properties of the oxygen molecule, and of the other reactants. The paper first describes the electronic structure of oxygen and of two low-lying electronically excited states. Concern with the low-lying electronically excited states is no longer the sole property of spectroscopists; recently, evidence has been presented for the participation of such activated molecules in chemical reactions. The chemistry of oxygen is dominated by the fact that the molecule in the ground state has two unpaired electrons, whereas the products of oxidation in many important reactions have zero spin. In its reactions with transition metal ions the restrictions imposed by the spin state of the oxygen molecule are easily circumvented. A number of reactions of oxygen with metal ions have been studied in considerable detail; conclusions on basic aspects of the reaction mechanism are outlined. Among the most interesting reactions of oxygen are those in which it is reversibly absorbed by reducing agents. Reversible absorption to form a peroxide in the bound state is possible; some of the conditions which must be fulfilled by a reducing system to qualify as storing oxygen in this way are reasonably well understood and are here enunciated. Little has been done on the formation of oxygen from water; some factors involved in this process are discussed.     

An oxygen model for Lake Haukivesi

A simple water quality model for Lake Haukivesi, heavily loaded by pulp and paper mill effluents, has been developed. The main purpose of the model is to predict the concentration of dissolved oxygen in the hypolimnion. The lake is divided into seven sub-basins, and also into epilimnion and hypolimnion. Transfers between sub-basins are calculated using water balance equations. The state variables of the model are dissolved oxygen concentration, biochemical oxygen demand, phytoplankton biomass, and total phosphorus concentration. The effect of temperature on reaction rate coefficients has been taken into account. Temperature is calculated in the model using a second degree polynomial function. The processes affecting hypolimnetic oxygen consumption are BOD decay, decomposition of phytoplankton, benthic oxygen demand, and decomposition of slowly decaying organic matter.     

Modelling and detecting tumour oxygenation levels

Tumours that are low in oxygen (hypoxic) tend to be more aggressive and respond less well to treatment. Knowing the spatial distribution of oxygen within a tumour could therefore play an important role in treatment planning, enabling treatment to be targeted in such a way that higher doses of radiation are given to the more radioresistant tissue. Mapping the spatial distribution of oxygen in vivo is difficult. Radioactive tracers that are sensitive to different levels of oxygen are under development and in the early stages of clinical use. The concentration of these tracer chemicals can be detected via positron emission tomography resulting in a time dependent concentration profile known as a tissue activity curve (TAC). Pharmaco-kinetic models have then been used to deduce oxygen concentration from TACs. Some such models have included the fact that the spatial distribution of oxygen is often highly inhomogeneous and some have not. We show that the oxygen distribution has little impact on the form of a TAC; it is only the mean oxygen concentration that matters. This has significant consequences both in terms of the computational power needed, and in the amount of information that can be deduced from TACs.     

Similarities and differences of hyperbaric oxygen and medical ozone applications

Hyperbaric oxygen (HBO) treatment is based on the principle of having the patient breath 100% oxygen in an environment above atmospheric pressure. Ozone (O(3)) is a colourless gas with a specific odour and consists of three oxygen atoms. The classical scientific understanding is that the world has become a place suitable for life for aerobic organisms with the increasing oxygen in the atmosphere billions of years ago. The formation of ozone after oxygen has then protected aerobic creatures from harmful rays. We now use these two gases for treatment purposes. It is noteworthy that the oxygen and ozone molecules that are formed by the same atom in different numbers are used for similar medical indications. We will try to emphasize the similarities and differences of HBO and medical ozone applications in this article.     

Measurement of low levels of oxygen and their effect on respiration in cell suspensions maintained in an open system

The presence of low levels of oxygen may have profound effects on the cytotoxic activity of radiation, radiosensitizers, and bioreductive alkylating agents. As others have shown, low oxygen tensions may significantly alter rates of cellular and chemical oxygen consumption. When experiments are performed at very low oxygen concentrations, the opposing effects of oxygen leakage into and cellular/chemical oxygen consumption from the system can lead to unpredictable results. Use of a newly designed, highly sensitive Clark-type oxygen sensor has permitted accurate and reproducible measurement of low levels of oxygen. Cellular depletion of oxygen at various cell densities has been monitored for a series of oxygen tensions in solution and the corresponding respiration rates have been calculated. Although oxygen depletion was found to be quite significant at low oxygen tensions, not all oxygen present could be removed by cellular respiration. Respiration rate decreased as oxygen tension decreased and approached zero at low oxygen tensions. This result was independent of cell density. A model is presented to account for the observed effect of oxygen tension on cellular oxygen utilization.     

AN INVESTIGATION INTO THE CAUSE OF THE SPONTANEOUS AGGREGATION OF FLAGELLATES AND INTO THE REACTIONS OF FLAGELLATES TO DISSOLVED OXYGEN : PART II.

Spontaneous aggregations of flagellates are formed under the cover-glass because the organisms are attracted to and remain in regions where the concentration of dissolved oxygen is less than the saturation concentration under atmospheric partial pressure. These regions of lessened oxygen content arise towards the center of the liquid beneath the cover-glass, owing to the oxygen consumed by the flagellates in respiration not being replaced here by the solution of atmospheric oxygen, as it is along the edges of the liquid. The flagellates, however, are insensitive to the attraction of regions of lessened oxygen concentration when the oxygen concentration throughout the liquid is above a certain value. Therefore, for the aggregations to form, either the initial concentration of dissolved oxygen must be below this limiting value, or an interval of time must first elapse after the making of the preparation until the respiration of the organisms has reduced the oxygen concentration throughout the liquid down to this limiting value. The aggregations will then form because the flagellates have become positively chemotropic to the lower concentration of oxygen at the center of the liquid. Once established, such an aggregation of flagellates does not remain long in the same form. An area free from flagellates appears at the center of the aggregation so that the organisms lie in a circular band surrounding the clear area. The latter increases in size and its bordering band of flagellates in diameter, the band gradually becoming less circular and more square in shape, if the cover-glass is a square one. The clear central area is a region where the oxygen consumption of the flagellates has reduced the oxygen content to such a low value that the organisms are forced to leave the region. They collect in a band where the concentration of dissolved oxygen is an optimum for them. It is the equilibrium position between the oxygen consumed at the center and that diffusing in from the edges of the liquid. As the consumption at the center is more rapid than the replacement from the edge, the flagellate band moves outwards until it becomes stationary at a position where the rates of consumption and replacement of oxygen are equal. Although the flagellates collect in this manner in regions of optimum oxygen concentration, yet greater concentrations of dissolved oxygen have no injurious effect on them. Concentrations of dissolved oxygen lower than the optimum have the effect of inhibiting the movement of the flagellates. They recover their activity, however, immediately they are given access to dissolved oxygen again. Work done in the past on chemotropism of flagellates will have to be revised in the light of the above facts, since the oxygen content of solutions used has never been taken into account.     

Similarities and differences of hyperbaric oxygen and medical ozone applications

Abstract

Hyperbaric oxygen (HBO) treatment is based on the principle of having the patient breath 100% oxygen in an environment above atmospheric pressure. Ozone (O₃) is a colourless gas with a specific odour and consists of three oxygen atoms. The classical scientific understanding is that the world has become a place suitable for life for aerobic organisms with the increasing oxygen in the atmosphere billions of years ago. The formation of ozone after oxygen has then protected aerobic creatures from harmful rays. We now use these two gases for treatment purposes. It is noteworthy that the oxygen and ozone molecules that are formed by the same atom in different numbers are used for similar medical indications. We will try to emphasize the similarities and differences of HBO and medical ozone applications in this article.     

No single way to understand singlet oxygen signalling in plants

When plant cells are under environmental stress, several chemically distinct reactive oxygen species (ROS) are generated simultaneously in various intracellular compartments and these can cause oxidative damage or act as signals. The conditional flu mutant of Arabidopsis, which generates singlet oxygen in plastids during a dark-to-light transition, has allowed the biological activity of singlet oxygen to be determined, and the criteria to distinguish between cytotoxicity and signalling of this particular ROS to be defined. The genetic basis of singlet-oxygen-mediated signalling has been revealed by the mutation of two nuclear genes encoding the plastid proteins EXECUTER (EX)1 and EX2, which are sufficient to abrogate singlet-oxygen-dependent stress responses. Conversely, responses due to higher cytotoxic levels of singlet oxygen are not suppressed in the ex1/ex2 background. Whether singlet oxygen levels lower than those that trigger genetically controlled cell death activate acclimation is now under investigation.     

Cartesian-diver microrespirometry: a technique for the accurate measurement of respiratory flux in plant cells

Abstract. A Cartesian-diver microrespirometer system is described which can be used to measure respiratory fluxes of oxygen accurately for cells of higher plants in a liquid phase. This microrespirometry technique has been adapted from protozoological and microfaunal studies to plant physiology. The Cartesian-diver has considerable scope for investigation of oxygen flux in plant cells and has several advantages compared to the oxygen electrode in terms of sensitivity to changing oxygen levels in respiring material. Because the volumes of liquid are small in the Cartesian-divers, diffusional distances arc measured in micrometres and there is no need for stirring to overcome diffusional problems, thus minimizing the risk of mechanical damage to the experimental material. In addition, only very small quantities of experimental material are required for the Cartesian-diver which is invaluable where only limited amounts of tissue or numbers of cells can be obtained. Examples of respiratory oxygen consumption by protoplasts from intercalary meristematic regions of light-grown barley ( Hordeum vulgare L.c.v. Patty) seedlings, in response to abscisic and gibberellic acids, are presented. The advantages and disadvantages of Cartesian-diver microrespirometry compared to oxygen electrodes are also discussed.     

Ligand binding kinetics of des arginine haemoglobin

Des arginine 141 a haemoglobin (the haemoglobin in which the C-terminal arginine of the a chain has been removed) has a high affinity for oxygen and a reduced co-operativity in its oxygen equilibrium binding. The kinetic consequences of this modification are investigated in this paper. Deoxy des Arg haemoglobin binds carbon monoxide faster than does haemoglobin A, whilst oxy des Arg haemoglobin loses oxygen more slowly. These results are correlated with the oxygen equilibrium binding properties of des Arg haemoglobin. The carbon monoxide binding kinetics have been interpreted as implying a change in the parameter c (of the allosteric model), as well as L, when this arginine is removed from haemoglobin.     

New Method for Oxygen Therapy in the Home using an Oxygen Concentrator

Patients with pulmonary hypertension due to chronic bronchitis may improve during long-term treatment with oxygen. The methods of administration which are currently available are expensive and present practical difficulties. The Rimer-Birlec domiciliary oxygen concentrator produces an oxygen concentration of 92% at a flow of 21./min. It has been used successfully in a patient''s home but further use will require an increase in mechanical reliability and a decrease in noise. In view of its convenience and the economic advantages the oxygen concentrator is an important advance in treatment with oxygen and could prove to be the method of choice in the home.     

The aging brain, metals and oxygen free radicals.

In this overview we bring together certa in facts and concepts that support the theory that the aging of "disease-free" brain is a consequence of the accumulated cellular-molecular modifications caused by oxygen free radicals. The relevance of transition metals, especially iron ions, in the production of oxygen free radicals, initiation of oxidative chain-reactions and in site-specific molecular modifications is documented. Mitochondria are identified as the major source of oxygen free radicals, and mitochondrial DNA is a likely target. Special attention is given to iron-sulfur clusters as sources of reactive iron and sites of modifications. Potential mechanisms by which oxygen free radicals can alter membrane receptors and intracellular signaling are cited. Although the evidence is still correlative, the oxygen free radical theory has strong experimental support and has promise for facilitating a better understanding of the "disease-free", aging brain.     

Oxygen and the production of formazan from neotetrazolium chloride

Summary Electrons, generated from dehydrogenase reactions, are transferred to oxygen in preference to neotetrazolium chloride. In model systems in solution the presence of a small amount of oxygen drastically reduces the rate of formazan production. The rate of reaction in tissue sections has been followed using scanning and integrating microdensitometry. As in solution, electrons are transferred preferentially to oxygen. However, oxygen seems unable to diffuse through the incubation medium and thus the supply of oxygen at the site of the enzyme activity becomes exhausted; the time taken to use up the oxygen will depend on the rate of the enzyme activity. It is only then that electrons are passed to the tetrazolium salt and formazan is precipitated.     

A Detoxifying Oxygen Reductase in the Anaerobic Protozoan Entamoeba histolytica

We report the characterization of a bacterial-type oxygen reductase abundant in the cytoplasm of the anaerobic protozoan parasite Entamoeba histolytica. Upon host infection, E. histolytica is confronted with various oxygen tensions in the host intestine, as well as increased reactive oxygen and nitrogen species at the site of local tissue inflammation. Resistance to oxygen-derived stress thus plays an important role in the pathogenic potential of E. histolytica. The genome of E. histolytica has four genes that encode flavodiiron proteins, which are bacterial-type oxygen or nitric oxide reductases and were likely acquired by lateral gene transfer from prokaryotes. The EhFdp1 gene has higher expression in virulent than in nonvirulent Entamoeba strains and species, hinting that the response to oxidative stress may be one correlate of virulence potential. We demonstrate that EhFdp1 is abundantly expressed in the cytoplasm of E. histolytica and that the protein levels are markedly increased (up to ～5-fold) upon oxygen exposure. Additionally, we produced fully functional recombinant EhFdp1 and demonstrated that this enzyme is a specific and robust oxygen reductase but has poor nitric oxide reductase activity. This observation represents a new mechanism of oxygen resistance in the anaerobic protozoan pathogen E. histolytica.     

Oxygen-dependent cellular functions--why fishes and their aquatic environment are a prime choice of study

Owing to the variability of oxygen tension in aquatic, especially the freshwater environment, oxygen has been a major force in the evolution of fishes. Their long evolutionary history, and the present different oxygen requirements between species, and acclimatory responses to hypoxic and hyperoxic conditions make fishes prime models in the study of oxygen-dependent cellular functions and their regulation. In the present article oxygen-dependent membrane transport, cellular signalling, energy metabolism, gene expression and apoptosis are reviewed with an emphasis on available results on fish. Available data on oxygen sensing are described and examples on the cascade from sensing oxygen to its physiological effects are given. From the data it is clear that hitherto fish have not been utilised in the study of oxygen-dependent cellular regulation as much as their evolutionary history and present oxygen requirements would give possibilities to. Even more generally, oxygen has hitherto seldom been a carefully controlled key variable in experimental cell biology.     

Optimization of 2,3-butanediol production by Klebsiella oxytoca through oxygen transfer rate control

Production of 2,3-butanediol by Klebsiella oxytoca is influenced by the degree of oxygen limitation. During batch culture studies, two phases of growth are observed: energy-coupled growth, during which cell growth and oxygen supply are coupled; and, energy-uncoupled growth, which arises when the degree of oxygen limitation reaches a critical value. Optimal 2,3-butanediol productivity occurs during the energy-coupled growth phase. In this article, a control system which maintains the batch culture at a constant level of oxygen limitation in the energy-coupled growth regime has been designed. Control, which involves feedback control on the oxygen transfer coefficient, is achieved by continually increasing the partial pressure of oxygen in the feed gas, which in turn continually increases the oxygen transfer rate. Control has resulted in a balanced state of growth, a repression of ethanol formation, and an increase in 2,3-butanediol productivity of 18%. (c) 1993 John Wiley & Sons, Inc.     

Pericellular oxygen depletion during ordinary tissue culturing, measured with oxygen microsensors

Recent research has found important differences in oxygen tension in proximity to certain mammalian cells when grown in culture. Oxygen has a low diffusion rate through cell culture media, thus, as a result of normal respiration, a decrease in oxygen tension develops close to the cells. Therefore, for the purpose of standardization and optimization, it is important to monitor pericellular oxygen tension and cell oxygen consumption. Here, we describe an integrated oxygen microsensor and recording system that allows measurement of oxygen concentration profiles in vertical transects through a 1.6-mm deep, stagnant, medium layer covering a cell culture. The measurement set-up reveals that, when confluent, a conventional culture of adherent cells, although exposed to the constant oxygen tension of ambient air, may experience pericellular oxygen tensions below the level required to sustain full oxidative metabolism. Depletions reported are even more prominent and potentially aggravating when the cell culture is incubated at reduced oxygen tensions (down to around 4% oxygen). Our results demonstrate that, if the pericellular oxygen tension is not measured, it is impossible to relate in vitro culture results (for example, gene expression to the oxygen tension experienced by the cell), as this concentration may deviate very substantially from the oxygen concentration recorded in the gas phase.     

Hypoxia and the regulation of gene expression

The optimal delivery of oxygen to tissues is essential both to ensure adequate energy provision and to avoid the toxic effects of higher oxygen concentrations. For this to occur, organisms must be able to sense oxygen and respond to changes in oxygen tension by altering gene expression. The analysis of the regulation of erythropoiesis has provided important insights into the mechanisms of oxygen-regulated gene expression. These mechanisms have a role in the regulation of many genes, in many cell types and appear to be of relevance to many common pathologies in which disturbances of oxygen supply are central.     

Kinetic Stability of Bulk LiNiO2 and Surface Degradation by Oxygen Evolution in LiNiO2‐Based Cathode Materials

Capacity degradation by phase changes and oxygen evolution has been the largest obstacle for the ultimate commercialization of high‐capacity LiNiO₂‐based cathode materials. The ultimate thermodynamic and kinetic reasons of these limitations are not yet systematically studied, and the fundamental mechanisms are still poorly understood. In this work, both phenomena are studied by density functional theory simulations and validation experiments. It is found that during delithiation of LiNiO₂, decreased oxygen reduction induces a strong thermodynamic driving force for oxygen evolution in bulk. However, oxygen evolution is kinetically prohibited in the bulk phase due to a large oxygen migration kinetic barrier (2.4 eV). In contrast, surface regions provide a larger space for oxygen migration leading to facile oxygen evolution. These theoretical results are validated by experimental studies, and the kinetic stability of bulk LiNiO₂ is clearly confirmed. Based on these findings, a rational design strategy for protective surface coating is proposed.