Radical-like Species Research Articles

Mild iron overload effect on rat liver nuclei

A single injection of iron-dextran significantly increased iron content in plasma, whole liver, cellular cytosol and liver nuclei. In vitro nuclear rate of Fe 3+-EDTA reduction was not affected by the treatment. Membrane-bound enzymatic activities in the nuclei were measured after iron overload. Both NADPH- and NADPH-dependent cytochrome c reductases were slightly decreased after iron overload, but cytochrome P 450 was undetectable after 6 h of iron supplementation. The contents of lipid- and water-soluble antioxidants were measured in isolated nuclei from control and iron-overloaded rats. α-Tocopherol and ß-carotene co-elutant were decreased by 40% and 83%, respectively after 6 h of treatment. Nuclear glutathione content was not affected. The rate of generation of superoxide anion (O 2 −, hydrogen peroxide (H 2O 2) and hydroxyl radical-like species by isolated rat liver nuclei, were decreased by 50%, 40% and 60%, respectively after 6 h of iron supplementation. An identical qualitative response to iron overload was observed with NADPH and NADH. The inactivation of nuclear cytochrome P 450, the significant loss in lipid-soluble antioxidants (α-tocopherol and ß-carotene) and the decrease in enzyme-dependent oxygen radical generation, suggest that the increase in catalytic active iron induced by iron overload could affect the cellular nuclei functionality.

Increased Production of Reactive Oxygen Species by Rat Liver Mitochondria After Chronic Ethanol Treatment

Rat liver microsomes and, to a lesser extent, nuclei were previously shown to produce reactive oxygen species at elevated rates after chronic ethanol treatment. The ability of intact rat liver mitochondria to interact with iron and either NADH or NADPH, and the effects of ethanol treatment, on production of reactive oxygen intermediates was determined. In the presence of ferric-ATP, NADH or NADPH catalyzed mitochondrial lipid peroxidation. Rates were elevated two- to threefold with mitochondria from ethanol-fed rats with both reductants. Mitochondrial lipid peroxidation was insensitive to superoxide dismutase, catalase, or hydroxyl radical scavengers but was sensitive to GSH and anti-oxidants such as trolox. Mitochondrial generation of hydroxyl radical-like species (assayed by oxidation of chemical scavengers) was increased after chronic ethanol treatment, as was H2O2 production. Modifiers of mitochondrial metabolism such as rotenone, cyanide, or an uncoupling agent, had no effect on mitochondrial production of reactive oxygen intermediates. The membrane-impermeable thiol reagent, p-chloromercuribenzoate, was completely inhibitory with both mitochondrial preparations. The activity of the rotenone-insensitive NADH-cytochrome c reductase, an enzyme of the outer mitochondrial membrane, was increased 40 to 60% by the ethanol treatment. These results suggest that NADH acting via the outer membrane NADH reductase can catalyze an iron-dependent production of oxygen radicals by rat liver mitochondria. The outer mitochondrial membrane fraction, prepared by digitonin fractionation, displayed increased rotenone-insensitive NADH-cytochrome c reductase activity after ethanol treatment and was more reactive in catalyzing scission of pBR322 DNA from the supercoiled form to the open circular forms. Rates of oxygen radical production by mitochondria and the extent of increase produced by chronic ethanol treatment are similar to those previously found with microsomes when NADH is the cofactor. Oxidation of ethanol by alcohol dehydrogenase generates NADH, and NADH-dependent production of reactive oxygen species by various organelles is increased after chronic ethanol treatment. These acute metabolic interactions coupled to induction by chronic ethanol treatment may play an important role in the development of a state of oxidative stress in the liver by ethanol.

Efficient H2O2 Oxidation of Alkanes and Arenes to Alkyl Peroxides and Phenols Catalyzed by the System Vanadate-Pyrazine-2-Carboxylic Acid

H 2 O 2 oxidizes cyclohexane and other alkanes in CH 3 CN at 20-70°C in the presence of the catalyst Bu 4 NVO 3 -pyrazine-2-carboxylic acid (ratio 1 : 4) to afford, after reduction with PPh 3 , the corresponding alcohol and carbonyl derivatives (the ratio decreases from 12 to 4 on raising the temperature from 20 to 70°C). The product yield reaches 46%, based on H 2 O 2 , and the turnover number is ca. 1000. The oxidation appears to give predominantly alkyl peroxides. which are partially decomposed in the course of the reaction. Peroxides can be formed when alkyl radicals, generated from alkanes, and radical-like species react with O 2 or abstract OOH groups from the vanadium complex. Benzene and its alkyl derivatives are oxidized to produce phenols as well as side-chain oxidation derivatives

Stimulation of microsomal production of reactive oxygen intermediates by rifamycin SV: Effect of ferric complexes and comparisons between NADPH and NADH

Rifamycins are antibacterial antibiotics which are especially useful for the treatment of tuberculosis. Reactive oxygen intermediates are produced in the presence of rifamycin SV and metals such as copper or manganese. Experiments were carried out to evaluate the interaction of rifamycin SV with rat liver microsomes to catalyze the production of reactive oxygen species. At a concentration of 1 m m, rifamycin SV increased microsomal production of superoxide with NADPH as cofactor 3-fold, and with NADH as reductant by more than 5-fold. Rifamycin SV increased rates of H 2O 2 production by the microsomes twofold with NADPH, and 4- to 8-fold with NADH. In the presence of various iron complexes, microsomes generated hydroxyl radical-like (· OH) species. Rifamycin SV had no effect on NADPH-dependent microsomal · OH production, irrespective of the iron chelate. A striking stimulation of · OH production was found with NADH as the reductant, ranging from 2- to 4-fold with catalyst such as ferric-EDTA and ferric-DTPA to more than 10-fold with ferric-ATP, -citrate, or -histidine. Catalase and competitive · OH scavengers lowered rates of · OH production (chemical scavenger oxidation) and prevented the stimulation by rifamycin. Superoxide dismutase had no effect on the NADH-dependent rifamycin stimulation of · OH production with ferric-EDTA or -DTPA, but was inhibitory with the other ferric complexes. In contrast to the stimulatory effects on production of O ⨪2, H 2O 2, and · OH, rifamycin SV was a potent inhibitor of microsomal lipid peroxidation. These results show that rifamycin SV stimulates microsomal production of reactive oxygen intermediates, and in contrast to results with other redox cycling agents, is especially effective with NADH as the microsomal reductant. These interactions may contribute to the hepatotoxicity associated with use of rifamycin, and, since alcohol metabolism increases NADH availability, play a role in the elevated toxic actions of rifamycin plus alcohol.

THE EFFECT OF CHRONIC ETHANOL CONSUMPTION ON NADH- AND NADPH-DEPENDENT GENERATION OF REACTIVE OXYGEN INTERMEDIATES BY ISOLATED RAT LIVER NUCLEI

Previous results have shown that microsomes from ethanol-treated rats generate reactive oxygen intermediates at elevated rates as compared to pair-fed controls in the presence of NADH and especially NADPH. Since isolated rat liver nuclei can produce oxygen radicals with NADH or NADPH as reductants, the effect of chronic ethanol treatment on nuclear generation of reactive oxygen intermediates was determined. Ethanol treatment increased the activity of NADH (+27%) and NADPH (+50%) cytochrome c reductase in the nucleus. Nuclear lipid peroxidation, H2O2 production, and generation of hydroxyl radical-like species were increased by about 25 to 40% after ethanol treatment. In contrast to microsomes, where NADPH-dependent rates were higher than the NADH-dependent rates, in nuclei, NADH was as effective as, or even more reactive than NADPH in promoting production of various oxidizing species. The increases in oxygen radical production by nuclei after ethanol treatment were less than the increases found previously for microsomes. Moreover, rates of oxygen radical production by nuclei were less than 10% of the corresponding rates found with microsomes, suggesting that it is unlikely that the small increases found with nuclei after ethanol treatment contribute significantly towards the development of a state of oxidative stress in the liver.

The oxidative conversion of methane to higher hydrocarbons

Many transition metal oxides have been evaluated as oxidative coupling catalysts for converting methane to C 2 and higher hydrocarbons. Reactions were done in a cyclic redox mode in which oxidized catalyst was reacted with methane in the absence of oxygen to form coupling products and reduced catalyst which was reoxidized with air in a separate step. Manganese, indium, germanium, antimony, tin, bismuth, and lead oxides were found to be effective coupling catalysts, giving 10 to 50% selectivity to higher hydrocarbons. Silica is a superior support compared to alumina. Mechanistic studies with manganese oxide on silica indicate that the initial coupling product is ethane which is formed via dimerization of a CH 3 radical-like species. The ethane is oxidatively dehydrogenated to ethylene which may react with CH 3 to give propylene. The major path for combustion involves sequential oxidation of products.

Ethylene production from α-keto-4-thiomethylbutyric acid by isolated rat liver cells, suspension medium, and perfusates in the absence and presence of iron

Experiments were carried out to evaluate the production of hydroxyl radical-like species by intact rat liver cells by assaying for the production of ethylene from α-keto-4-thiomethylbutyric acid in the absence and presence of added iron. In the absence of iron, a low rate of ethylene production, which was not sensitive to superoxide dismutase, catalase, or competitive scavengers was observed. Ethylene was produced when KMBA was incubated with perfusates of rat liver or the suspension medium obtained after incubating liver cells for varying periods of time, followed by removal of the liver cells. Boiling the perfusate or suspension medium had no effect on ethylene production. This ethylene production does not appear to reflect an oxygen radical-mediated event. The addition of ferric-EDTA, but not ferric-desferrioxamine, increased ethylene production by the hepatocyte suspensions in a reaction sensitive to inhibition by catalase, ascorbate oxidase, and competitive scavengers, but not superoxide dismutase. The sensitivity to externally added catalase and ascorbate oxidase suggested that the ethylene production reflected an extracellular oxygen radical generating system. Ferric alone and several ferric chelates, for example, ferric-ATP, ADP, AMP, histidine, and citrate stimulated ethylene production using perfusates of liver or suspension medium after removal of the hepatocytes. The sensitivity of the added iron system to ascorbate oxidase suggested that during perfusion or incubation of liver cells, efflux of ascorbate occurs, follwed by reduction of the iron and subsequently, extracellular production of oxygen radicals. Indeed, a strong correlation was observed between rates of ethylene production in the presence of iron and levels of ascorbate in the perfusates. Thus, the use of KMBA plus iron is not suitable to detect intracellular production of hydroxyl radical-like species in cells such as hepatocytes, which leak ascorbate.

Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

The half-wave reduction potentials, measured by cyclic voltammetry, and n,π* triplet energies, measured by phosphorescence spectroscopy, were determined for a series of eighteen symmetrically and unsymmetrically substituted benzophenones. Attempts are made to correlate the results with Hammett substituent constants. Better correlations are observed when the data are correlated with a two-parameter function consisting of Hammett substituent constants and a set of substituent parameters describing variations in free radical stability. Significant deviations from "normal" behaviour are observed for benzophenones substituted by both electron-donating and electron-withdrawing substituents. These deviations are attributed to merostabilization of the radical-like species, and an empirical approach designed to evaluate the importance of this effect is developed. Abinitio calculations of molecular orbital energies in meta- and para-substituted benzaldehydes are used to evaluate the substituent effects on E1/2red and ETn,π* in terms of the effect on the energies of the n- and π*-orbitals.