Reactions of boron with soils

Boron is an essential micronutrient for plants, but the range between deficient and toxic B concentration is smaller than for any other nutrient element. Plants respond directly to the activity of B in soil solution and only indirectly to B adsorbed on soil constituents. Soil factors affecting availability of B to plants are: pH, texture, moisture, temperature, organic matter and clay mineralogy. Boron adsorbing surfaces in soils are: aluminium and iron oxides, magnesium hydroxide, clay minerals, calcium carbonate, and organic matter. Boron adsorption reactions can be described empirically using the Langmuir adsorption isotherm equation, the Freundlich adsorption isotherm equation, and the phenomenological Keren model. Chemical models such as the constant capacitance model, the triple layer model, and the Stern VSC-VSP model can describe B adsorption over changing conditions of solution pH and B concentration. Boron desorption reactions often exhibit hysteresis. The rate of B desorption can be described using the first order rate equation, the Elovich reaction rate equation, and the power function equation.     

Reactions of aminomalononitrile with electrophiles

Aminomalononitrile (HCN trimer) reacts with electrophiles such as aldehydes and acrylonitrile under very mild conditions of temperature and pH to produce intermediates which, after acid hydrolysis, yield amino acids. The following amino acids have been identified and quantitated: glycine, D, L-erythro- and D, L-threo-beta - hydroxyaspartic acids, D, L glutamic acid, and D, L-threonine and allo-threonine. The mechanism of their formation and the possible significance of these reactions in prebiotic syntheses are discussed.     

Reactions of superoxide with myeloperoxidase

When neutrophils ingest bacteria, they discharge superoxide and myeloperoxidase into phagosomes. Both are essential for killing of the phagocytosed micro-organisms. It is generally accepted that superoxide is a precursor of hydrogen peroxide which myeloperoxidase uses to oxidize chloride to hypochlorous acid. Previously, we demonstrated that superoxide modulates the chlorination activity of myeloperoxidase by reacting with its ferric and compound II redox states. In this investigation we used pulse radiolysis to determine kinetic parameters of superoxide reacting with redox forms of myeloperoxidase and used these data in a steady-state kinetic analysis. We provide evidence that superoxide reacts with compound I and compound III. Our estimates of the rate constants for the reaction of superoxide with compound I, compound II, and compound III are 5 x 10(6) M-1 s-1, 5.5 +/- 0.4 x 10(6) M-1 s-1, and 1.3 +/- 0.2 x 10(5) M-1 s-1, respectively. These reactions define new activities for myeloperoxidase. It will act as a superoxide dismutase when superoxide reacts consecutively with ferric myeloperoxidase and compound III. It will also act as a superoxidase by using hydrogen peroxide to oxidize superoxide via compound I and compound II. The favorable kinetics of these reactions indicate that, within the confines of a phagosome, superoxide will react with myeloperoxidase and affect the reactions it will catalyze. These interactions of superoxide and myeloperoxidase will have a major influence on the way neutrophils use oxygen to kill bacteria. Consequently, superoxide should be viewed as a cosubstrate that myeloperoxidase uses to elicit bacterial killing.     

Reactions of hypochlorite with catalase

OCl-/HOCl imposed a rapid inactivation of catalase (hydrogen-peroxide: hydrogen-peroxide oxidoreductase, EC 1.11.1.6), some of which was slowly reversible upon subsequent exposure to H2O2. Ethanol accelerated this restoration of activity by H2O2. OCl- caused biphasic changes in the visible absorption spectrum of catalase, which were partially reversed by dithionite. A scheme of reactions involving axial ligation of one or two OCl- to heme iron, followed by heterolytic or homolytic cleavages of the O-Cl bond, is proposed to account for the behavior of the system.     

Reactions of heme peroxidases with peroxynitrite

The interaction of peroxynitrite, produced by ozonation of azide, with two heme peroxidases (horseradish peroxidase and lactoperoxidase) was studied. Enzymes retained full activity after incubation with peroxynitrite at neutral pH. Lactoperoxidase alone was found to catalyze peroxynitrite decomposition, whereas horseradish peroxidase accelerated peroxynitrite decomposition only in the presence of certain substrates. For example, in the presence of guaiacol the catalyzing effect was clear, but in the presence of trolox was only noticeable.     

Reactions of oxyl radicals with DNA

The importance of radical-induced damage to DNA is apparent from the ever-increasing number of publications in this area. This review focuses on the damage caused to DNA by reactive oxygen-centred radicals, however formed. These may be hydroxyl radicals, which arise either from the radiolysis of water by ionizing radiation (γ-rays or X-rays), or from a purely chemical source. Alternatively, metal-bound oxyl radicals (M–O·) are also active intermediates in DNA-cleaving reactions and may be formed from synthetic compounds or from natural products such as bleomycin (BLM). Chemical mechanisms leading to the observed degradation products are covered in detail. The biological effects of some of the DNA base lesions formed are touched upon, concentrating on the molecular mechanisms behind the initial events that lead to mutagenesis.     

Reactions of reducing radicals with ribonuclease

Radiation-induced reactions of hydrated electrons, formate- and ethanol radicals with ribonuclease were studied by pulse radiolysis and by electrophoresis. Initially formate radicals react rapidly and very specifically with the disulphide bonds of ribonuclease. This reaction leads to aggregation by intermolecular S-S-interchange, the process being more effective at pH 4, since formation and decay of S-S-.-radical anions increases with decreasing pH. With high doses additional unreducible aggregates are formed. Radical formation at the positively charged histidine residues seems to be involved. Hydrated electrons do not react as selectively as the formate radicals, but with several sites in native ribonuclease. Thus with low doses unreducible aggregates are formed. Electrophoresis shows that reaction of the electrons causes fragmentation of the peptide chain, when OH-radicals are scavenged. Very weak transient spectra and very little degradation result on reaction of ethanol radicals with ribonuclease.     

Halogenation and Oxidation Reactions with Haloperoxidases

Haloperoxidases are enzymes which catalyze the incorporation of halogen atoms into organic molecules. They are found throughout nature, playing a major role in the defence system of many organisms. Their reaction mechanisms as well as their use as catalysts for halogenation and oxidation reactions on laboratory and industrial scales are discussed. Up to now, selective halogenation reactions have only been reported for the chloroperoxidase from Pseudomonas pyrrocinia. The usefulness of the other enzymes is based on their ability to produce hypohalous acid (HOX) in a controllable way, allowing the smooth (yet nonselective) halogenation of electron-rich substrates. On the other hand, it has been shown recently that some haloperoxidases can stereoselectively convert sulfides and alkenes into their corresponding homochiral oxides. Therefore, these enzymes will undoubtedly gain importance in the near future.     

Halogenation and Oxidation Reactions with Haloperoxidases

Haloperoxidases are enzymes which catalyze the incorporation of halogen atoms into organic molecules. They are found throughout nature, playing a major role in the defence system of many organisms. Their reaction mechanisms as well as their use as catalysts for halogenation and oxidation reactions on laboratory and industrial scales are discussed. Up to now, selective halogenation reactions have only been reported for the chloroperoxidase from Pseudomonas pyrrocinia. The usefulness of the other enzymes is based on their ability to produce hypohalous acid (HOX) in a controllable way, allowing the smooth (yet nonselective) halogenation of electron-rich substrates. On the other hand, it has been shown recently that some haloperoxidases can stereoselectively convert sulfides and alkenes into their corresponding homochiral oxides. Therefore, these enzymes will undoubtedly gain importance in the near future.     

Reactions of dysprosium with adenine nucleotides

Dysprosium catalyzes a rapid hydrolysis of both ATP and ADP, at ambient temperatures, pH 7.0, where no hydroxide precipitates. The reactive complexes, at pH 6.7, were found to contain 2Dy:1ATP and 3Dy:2ADP. AMP forms an insoluble complex containing 1Dy:2AMP, which does not hydrolyze. ATP also forms a soluble 1Dy:1ATP complex, which does not react. Dysprosium only catalyzes the hydrolysis of ATP above pH 5.8, where it has been titrated to the hydroxide. At the optimum pH (pH 7) the stoichiometric composition is Dy2.ATP.(OH)2, indicating that the active complex is neutral, whereas at pH 5.8 the stoichiometric composition is Dy2.ATP.(OH)+, indicating an inactive cationic complex. The mechanism proposed for the hydrolysis is consistent with those proposed for other in vitro systems known to catalyze the hydrolysis of ATP.     

Reactions of the HCN-tetramer with aldehydes

The HCN-tetramer, a 'classic' of the prebiotic chemistry of HCN, is shown to undergo a remarkable reaction with acetaldehyde in slightly basic or neutral aqueous solution at room temperature. The reaction consists in an aldolization-type C,C-bond formation, accompanied by a (presumably aldehyde-catalyzed) hydration of one of the two nitrile groups and the formation of two cyclic aminal-type groupings, each of the latter incorporating an additional molecule of the aldehyde. Should this so far unexplored type of chemistry of the HCN-tetramer prove to have some generality, the finding might add a new dimension to the potential etiological relevance of this HCN-oligomer.     

Endotoxin Induces Severe Inflammatory Reactions with Necrosis at Sites Primed with Delayed-Type Hypersensitivity Reactions in Guinea Pigs

Guinea pigs immunized with Freund's complete adjuvant received challenge injection of the purified protein derivative of Mycobacterium tuberculosis in the flanks and the corneas to prepare delayed-type hypersensitivity (DTH) reactions. The animals were injected subcutaneously with lipopolysaccharide (LPS) or a synthetic lipid A (LA-15-PP). At the skin site primed with DTH reaction, increased swelling and hemorrhagic reaction followed by a definite necrotic reaction occurred. Severe corneal reactions were also observed in the animals. These findings indicate that bacterial endotoxin modulates DTH reactions and induces severe inflammatory reactions.