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Cyclic oxidation and reduction of protein methionine residues is an important antioxidant mechanism.

Almost all forms of reactive oxygen species (ROS) oxidize methionine residues of proteins to a mixture of the R- and S-isomers of methionine sulfoxide. Because organisms contain methionine sulfoxide reductases (Msr's) that can catalyze the thioredoxin-dependent reduction of the sulfoxides back to methionine, it was proposed that the cyclic oxidation/reduction of methionine residues might serve as antioxidants to scavenge ROS, and also to facilitate the regulation of critical enzyme activities. We summarize here results of studies showing that organisms possess two different forms of Msr--namely, MsrA that catalyzes reduction of the S-isomer and MsrB that catalyzes the reduction of the R-isomer. Deletion of the msrA gene in mice leads to increased sensitivity to oxidative stress and to a decrease (40%) in the maximum lifespan. This suggests that elimination of both Msr's would have more serious consequences.

Animals↗
PubMed1981

Iron-sulfur N-1 clusters studied in NADH-ubiquinone oxidoreductase and in soluble NADH dehydrogenase.

Two N-1 type iron-sulfur clusters in NADH-ubiquinone oxidoreductase (Complex I, EC 1.6.5.3) were potentiometrically resolved: one was titrated as a component with a midpoint oxidation-reduction potential of -335 mV at pH 8.0, and with an n-value equal to one; the other as an extremely low midpoint potential component (Em 8.0 less than -500 mV). These two clusters are tentatively assigned to N-1b and N-1a, respectively. Cluster N-1b is completely reducible with NADH and has a spin concentration of about 0.8/FMN. Its EPR spectrum can be simulated as a single rhombic component with principal g values of 2.019, 1.937, and 1.922, which correspond to the Center 1 reported earlier by Orme-Johnson, N. R., Hansen, R. E., and Beinert, H. (1974) J. Biol. Chem. 249, 1922-1927. At extremely low oxidation-reduction potentials (less than -450 mV), additional EPR signals emerge with apparent g values of gz = 2.03, gy = 1.95, and gx = 1.91, which we assign to cluster N-1a. It is difficult, however, to simulate the detailed spectral line shape of this component as a single rhombic component, suggesting some degree of protein modification or interaction with a neighboring oxidation-reduction component. EPR spectra of soluble NADH dehydrogenase, containing 5-6 g atoms of non-heme iron and 5-6 mol of acid-labile sulfide/mol of FMN, were examined. Signals from at least two iron-sulfur species could be distinguished in the NADH-reduced form: one of an N-1b type spectrum; the other of a spectrum with g values of 2.045, 1.95, and 1.87 (total of about 0.5 spin equivalents/FMN). This is the first example of an N-1 type signal detected in isolated soluble NADH dehydrogenase.

Binding Sites↗
PubMed1975

The non-light-dependent reduction of 2,6-dichlorophenolindophenol by cells of the blue-green alga Anacystis nidulans.

Whole cells of the blue-green alga Anacystis nidulans reduced, in the dark, the oxidation-reduction dye, 2,6-dichlorophenolindophenol at rates severalfold higher than those of the other algae tested. Under anaerobiosis, the endogenous reductant was depleted after up to 80 nmol of dye were reduced per microliter of cells. Cells held in darkness for several hours exhibited lowered dark reduction rates relative to cells held in light. Treatment with lysozyme and ethylenediaminetetraacetic acid yielded cells that would photoreduce the dye, whereas untreated cells would not. Comparisons of photoreduction and dark reduction revealed that the dark reduction proceeded independently of the photoreduction. It was concluded that the dark reduction represents a pool of endogenous reductant of sufficiently low oxidation-reduction potenital to reduce completely 2,6-dichlorophenolindophenol. Additionally, untreated cells were shown to be permeable to the dye although they did not photoreduce it; thus lysozyme/ethylenediaminetetraacetic acid treatment was considered to make the oxidant accessible to the photosynthetic machinery.

Anaerobiosis↗
PubMed1992

Site-directed mutagenesis of tetraheme cytochrome c3. Modification of oxidoreduction potentials after heme axial ligand replacement.

The nature of the axial ligands of a heme group is an important factor in maintaining the oxidation-reduction potential of a c-type cytochrome. Cytochrome c3 from Desulfovibrio vulgaris Hildenborough contains four bis-histidinyl coordinated hemes with low oxidation-reduction potentials. Site-directed mutagenesis was used to generate a mutant in which histidine 70, the sixth axial ligand of heme 4, has been replaced by a methionine. The mutant protein was expressed in Desulfovibrio desulfuricans G200 at a level similar to the wild type cytochrome. A model for the three-dimensional structure of D. vulgaris Hildenborough cytochrome c3 was generated on the basis of the crystal structure of D. vulgaris Miyazaki cytochrome c3 in order to investigate the effects of the H70M mutation. The model, together with NMR data, suggested that methionine 70 has effectively replaced histidine 70 as the sixth axial ligand of heme 4 without significant alteration of the structure. A large increase of at least 200 mV of one of the four oxidation-reduction potentials was observed by electrochemistry and is interpreted in terms of structure/potential relationships.

Amino Acid Sequence↗
PubMed2001

Redox paradox: effect of N-acetylcysteine and serum on oxidation reduction-sensitive mitogen-activated protein kinase signaling pathways.

The thiol reducing agent N-acetylcysteine (NAC) is commonly used as an "antioxidant" in studies examining gene expression, signaling pathways, and outcome in acute and chronic models of lung injury. It is less widely appreciated that NAC can also undergo auto-oxidation and behave as an oxidant. We showed previously that NAC can have opposite effects on the activation of nuclear factor-kappaB depending on whether or not serum is present, and that the effects of NAC in the absence of serum are mimicked by various oxidants. Here we show that in a serum-depleted environment (0.1% fetal bovine serum), NAC substantially inhibited lipopolysaccharide (LPS) activation of the mitogen-activated protein kinases (MAPKs), namely extracellular signal-regulated kinase (ERK), p38mapk, and c-Jun NH2-terminal kinase (JNK). By contrast, in the presence of 10% serum, NAC had no effect on LPS activation of p42 and p44 ERK and in fact enhanced LPS induction of p38mapk and JNK phosphorylation. Because serum can significantly alter the redox state, these findings highlight the importance of the local redox milieu in signal transduction.

Acetylcysteine↗
PubMed1975

Cytochrome b and photosynthetic sulfur bacteria.

Chromatophores isolated from the purple sulfur bacterium Chromatium and the green sulfur bacterium Chlorobium exhibit absorbance changes in the cytochrome alpha-band region consistent with the presence of a b-type cytochrome. Cytochrome content determined by reduced minus oxidized difference spectra and by heme photochemically active bacteriochlorophyll (reaction-center bacteriochlorophyll). The b-type cytochrome in Chromatium has an alpha-band maximum at 560 nm and a midpoint oxidation-reduction potential of -5 mV at pH 8.0. The b-type cytochrome in Chlorobium has an alpha-band maximum at 564 nm and an apparent midpoint oxidation-reduction potential near -90 mV. Chromatophores isolated from both Chromatium and Chlorobium cells catalyze a photoreduction of cytochrome b that is enhanced in the presence of antimycin A. Antimycin A and 2-n-heptyl-4-hydroxyquinoline-N-oxide inhibit endogenous (but not phenazine methosulfate-mediated) cyclic photophosphorylation in Chromatium chromatophores and non-cyclic electron flow from Na-2S to NADP in Chlorobium chromatophores. These observations suggest that b-type cytochromes may function in electron transport reactions in photosynthetic sulfur bacteria.

Antimycin A↗
PubMed1980

Mammalian histidine decarboxylase; changes in molecular properties induced by oxidation and reduction.

Histidine decarboxylase from a murine mastocytoma has been submitted to different separation methods. In these experiments the activity peaks were often very broad. This heterogeneity of the enzyme is traced back to the formation of aggregates, differing in apparent molecular weight by a multiple of about 55,000, as a result of oxidation. Under non-oxidative conditions the histidine decarboxylase activity is confined to one peak in both molecular sieve chromatography, hydrophic interaction chromatography, chromatography on hydroxy apatite, pore gradient electrophoresis and electrofocusing. The molecular weight of the enzyme is estimated to be 110,000 by pore gradient electrophoresis (alkylated enzyme). The isoelectric point is pH 4.9--5.0, determined by electrofocusing under reducing conditions.

Animals↗