Inactivation of the human CuZn superoxide dismutase during exposure to O-2 and H2O2

Human copper-zinc superoxide dismutase undergoes inactivation when exposed to O₂^? and H₂O₂ generated during the oxidation of acetaldehyde by xanthine oxidase at pH 7.4 and 37° C. In contrast, human manganese superoxide dismutase is not inactivated under the same conditions. Catalase and Mn-superoxide dismutase protect CuZn superoxide dismutase from inactivation. Similar protection is observed with hydroxyl radical (OH.) scavengers, such as formate and mannitol. In contrast, other OH. scavengers such as ethanol and tert-butyl alcohol, have no protective action. The latter results indicate that “free OH.” is not responsible for the inactivation. Furthermore, H₂O₂ generated during the oxidation of glucose by glucose oxidase, i.e., without production of O₂^?, does not induce CuZn superoxide dismutase inactivation. A mechanism accounting for this $\text{O}{}_2{}^{\mathrm{?}}\text{H}{}_2\text{O}{}_2$-dependent inactivation of CuZn superoxide dismutase is proposed.     

Kinetic studies of 18O exchange from inorganic phosphate catalyzed by Mg2+-activated unadenylylated glutamine synthetase (E. coli w).

Oxygen-18 exchange out of [¹⁸O]Pi catalyzed by Mg²⁺-activated unadenylated glutamine synthetase from $\text{E.}$$\text{coli}$ was followed by ³¹P-NMR in the presence of the other substrates, ADP and L-glutamine. The pattern of the ${}^{16}\text{O}{}^{18}\text{O}$ in the species P¹⁸O₄, P¹⁸O₃¹⁶O₁, P¹⁸O₂¹⁶O₂, P¹⁸O₁¹⁶O₃, P¹⁶O₄ during the exchange followed a binomial distribution consistent with indiscriminate removal of any of the four oxygens of P_i. The rate constant for ${}^{16}\text{O}{}^{18}\text{O}$ exchange was 410±40 min^?1 while the rate constant for net reaction (ATP formation) was 62±4 min^?1. Thus exchange proceeds ～7 times faster than net reaction, a finding in accord with that of Stokes and Boyer ($\text{J.}$$\text{Biol.}$$\text{Chem.}$ (1976) $\text{251}$, 5558) for the Mn²⁺-activated adenylylated glutamine synthetase. A model for the overall catalytic events first derived from rapid kinetic fluorescence experiments (Rhee and Chock, Proc. Natl. Acad. Sci. USA, (1976) $\text{73}$, 476) was successfully used to fit the oxygen exchange data in this paper.     

Oxidation of ascorbic acid with superoxide anion generated by the xanthine-xanthine oxidase system.

Ascorbic acid was found to be oxidized by O₂^$\text{?}$ which was generated by the xanthine-xanthine oxidase system. From a kinetic analysis of the inhibition of this reaction by superoxide dismutase, the second-order rate constant for the reaction between ascorbic acid and O₂^? at pH 7.4 was estimated to be 2.7 × 10⁵ M^?1 sec^?1. A function of ascorbic acid as a defense against O₂^? is presented.     

The relative effectiveness of .OH,H2O2,O2−, and reducing free radicals in causing damage to biomembranes: A study of radiation damage to erythrocyte ghosts using selective free radical scavengers

The relative effectiveness of oxidizing (^.OH, H₂O₂), ambivalent (O₂^?) and reducing free radicals (e^? and CO₂^?) in causing damage to membranes and membrane-bound glyceraldehyde-3-phosphate dehydrogenase of resealed erythrocyte ghosts has been determined. The rates of damage to membranebound glyceraldehyde-3-phosphate dehydrogenase ($\text{R}$(enz)) were measured and the rates of damage to membranes ($\text{R}$(mb)) were assessed by measuring changes in permeability of the resealed ghosts to the relatively low molecular weight substrates of glyceraldehyde-3-phosphate dehydrogenase. Each radical was selectively isolated from the mixture produced during gamma-irradiation, using appropriate mixtures of scavengers such as catalase, superoxide dismutase and formate. ^.OH, O₂^? and H₂ O₂ were approximately equally effective in inactivating membrane-bound glyceraldehyde-3-phosphate dehydrogenase, while e^? and CO₂^? were the least effective. $\text{R}$(enz) values of O₂^? and H₂O₂ were 10-times and of ^.OH 15-times that of e^?. $\text{R}$(mb) values were quite similar for e^? and H₂O₂ (about twice that of O₂^?), while that of ^.OH was 3-times that of O₂^?. Hence, with respect to $\text{R}$(mb): ${}^{\mathrm{.}}\text{OH}\text{>}\text{e}{}^{\mathrm{?}}\text{=}\text{H}{}_2\text{O}{}_2\text{>}\text{O}{}_2{}^{\mathrm{?}}$ , and with respect to $\text{R}$(enz): ${}^{\mathrm{.}}\text{OH}\text{>}\text{O}{}_2{}^{\mathrm{?}}\text{=}\text{H}{}_2\text{O}{}_2\text{>}\text{e}{}^{\mathrm{?}}$. The difference between the effectiveness of the most damaging and the least damaging free radicals was more than 10-fold greater in damage to the enzyme than to the membranes. Comparison between H₂O₂ added as a chemical reagent and H₂O₂ formed by irradiation showed that membranes and membrane-bound glyceraldehyde-3-phosphate dehydrogenase were relatively inert to reagent H₂O₂ but markedly susceptible to the latter.     

Photosystem II energy coupling in chloroplasts with H2O2 as electron donor

NH₂OH-treated, non-water-splitting chloroplasts can oxidize H₂O₂ to O₂ through Photosystem II at substantial rates (100–250 μequiv · h^?1 · mg^?1 chlorophyll with 5 mM H₂O₂) using 2,5-dimethyl-p-benzoquinone as an electron acceptor in the presence of the plastoquinone antagonist dibromothymoquinone. This H₂O₂ → Photosystem II → dimethylquinone reaction supports phosphorylation with a $\text{P}\text{e}{}_2$ ratio of 0.25–0.35 and proton uptake with $\text{H}{}^{\mathrm{+}}\text{e}$ values of 0.67 (pH 8)–0.85 (pH 6). These are close to the $\text{P}\text{e}{}_2$ value of 0.3–0.38 and the $\text{H}{}^{\mathrm{+}}\text{e}$ values of 0.7–0.93 found in parallel experiments for the H₂O → Photosystem II → dimethylquinone reaction in untreated chloroplasts. Semi-quantitative data are also presented which show that the donor → Photosystem II → dibromothymoquinone (→O₂) reaction can support phosphorylation when the donor used is a proton-releasing reductant (benzidine, catechol) but not when it is a non-proton carrier (I^?, ferrocyanide).     

Iron(II)-bleomycin. Biochemical and spectral properties in the presence of radical scavengers

Consistent with a recent literature report (Repine, J. E. $\text{et}$$\text{al.}$ (1981) $\text{Proc.}$$\text{Nat.}$$\text{Acad.}$$\text{Sci.}$$\text{USA}$$\text{7}\text{?}$$\text{8}\text{?}$, 1001–1003), the release of [³H]-thymine from PM-2 DNA by Fe(II)-H₂O₂-generated ·OH was suppressed by dimethyl sulfoxide. In contrast, DMSO did not affect [³H]-thymine release mediated by Fe(II)-bleomycin. Under aerobic conditions in the presence of $\text{t}$-butyl phenylnitrone, Fe(II)-BLM produces an epr signal that has been presumed to arise by transfer of ·OH or $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ from the “active complex” of bleomycin to the spin trap. Remarkably, high concentrations (80 mM) of PBN had no effect on the ability of Fe(II)-BLM to solubilize [³H]-thymine, although the ability of authentic ·OH to degrade DNA was completely suppressed under these condition. The suproxide dismutase catalyst tetrakis(4-N-methylpyridyl)porphineiron(III) also failed to suppress BLM-mediated DNA degradation. Moreover, the epr signal observed with 1.6 mM Fe(II)-BLM in the presence of 80 mM PBN was found to be much less intense than that produced by 1.6 mM Fe(II) and 290 mM H₂O₂, but equivalent in intensity to that obtained with 45 mM Fe(II) and exoess H₂O₂. We conclude that the fragmentation of DNA produced by Fe(II)-BLM can be due neither to free ·OH nor to $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$. We suggest that DNA degradation is initiated by an “active complex” consisting of BLM, metal and oxygen that functions by abstracting H· from susceptible sites on DNA.     

Mycoplasma pneumoniae: A prokaryote which consumes oxygen and generates superoxide but which lacks superoxide dismutase

Superoxide dismutase and catalase were not detected in $\text{M}$. $\text{pneumoniae}$ and several other species of $\text{Mycoplasma}$ some of which consume oxygen and secrete H₂O₂. $\text{M}$. $\text{pneumoniae}$ in suspension formed O₂^? in the presence of NADH and flavins and extracts of $\text{M}$. $\text{pneumoniae}$ formed O₂^? in the presence of either NADH or NADPH. The lack of superoxide dismutase in $\text{M}$. $\text{pneumoniae}$ could not be attributed to superoxide dismutase in the complex medium in which the organisms were grown because organisms grown in medium in which the superoxide dismutase had been inactivated by heat still contained undetectable amounts. Mycoplasmas appear to be an exception to the rule that organisms which consume O₂ synthesize superoxide dismutase.     

Do copper ions influence the reduction of ferricytochrome C by O−2?

Recently, it was suggested that the measured rate of reduction of ferricyto chrome $\text{C}$ by O^?₂ below pH 8, was too high in the presence of high concentrations of formate (Koppenol, W.H., Van Buuren, K.J.H., Butler J. and Braams, R. (1976) Biochim. Biophys. Acta 449, 157–168).The high values were attributed to the presence of impurities of copper, which compete for O^?₂. This assumption is consistent with either a decrease in the reduction yield of ferricytochrome $\text{C}$ in the presence of copper, or with a very fast reaction of Cu(I) with ferricytochrome $\text{C}$.It was previously shown by us and by others that the reduction yield of ferricytochrome $\text{C}$ by O^?₂ is 100%. We measured the rate of reduction of ferricytochrome $\text{C}$ by Cu(I), and found that this reaction is slow: $\text{k = (1.5±0.5) · 10}{}^3\text{M}{}^{\mathrm{?1}}\text{) ·}\text{s}{}^{\mathrm{?1}}$.Therefore, our results rule out the possibility that below pH 8 copper impurities affect the measured rate constant of the reduction of ferricytochrome $\text{C}$ by O^?₂.     

The accumulation of superoxide radical during the aerobic action of xanthine oxidase A requiem for H2O4−

The action of xanthine oxidase upon acetaldehyde or xanthine at pH 10.2 has been shown to be accompanied by substantial accumulation of O₂^? during the first few minutes of the reaction. H₂O₂ decreases this accumulation of O₂^? presumably because of the Haber-Weiss reaction ($\text{H}{}_2\text{O}{}_2\text{+}\text{O}{}_2{}^{\mathrm{?}}\text{→}\text{OH}{}^{\mathrm{?}}\text{+}\text{OH}\text{+}\text{O}{}_2$) and very small amounts of superoxide dismutase eliminate it. This accumulation of O₂^? was demonstrated in terms of a burst of reduction of cytochrome $\text{c}$, seen when the latter compound was added after aerobic preincubation of xanthine oxidase with its substrate. The kinetic peculiarities of the luminescence seen in the presence of luminol, which previously led to the proposal of H₂O₄^?, can now be satisfactorily explained entirely on the basis of known radical intermediates.     

Energy-coupling mechanisms under aerobic and anaerobic conditions in autotrophically grown Pseudomonas saccharophila

The cell-free preparations from autotrophieally grown Pseudomonas saccharophila catalyzed the process of electron transport from H₂ or various other organic electron donors to either O₂ or NO₃^? with concomitant ATP generation. The respective $\text{P}\text{O}$ ratios with H₂ and NADH were 0.63 and 0.73, the respective $\text{P}\text{NO}{}_3{}^{\mathrm{?}}$ ratios were 0.57 and 0.54. In contrast, the $\text{P}\text{O}$ and $\text{P}\text{NO}{}_3{}^{\mathrm{?}}$ ratios with succinate were 0.18 and 0.11, respectively. ATP formation coupled to the oxidation of ascorbate, in the absence or presence of added N,N,N′,N′-tetramethyl-p-phenylenediamine or cytochrome c, could not be detected. Various uncouplers inhibited phosphorylation with either O₂ or NO₃^? as terminal electron acceptors without affecting the oxidation of H₂ or other substrates. The NADH oxidation at the expense of O₂ or NO₃^? reduction as well as the associated phosphorylation were inhibited by rotenone and amytal. The aerobic and anaerobic H₂ oxidation and coupled ATP synthesis, on the other hand, was unaffected by the flavoprotein inhibitors as well as by the NADH trapping system. The NADH, H₂, and succinate-linked electron transport to O₂ or NO₃^? and the associated phosphorylations were sensitive, however, to antimycin A or 2-n-nonyl-4-hydroxyquino-line-N-oxide, and cyanide or azide. The data indicated that although the phosphorylation sites 1 and II were associated with NADH oxidation by O₂ or NO₃^?, the energy conservation coupled to H₂ oxidation under aerobic or anaerobic conditions appeared to involve site II only.     

Superoxide-dependent formation of hydroxyl radicals: Detection of hydroxyl radicals by the hydroxylation of aromatic compounds

A mixture of xanthine or hypoxanthine and xanthine oxidase generates the superoxide radical, O₂^$\text{?}$, and H₂O₂. In the presence of iron salts, O₂^$\text{?}$ and H₂O₂ can interact to produce the hydroxyl radical, OH·. Superoxide-dependent formation of OH· can be measured by its ability to hydroxylate salicylate as followed by an improved colorimetric assay described in this paper. A more accurate analysis of OH· can be obtained using its ability to hydroxylate phenol, the hydroxylated products being separated and measured after derivatization using gas-liquid chromatography and electron-capture detection. The derivatization and separation techniques are described.     

Experimental determination of the redox potential of the superoxide radical O 2

The redox potential of ^?O₂^? was determined based on the dependence of the electron transfer reaction from ^?O₂^? upon the known redox potential of various acceptors A (including a range of quinones, dyes and ferricyanide). The efficiencies and the rates of these electron transfer processes were determined, using the technique of pulse radiolysis, by monitoring the formation kinetics of the semiquinone radical anions at the appropriate wavelength. From the percentage efficiency versus E^o′ plot, an E^o′ value of + 0.15 ± 0.01 V at pH 7.0 and 22°C, or E^o = + 0.57 ± 0.01 V, for the ${}^{\mathrm{?}}\text{O}{}_2{}^{\mathrm{?}}\text{O}{}_2$ couple was obtained. The rate k(^?O₂^? + A → O₂ + ^?A^?) = 9.8 × 10⁸ M^?1 sec^?1 where A = p-benzoquinone. The E^o value for the ^?HO₂ radical is > 1.0 V. It was also found that hydroquinone can quantitatively reduce ^?O₂^? to H₂O₂, k(^?O₂^? + QH₂ → QH^? + HO₂^?) = 1.6 ± 0.1 × 10⁷ M^?1 sec^?1 at pH 7.0 and 22°C.     

The one-electron transfer redox potentials of free radicals. I. The oxygen/superoxide system

The method of determination of Redox potentials of radicals, using the pulse radiolysis technique, is outlined. The method is based on the determination of equilibrium constants of electron transfer reactions between the radicals and appropriate acceptors. The limitations of this technique are discussed.The redox potentials of several quinones-semiquinones are calculated, as well as the standard redox potential of the peroxy radical. and the redox oxidation properties of the peroxy radical in various systems and pH are discussed. The value determined for the redox potentials of $\text{O}{}_2\text{O}{}_2{}^{\mathrm{?}}$ is higher by more than 0.2 V than earlier estimates, which has important implications on the possible role of O₂^? in biological processes of O₂ fixation.     

Dual action of ionophore A23187 on intact chloroplasts

1. Ionophore A23187 induces uncoupling of potassium ferricyanide-dependent O₂ evolution by envelope-free chloroplasts and oxaloacetate-dependent O₂ evolution by intact chloroplasts. The half maximal concentration ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for stimulation of oxygen evolution in both cases is approximately 4 μM · 100 μg chlorophyll · ml^?1.2. Ionophore A23187 also induces inhibition of CO₂ and 3-phosphoglycerate-dependent O₂ evolution by intact chloroplasts in the presence of 3 mM MgCl₂. The half maximal concentrations ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for inhibition of O₂ evolution are 3 μM and 5 μM respectively · 100 μg^?1 chlorophyll · ml^?1.3. A very high concentration of ionophore A23187 (10 μM · 20 μg^?1 chlorophyll · ml^?1) plus 0.1 mM EDTA lowers the fluorescence yield of intact chloroplasts suspended in a cation-free medium in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, indicating loss of divalent cation from the diffuse double layers of the thylakoid membranes.4. These results are discussed in relation to ionophore A23187-induced divalent cation/proton exchange at both the thylakoid and the envelope membranes of intact chloroplasts.     

H/2H isotope effect in redox reactions of cytochrome c

The rate of reaction of ferro- and ferricytochrome c (C(II) and C(III)) with ferri- and ferrocyanide and of C(III) with O₂^? and CO₂^? was determined in H₂O and in ²H₂O in the temperature range 5–35 °C. No isotope effect was evident in any of the reductions of C(III); the apparent energy of activation was identical in H₂O and ²H₂O. An isotope effect with , depending on pH for instance was observed in the oxidation of C(II), in the slow phase of oxidation which involves conformational changes. An interpretation (supported by evidence from previous work) involving water molecules in the close vicinity of the reaction site on the protein is discussed.     

Complete stoichiometry of free NADPH oxidation in liver microsomes

The stoichiometry of free NADPH oxidation in phenobarbital induced rabbit liver microsomes was measured by means of registering the rates of NADPH, H⁺ and O₂ consumption and $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ and H₂O₂ production. $\text{ΔO}{}_2{}^{\mathrm{<mtext>?</mtext>}}\text{:ΔH}{}_2\text{O}{}_2$ ratio is approximately I indicating that about half H₂O₂ results from $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ dismutation, the second half being formed directly. ΔNADPH:ΔH₂O₂ and ΔO₂:ΔH₂O₂ ratios exceed I and therefore another product of the reaction is water. The fact that the ratio (ΔNADPH-ΔH₂O₂):(ΔO₂-ΔH₂O₂) is 2 allows one to consider direct 4-electron O₂ reduction as the major way of water formation rather than endogenous substrate hydroxylation.     

Enzymatic formation of (20R, 22R)-20,22-dihydroxycholesterol from cholesterol and a mixture of 16O2 and 18O2: random incorporation of oxygen atoms

[4-¹⁴C]Cholesterol was incubated with an adrenocortical preparation in the presence of ¹⁶O₂ and ¹⁸O₂ devoid of significant ¹⁶O¹⁸O. Isolated (20R,22R)-20,22-dihydroxycholesterol was converted to a trimethylsilyl derivative and analyzed by gas chromatography - mass spectrometry to determine the isotope distribution of the oxygen atoms at C-20 and C-22. The ions of $\text{m}\text{e}$ 289, 291, and 293 (comprising the C₈ C-20 to C-27 side-chain and containing, respectively, ¹⁶O₂, ¹⁶O¹⁸O, and ¹⁸O₂) exhibited a binomial distribution indicating that the oxygen atoms of the vicinal glycol were drawn at random from the atomic pool of the oxygen molecules. If both side-chain hydroxyl groups had originated from the atoms of the same oxygen molecule, the ion of $\text{m}\text{e}$ 291 would have been absent.     

Increase in glutathione peroxidase activity in erythrocytes from trisomy 21 subjects.

Glutathione peroxidase activity has been measured in erythrocytes from normal subjects and from trisomy 21 patients. The latter cases show about 50 % increase of this enzyme similar to the increase observed for superoxide dismutase (erythrocuprein) suggesting either localisation of the gene for glutathione peroxidase on chromosome 21 (as is the case for erythrocuprein) or regulation of this enzyme by intracellular levels of O₂^$\text{?}$, H₂O₂ or superoxide dismutase.     

5-Nitro-2'-deoxyuridine 5'-monophosphate is a potent irreversible inhibitor of Lactobacillus caesi thymidylate synthetase.

5-Nitro-2′-deoxyuridine 5′-monophosphate was found to be an active sitedirected irreversible inhibitor of thymidylate synthetase from $\text{Lactobacillus caesi}$. It's K_I was determined as 2.9 × 10^?8$\text{M}$ from a double-reciprocal plot of velocity $\text{vs}$ substrate concentration.     

Cyanide-resistant respiration in yeast. II. Characterization of a cyanide-insensitive NAD(P)H oxidoreductase

A single protein species isolated from yeast (preceding paper) which catalyzes the cyanide-resistant reduction of molecular oxygen by reduced pyridine nucleotides has been characterized using spectral and chemical criteria. This NAD(P)H:O₂ oxidoreductase is a metalloflavoprotein containing FMN as a prosthetic group and Cu²⁺ as the metal ion. The enzyme which is devoid of nonheme iron centers is inhibited by the chelators m-chlorohydroxamic acid; salicylhydroxamic acid, and bathophenathroline sulfonate. Of the many substrates tested only NADH and NADPH were found to be active with the latter having a higher apparent K_m (2.1 of 10^?5 vs 1.4 × 10^?6m). Stopped-flow kinetics established that the $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for these substrates was 0.41 and 0.32, respectively. It is suggested that the function of this enzyme is associated with the nonmitochondrial P-450 system. A tentative pathway of electron flow has been proposed to NADPH → FMN → Cu²⁺ → H₂O₂.