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Query: UMLS:C0027960 (mole)
 21,279 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

In the presence of hexacyanoferrate(III), or other suitable oxidants, transketolase catalyzes the oxidative cleavage of its donor substrates xylulose 5-phosphate or fructose 6-phosphate into glycolate and glyceraldehyde 3-phosphate or erythrose 4-phosphate, respectively. Two moles of hexacyanoferrate(III) are reduced per mole of oxidatively cleaved donor substrate. In analogy to the oxidative trapping of carbanion intermediates of other enzymes [Healy, M. J. & Christen, P. (1973) Biochemistry, 12, 35-41], the kinetic features of the reaction indicate that the 1,2-dihydroxyethylthiamin diphosphate intermediate is the oxidation-susceptible species. The molecular activity for the oxidative cleavage of fructose 6-phosphate at a hexacyanoferrate(III) concentration of 0.5 mM is 0.2% of that for the normal transfer reaction with erythrose 4-phosphate as acceptor substrate. Glycolate is also produced with H2O2 as oxidant; however, the reaction is at least two orders of magnitude slower than with hexacyanoferrate(III).
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PMID:Production of glycolate by oxidation of the 1,2-dihydroxyethyl-thamin-diphosphate intermediate of transketolase with hexacyanoferrate(III) or H2O2. 699 16

At physiological pH values, oxidation of the neurotransmitter dopamine (DA) by the peroxidase/H2O2 system leads to, besides dopaminochrome and 5,6-dihydroxyindole resulting from oxidative cyclization of dopaminequinone (DQ), significant amounts of the neurotoxin 6-hydroxydopamine (6-OHDA) in the oxidized quinonoid form (topaminequinone, TQ). Formation of TQ was shown to depend critically on the presence of hydrogen peroxide in the reaction medium and was not observed when DA oxidation was carried out using the tyrosinase/O2 system or chemical agents such as periodate or ferricyanide. These and other data suggest that, under the conditions adopted, nucleophilic attack of the hydrogen peroxide anion on DQ leading to TQ significantly competes with the intramolecular cyclization path. In line with this mechanism, the reaction course was not affected by the presence of hydroxyl radical scavengers. Peroxidase/H2O2 oxidation of the model N-acetyldopamine (1) gave, as expected, the 2-hydroxy-1,4-benzoquinone 3 in yields up to 55%, depending on the catecholamine/H2O2 mole ratio. Likewise, reaction of 4-methyl-1,2-benzoquinone (4) with hydrogen peroxide afforded 2-hydroxy-5-methyl-1,4-benzoquinone (5) in good yields. Collectively, these results would point to the possibility that intraneuronal formation of 6-OHDA is associated with an increased production of hydrogen peroxide under oxidative stress conditions.
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PMID:Generation of the neurotoxin 6-hydroxydopamine by peroxidase/H2O2 oxidation of dopamine. 769 8

Addition of HS- enhanced the O(2-)-scavenging activity of bovine erythrocyte Cu,Zn superoxide dismutase (EC 1.15.1.1) by about twofold. The positive effect was measured using a diverse selection of SOD activity assays, and cannot be an artifact restricted to any single technique. Km values for HS- varied in different assay techniques, but we estimate Km approximately 80 microM HS-. In contrast to HS-, other small molecules tested with SOD either had little effect or were inhibitory. Consumption of HS- and O2- occurred in nearly 1:1 mole ratio. The products were H2O2 and sulfane sulfur, such as either elemental sulfur or polysulfide. Binding of HS- to the enzyme was rapid, with k > 10(7) M-1 s-1. The resulting complex exhibited a Cu-to-S charge-transfer absorbance band at 345 nm and an altered Cu(II) EPR spectrum. Taken together, these observations suggest that HS- binds at the catalytic Cu center of SOD and can be a genuine substrate of the enzyme.
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PMID:Interaction of Cu,Zn superoxide dismutase with hydrogen sulfide. 773 52

Myeloperoxidase and eosinophil peroxidase catalyzed the oxidation of bromide ion by hydrogen peroxide (H2O2) and produced a brominating agent that reacted with amine compounds to form bromamines, which are long-lived oxidants containing covalent nitrogen-bromine bonds. Results were consistent with oxidation of bromide to an equilibrium mixture of hypobromous acid (HOBr) and hypobromite ion (OBr-). Up to 1 mol of bromamine was produced per mole of H2O2, indicating that bromamine formation prevented the reduction of HOBr/OBr- by H2O2 and the loss of oxidizing and brominating activity. Bromamines differed from HOBr/OBr- in that bromamines reacted slowly with H2O2, were not reduced by dimethyl sulfoxide, and had absorption spectra similar to those of chloramines, but shifted 36 nm toward higher wavelengths. Mono- and di-bromo derivatives (RNHBr and RNHBr2) of the beta-amino acid taurine were relatively stable with half-lives of 70 and 16 h at pH 7, 37 degrees C. The mono-bromamine was obtained with a 200-fold excess of amine over the amount of HOBr/OBr- and the di-bromamine at a 2:1 ratio of HOBr/OBr- to the amine. In the presence of physiologic levels of both bromide (0.1 mM) and chloride (0.1 M), myeloperoxidase and eosinophil peroxidase produced mixtures of bromamines and chloramines containing 6 +/- 4% and 88 +/- 4% bromamine. In contrast, only the mono-chloramine derivative (RNHCl) was formed when a mixture of hypochlorous acid (HOCl) and hypochlorite ion (OCl-) was added to solutions containing bromide and excess amine. The rapid formation of the chloramine prevented the oxidation of bromide by HOCl/OCl-, and the chloramine did not react with bromide within 1 h at 37 degrees C. The results indicate that when enzyme-catalyzed bromide or chloride oxidation took place in the presence of an amine compound at 10 mM or higher, bromamines were not produced in secondary reactions such as the oxidation of bromide by HOCl/OCl- and the exchange of bromide with chlorine atoms of chloramines. Therefore, the amount of bromamine produced by myeloperoxidase or eosinophil peroxidase was equal to the amount of bromide oxidized by the enzyme. Bromide was preferred over chloride as the substrate for both enzymes.
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PMID:Oxidation of bromide by the human leukocyte enzymes myeloperoxidase and eosinophil peroxidase. Formation of bromamines. 785 68

A manganese-dependent peroxidase (MnP) from Phanerochaete chrysosporium catalyzed the reduction of cytochrome c in a reaction mixture containing H2O2, Mn(II)-tartrate, and p-hydroquinone. Electron spin resonance studies have shown that the hydroquinone-dependent reductive activity of MnP is due to the benzosemiquinone formed upon the one-electron oxidation of p-hydroquinone by Mn(III)-tartrate, which is formed upon the oxidation of Mn(II) by MnP. The reductive activity increased linearly with an increase in the concentration of p-hydroquinone. The reductive activity was also observed using other hydroquinones such as methylhydroquinone, 2,5-dimethylhydroquinone, and trimethylhydroquinone. The apparent Km values for Mn(II) and H2O2 for the hydroquinone-dependent reductive activity were similar to those for oxidative reactions of MnP. A stoichiometry study showed that about 1.5 mol of cytochrome c was reduced per mole of H2O2 consumed. The stoichiometry decreased with an increase in the concentration of H2O2. The optimal pH for the reductive activity was 5.0, approximately the physiological pH of the fungus. The reduction of cytochrome c was also observed using a quinone and cellobiose:quinone oxidoreductase isolated from the extracellular medium of the fungus.
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PMID:Reductive activity of a manganese-dependent peroxidase from Phanerochaete chrysosporium. 821 23

The one-electron oxidation of a reduced nitroxide (2,2,6,6-tetramethyl-1,4-dihydroxypiperidine, TOLH), detected by ESR, was used to resolve and quantify oxidants arising from the reaction of heme proteins with hydroperoxides, including chelatable iron released subsequent to oxidative cleavage of the porphyrin ring. Released iron was distinguished from protein radicals and ferryl heme by analyzing TOLH oxidation in the presence of different chelating agents. Metmyoglobin (metMb) treatment with one mole of H2O2 per mole of heme produced protein-bound oxidants that oxidized about two molecules of TOLH per heme. Some of the oxidizing species responsible for TOLH oxidation were highly persistent (t1/2 for the decay was 3 hrs at 25 degrees C). Iron release, metMb bleaching and the catalysis of Fenton-type chemistry were compared in metMb solutions treated with tert-butyl hydroperoxide (tBH). Iron release required about five-fold higher hydroperoxide concentrations than did metMb bleaching. Alkoxyl and methyl radical production was catalyzed by iron released from metMb but not by protein-bound iron in oxidized metMb solutions treated with tBH and ascorbic acid. The results suggest that ascorbate-mediated hydroxyl and alkoxyl radical production by hydroperoxide-treated metMb is due to released iron and that the protein-bound non-heme iron that arises during bleaching is at most a weak Fenton reagent.
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PMID:Hydroxyl and alkoxyl radical production by oxidation products of metmyoglobin. 839 72

Colominic acid (CA), an alpha-(2-->8) N-acetylneuraminic acid (sialic acid) polymer (average molecular weight of 10 kDa) was activated by periodate oxidation of carbon 7 at the non-reducing end of the saccharide. The oxidized CA was then coupled to catalase by reductive amination in the presence of sodium cyanoborohydride. The extent of sialylation of catalase, estimated by ammonium sulfate precipitation as 3.8+/-0.4 (mean+/-S.D.) moles of CA per mole of catalase, did not improve significantly when depolymerized CA was used in the coupling reaction. At the end of the coupling reaction, sialylated catalase exhibited a two-fold (70%) retention of initial activity compared to enzyme controls (29-35%) subjected to the same conditions. Formation of sialylated catalase was confirmed by ammonium sulfate or trichloroacetic acid precipitation, molecular sieve chromatography and SDS-PAGE electrophoresis. Enzyme kinetics studies revealed an increase in the apparent Km of the enzyme from 70.0 (native) to 122.9 mmol l-1 H2O2 (sialylated catalase) indicating a reduction of enzyme affinity for the substrate (hydrogen peroxide) on sialylation. Compared to native enzyme, sialylated catalase was much more stable in the presence of specific proteinases, completely resisting degradation by chymotrypsin and losing only some of its activity in the presence of trypsin. The increased stability conferred to catalase by sialylation agrees with similar observations on enzymes modified by other hydrophilic molecules (e.g., monomethoxypoly(ethyleneglycol)) and suggests that steric stabilization with the biodegradable polysialic acid may prove an alternative means to render therapeutic proteins more effective in vivo.
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PMID:Synthesis, characterization and properties of sialylated catalase. 865 33

The capacity of bovine serum amineoxidase (SAO) to oxidize free amino groups of nonconventional substrates, such as polylysine (up to 50 kDa) and some proteins as lysozyme and ribonuclease A, is described. The oxidation was quantified from the amount of H2O2 and NH3 enzymatically produced by SAO. Kinetic analysis indicated a stereospecific preference for L-configuration. Maximal oxidation rate was obtained with poly-L-lysine (9.6 kDa). After 10 h of incubation at 37 degrees C, the poly-L-lysine was partially oxidized generating 1.5 moles of H2O2 by one mole of polylysine. Denatured SAO presented very low oxidation rates with the mentioned substrates.
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PMID:Extended substrate specificity of serum amine oxidase: possible involvement in protein posttranslational modification. 866 Mar 85

A low-cost assay method that is able to measure H2O2 concentrations as low as the nano-molar range is described. The assay solution contains NADH, horseradish peroxidase, and superoxide dismutase at PH 7.5. After the addition of the sample, the decrease in NADH concentration measured by spectrophotometry is proportional to the H2O2 concentration. Because of superoxide dismutation, a high amplification factor defined as moles NADH oxidised per mole H2O2 added is obtained, which allows the sensitivity limit of the method to be greatly improved. We have established the conditions under which the amplification factor can be stabilised at a high level: the best compromise is to increase both the horseradish peroxidase and superoxide dismutase concentrations. Finally, we have also shown that coupled to specific oxidases, our assay method is suitable for measuring very low concentrations of biochemicals that can be oxidized by oxygen with H2O2 production.
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PMID:Experimental procedure for a hydrogen peroxide assay based on the peroxidase-oxidase reaction. 870 81

The role of reactive oxygen species in causing DNA damage through interaction of chromium (III) and hydrogen peroxide was examined using plasmid relaxation assay and EPR spectroscopy. Marked DNA strand breakage was induced by CrCl3 plus H2O2 in a phosphate buffer at pH 6-8.9; whereas, only slight DNA strand breakage was observed during similar treatment at pH less than 4. DNA breakage also increased as the reaction temperature and Cr(III)/H2O2 concentrations increased. Control experiments with Cr(III) or H2O2 alone did not cause DNA breakage. Sodium azide, D-mannitol, Tris-HCl, or catalase completely inhibited Cr(III)/H2O2-induced DNA breakage, but superoxide dismutase did not. The D2O enhancing effect on DNA breaks was not observed. Cr(III) pre-incubated with a 30-fold molar excess of EDTA did not cause any significant DNA breakage in the presence of H2O2. In a phosphate buffer containing Cr(III) and H2O2, singlet oxygen and hydroxyl radicals were detected using EPR spectrometry with the spin traps 2,2,6,6-tetramethyl-4-piperidone and 5,5-dimethyl-1-pyrroline 1-oxide (DMPO), respectively. DMPO/.OH adducts and DNA breakage induced by Cr(III)/H2O2 were markedly higher than those induced by Cr(VI)/H2O2. Furthermore, ascorbate decreased Cr(III)/H2O2-induced DNA breakage. EPR studies revealed that ascorbate (mole ratio to Cr(III) = 0.5:1) attenuated the DMPO/.OH signal generated by Cr(III)/H2O2/DMPO, but a Cr(V) signal and ascorbate radicals were detected. NADPH, GSH, and GSSG also decreased DMPO/.OH generated by Cr(III)/H2O2/DMPO; however, they were less efficient than ascorbate and no Cr(V) signals were detected. This study shows that Cr(III)/H2O2 generates oxidative damage to DNA through a Fenton-like reaction: Cr(III) + H2O2-->Cr(IV) + .OH + OH.
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PMID:Formation of reactive oxygen species and DNA strand breakage during interaction of chromium (III) and hydrogen peroxide in vitro: evidence for a chromium (III)-mediated Fenton-like reaction. 902 Nov 67


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