Molecular oxygen in biological oxidations--an overview

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.

Hypoxia--implications for pharmaceutical developments

Cells sense oxygen availability using not only the absolute value for cellular oxygen in regard to its energetic and metabolic functions, but also the gradient from the cell surface to the lowest levels in the mitochondria. Signals are used for regulatory purposes locally as well as in the generation of cellular, tissue, and humoral remodeling. Lowered oxygen availability (hypoxia) is theoretically important in the consideration of pharmacology because (1) hypoxia can alter cellular function and thereby the therapeutic effectiveness of the agent, (2) therapeutic agents may potentiate or protect against hypoxia-induced pathology, (3) hypoxic conditions may potentiate or mitigate drug-induced toxicity, (4) hypoxia may alter drug metabolism and thereby therapeutic effectiveness, and (5) therapeutic agents might alter the relative coupling of blood flow and energy metabolism in an organ. The prototypic biochemical effect of hypoxia is related to its known role as a cofactor in a number of enzymatic reactions, e.g., oxidases and oxygenases, which are affected independently from the bioenergetic effect of low oxygen on energetic functions. The cytochrome P-450 family of enzymes is another example. Here, there is a direct effect of oxygen availability on the conformation of the enzyme, thereby altering the metabolism of drug substrates. Indirectly, the NADH/NAD+ ratio is increased with 10% inspired oxygen, leading not only to reduced oxidation of ethanol but also to reduction of azo- and nitro-compounds to amines and disulfides to sulfhydryls. With chronic hypoxia, many of these processes are reversed, suggesting that hypoxia induces the drug-metabolizing systems. Support for this comes from observations that hypoxia can induce the hypoxic inducible factors which in turn alters transcription and function of some but not all cytochrome P-450 isoforms. Hypoxia is identified as a cofactor in cancer expression and metastatic potential. Thus, the effects of hypoxia play an important role in pharmacology, and the signaling pathways that are affected by hypoxia could become new targets for novel therapy or avenues for prevention.

Reactive oxygen intermediate production by oyster hemocytes exposed to hypoxia

Oysters are frequently exposed to severely hypoxic conditions, especially during summer months. During the summer, there are also large numbers of disease-related oyster mortalities. This research was conducted to determine whether exposure to environmental hypoxia reduces the ability of oyster hemocytes to produce reactive oxygen intermediates (ROIs), an important part of their defense system. Oysters of the species Crassostrea virginica were held in normoxic (P(O)(2)=20.0-20.7 kPa, pH 7.8-8.0) and hypoxic conditions (P(O)(2)=4.0-6.7 kPa, pH 7.1-7.4). In vivo hemolymph variables (P(O)(2), P(CO)(2) and pH) were measured after both 1 hour and 2 days in each treatment to determine the appropriate environment for subsequent hemocyte experiments. Production of reactive oxygen intermediates by hemocytes was measured using luminol-enhanced chemiluminescence (CL). During CL tests, hemocytes were held under the following conditions: air (P(O)(2)=20.7, P(CO)(2)<0.07, pH 7.6), in vivo hemolymph conditions of normoxic oysters (P(O)(2)=5.2, P(CO)(2)=0.27, pH 7.6), and in vivo hemolymph conditions of hypoxic oysters (P(O)(2)=1.47, P(CO)(2)=0.53, pH 7.1). Production of ROIs under hypoxic conditions was 33 % of that under normoxia. This decrease was the result of specific and independent effects of lower oxygen levels and decreased pH. It was not due to any direct effect of CO(2).

Effect of hypoxia and reoxygenation on the formation and release of reactive oxygen species by porcine pulmonary artery endothelial cells

Endothelial cells are critical targets in both hypoxia- and reoxygenation-mediated lung injury. Reactive O2 species (ROS) have been implicated in the pathogenesis of hypoxic and reoxygenation lung injury, and xanthine dehydrogenase/oxidase (XDH/XO) is a major generator of the ROS. Porcine pulmonary artery endothelial cells (PAEC) have no detectable XDH/XO. This study was undertaken to examine 1) ROS production by hypoxic porcine PAEC and their mitochondria and 2) ROS production and injury in reoxygenated PAEC lacking XDH/XO activity. Intracellular H2O2 generation and extracellular H2O2 and O2 divided release were measured after exposure to normoxia (room air-5% CO2), hypoxia (0% O2-95% N-5% CO2), or hypoxia followed by normoxia or hyperoxia (95% O2-5% CO2). Exposure to hypoxia results in significant reductions in intracellular H2O2 formation and extracellular release of H2O2 and O2 by PAEC and mitochondria. The reductions occur with as little as a 2 h exposure and progress with continued exposure. During reoxygenation, cytotoxicity was not observed, and the production of ROS by PAEC and their mitochondria never exceeded levels observed in normoxic cells. The absence of XDH/XO may prevent porcine PAEC from developing injury and increased ROS production during reoxygenation.

Autoxidation of oxymyoglobin: a meeting point of the stabilization and the activation of molecular oxygen

1. The primary events of haemoprotein reactions with molecular oxygen have been re-examined by placing special emphasis upon the reduction properties of dioxygen. 2. In the stepwise reduction of O2 to water via hydrogen peroxide, the addition of the first electron is an unfavourable, uphill process with the midpoint potential of -0.33 V, all the subsequent steps being downhill. This thermodynamic barrier to the first step is, therefore, a most crucial ridge located between the stabilization and the activation of dioxygen performed by haemoproteins. 3. If the proteins have a redox potential much higher than -0.33 V, molecular oxygen must bind to the proteins stably and reversibly. In Mb or Hb, however, the FeO2 centre is always subject to a nucleophilic attack of the water molecule or hydroxyl ion, which can enter the haem pocket from the surrounding solvent. These can cause irreversible oxidation of the FeO2 bonding to the ferric met-form with generation of the superoxide anion. 4. In cases of the oxygen activation, if haemoproteins have a redox potential lower than or close to -0.33 V, the first reduction of O2 to O2- would be a spontaneous process. Cytochrome P-450 provides such an example and can facilitate the subsequent addition of electrons that leads to the breaking of the O-O bond to yield the hydroxylating species. 5. As to the proteins whose redox potential is not facilitative and appreciably higher than -0.33 V, a bimetallic, concerted, two-equivalent reduction of the bound dioxygen to the peroxide level would be much more favoured without the intermediate formation of O2-. This is probably the case of cytochrome c oxidase for the reduction of O2 to water. 6. The redox potential diagrams thus visualize various aspects of the ways haemoproteins overcome their thermodynamic constraints and carry out their specific functions in the stabilization and the activation of molecular oxygen.

O2 effect on composition of chick embryonic heart and brain

Heart ventricles from chick embryos incubated in 60% O2 (hyperoxia) on the 16th through the 18th days of incubation were 21% heavier than those from control embryos maintained in 21% O2 (normoxia). Heart ventricles from embyros incubated in 15% O2 (hypoxia) were 8% lighter than controls. Changes in ventricular weight were accompanied by proportional changes in protein content (21% more in hyperoxic ventricles; 8% less in hypoxic ventricles). Ventricular tissue DNA content showed a significant increase in hyperoxia. Tissue protein/DNA ratios were significantly higher in hyperoxia and lower in hypoxia. These data suggest that increased O2 availability led to hypertrophy of chick embryo ventricular cells and an increase in the level of DNA synthesis. Cytochrome oxidase activity per mg DNA was 15-25% higher in hyperoxic ventricles than in hypoxic ventricles. This result is consistent with our previous findings that alterations in O2 availability affect the O2 consumption rate of the chick emryo in ovo, and it provides direct evidence that a phenomenon repeatedly observed in vitro is of importance in vivo. In contrast to the heart, O2 availability did not affect the wet weight, protein or DNA contents, or cytochrome oxidase activity of the chick embryo brain.

Liposome-entrapped tyrosinase: a tool to investigate the regulation of the Raper-Mason pathway

The effect of the entrapment of mushroom tyrosinase (EC 1.14.18.1) within liposomes on the enzyme activity and Km vs. L-3,4-dihydroxyphenylalanine is reported in the present work; the effect of cholesterol insertion within liposome membranes on the enzyme activity has also been studied. The oxidation rates of various monophenols and diphenols by free and liposome-integrated mushroom tyrosinase were measured and the oxidation latencies vs. different substrates investigated. The different substrates are apparently oxidized according to the properties of the substituents as electron donors or acceptors; the Km values vs. L-3,4-dihydroxyphenylalanine calculated on measuring O2 consumption are higher than those calculated on measuring the dopachrome production rates. It is interesting that natural substrates of tyrosinase are oxidized according to a negative catalysis by the liposome-entrapped enzyme; this point is discussed in relation to the well known cytotoxicity of some intermediates of the Raper-Mason pathway.

Nature of the FeO2 bonding in myoglobin: an overview from physical to clinical biochemistry

The iron(II)-dioxygen bond in myoglobin and hemoglobin is a subject of wide interest. Studies range from examinations of physical-chemical properties dependent on electronic structure, to investigations of stability as a function of oxygen supply. Stability properties are of particular importance in vivo, since the oxygenated form is known to be oxidized easily to the ferric form, which cannot be oxygenated and is therefore physiologically inactive. Kinetic and thermodynamic studies of the stability of native oxymyoglobin have revealed a new feature in FeO2 bonding. In vivo, the iron center is always subject to a nucleophilic attack of the water molecule or hydroxyl ion, which can enter the heme pocket from the surrounding solvent, and thereby irreversibly displace the bound dioxygen from MbO2 in the form of O2- so that the iron is converted to the ferric form. A free energy diagram for the potential reactions of FeO2 visualizes myoglobin as a molecular structure that can provide in solution the delicate balance of kinetic and thermodynamic factors necessary to stabilize reversible oxygenation, as opposed to irreversible autoxidation to metmyoglobin.

Purification and biochemical characterization of pyruvate oxidase from Lactobacillus plantarum

Pyruvate oxidase (EC 1.2.3.3) was isolated and characterized from Lactobacillus plantarum. The enzyme catalyzes the oxidative decarboxylation of pyruvate in the presence of phosphate and oxygen, yielding acetyl phosphate, carbon dioxide, and hydrogen peroxide. This pyruvate oxidase is a flavoprotein, with the relatively tightly bound cofactors flavin adenine dinucleotide, thiamine pyrophosphate, and a divalent metal ion, with Mn2+ being the most effective. The enzyme is only slightly inhibited by EDTA, implying that the enzyme-bound metal ion is poorly accessible to EDTA. Only under relatively drastic conditions, such as acid ammonium sulfate precipitation, could a colorless and entirely inactive apoenzyme be obtained. A partial reactivation of the enzyme was only possible by the combined addition of flavin adenine dinucleotide, thiamine pyrophosphate, and MnSO4. The enzyme has a molecular weight of ca. 260,000 and consists of four subunits with apparently identical molecular weights of 68,000. For catalytic activity the optimum pH is 5.7, and the optimum temperature is 30 degrees C. The Km values for pyruvate, phosphate, and arsenate are 0.4, 2.3, and 1.2 mM, respectively. The substrate specificity revealed that the enzyme reacts also with certain aldehydes and that phosphate can be replaced by arsenate. In addition to oxygen, several artificial compounds can function as electron acceptors.

Purification and characterization of a cysteine dioxygenase from the yeast phase of Histoplasma capsulatum

A cysteine dioxygenase, cysteine oxidase (EC 1.13.11.20), has been purified from the cytosolic fraction of yeast phase cells of the dimorphic fungus Histoplasma capsulatum. The cysteine oxidase is an iron-containing dioxygenase with a molecular weight of 10500 (+/- 1500) and is present only in the yeast phase of the fungus. The enzyme is highly specific for L-cysteine, with a Km of 2 X 10(-5) M in vitro. The product of cysteine oxidation is cysteinesulfinic acid, as analyzed by thin-layer chromatography and mass spectroscopy. To our knowledge, this is the first cysteine oxidase isolated from a fungus, and it probably plays an important role in the mycelial to yeast phase transition of H. capsulatum during which redox potential and cysteine levels are crucial factors.