In vitro preparation of iron-substituted human manganese superoxide dismutase: possible toxic properties for mitochondria

Antioxidant Capacity Is Decreased in Wilson's Disease and Correlates to Liver Function

The metabolic disorder Wilson's disease (WD) is caused by copper accumulation in the tissues due to a biallelic pathogenic mutation of the gene ATP7B, encoding intracellular copper transporter ATPase-7B. As copper is a redox active metal; aberrations in its homeostasis may create favourable conditions for superoxide-yielding redox cycling and oxidative damage to the cells. We tried to characterise antioxidant defence in WD patients and to evaluate whether it is related to liver function. The blood glutathione concentration, the activity of manganese-SOD (MnSOD), catalase (Cat), glutathione peroxidase, and glutathione S-transferase glutathione (GST), and serum antioxidant potential (AOP-450) were measured in WD treatment-naive patients and healthy controls and correlated with clinical data. The blood glutathione concentration, the activity of MnSOD, Cat, glutathione peroxidase, and GST and AOP-450 are significantly decreased in WD patients. There was a positive correlation of AOP-450 with AST. Moreover, the Cat and GST activity as well as AOP-450 strongly correlated with parameters of synthetic liver function. MnSOD activity correlated positively with ALT and AST.The blood glutathione concentration, the activity of MnSOD, Cat, glutathione peroxidase, and GST and AOP-450 are significantly decreased in WD patients. There was a positive correlation of AOP-450 with AST. Moreover, the Cat and GST activity as well as AOP-450 strongly correlated with parameters of synthetic liver function. MnSOD activity correlated positively with ALT and AST. Liver injury in course of WD is linked with decreased antioxidant capacity.

Insights into Manganese Superoxide Dismutase and Human Diseases

Redox equilibria and the modulation of redox signalling play crucial roles in physiological processes. Overproduction of reactive oxygen species (ROS) disrupts the body's antioxidant defence, compromising redox homeostasis and increasing oxidative stress, leading to the development of several diseases. Manganese superoxide dismutase (MnSOD) is a principal antioxidant enzyme that protects cells from oxidative damage by converting superoxide anion radicals to hydrogen peroxide and oxygen in mitochondria. Systematic studies have demonstrated that MnSOD plays an indispensable role in multiple diseases. This review focuses on preclinical evidence that describes the mechanisms of MnSOD in diseases accompanied with an imbalanced redox status, including fibrotic diseases, inflammation, diabetes, vascular diseases, neurodegenerative diseases, and cancer. The potential therapeutic effects of MnSOD activators and MnSOD mimetics are also discussed. Targeting this specific superoxide anion radical scavenger may be a clinically beneficial strategy, and understanding the therapeutic role of MnSOD may provide a positive insight into preventing and treating related diseases.

Association between serum manganese levels and diabetes in chinese adults with hypertension

Manganese (Mn) is an essential trace metal element that is associated with diabetes; however, the results of previous studies are inconsistent. Furthermore, few studies have been conducted in a hypertensive population. The purpose of this study is to explore the relationship between manganese and diabetes in a population with hypertension. A cross‐sectional study was conducted, including 2575 hypertensive individuals from 14 provinces in China. Serum manganese concentrations were measured by the inductively coupled plasma mass spectrometry (ICP‐MS) method. And logistic regression models were used to analyze the association between serum manganese and the risk of diabetes. The prevalence of diabetes was 27.0% in this hypertensive population. In logistic regression models, the odds ratios (95% confidence interval) for diabetes in tertile subgroups were 1.40 (1.12, 1.76) and 1.32 (1.05, 1.65) for tertiles 1 and tertiles 3, respectively, compared to tertile 2 (reference). Additionally, an interaction between sex and manganese was observed. The odds ratios (95% confidence interval) for diabetes were 1.29 (0.95, 1.75) and 0.96 (0.70, 1.31) for tertiles 1 and tertiles 3 among males, and 1.44 (1.01, 2.04) and 1.81 (1.29, 2.55) for tertiles 1 and tertiles 3 among females, respectively, compared to tertile 2. In conclusion, a U‐shaped association between serum manganese and diabetes was observed in a Chinese population with hypertension, and the association was modified by sex.

Manganese Superoxide Dismutase Acetylation and Regulation of Protein Structure in Breast Cancer Biology and Therapy

The loss and/or dysregulation of several cellular and mitochondrial antioxidants' expression or enzymatic activity, which leads to the aberrant physiological function of these proteins, has been shown to result in oxidative damage to cellular macromolecules. In this regard, it has been surmised that the disruption of mitochondrial networks responsible for maintaining normal metabolism is an established hallmark of cancer and a novel mechanism of therapy resistance. This altered metabolism leads to aberrant accumulation of reactive oxygen species (ROS), which, under specific physiological conditions, leads to a potential tumor-permissive cellular environment. In this regard, it is becoming increasingly clear that the loss or disruption of mitochondrial oxidant scavenging enzymes may be, in specific tumors, either an early event in transformation or exhibit tumor-promoting properties. One example of such an antioxidant enzyme is manganese superoxide dismutase (MnSOD, also referred to as SOD2), which detoxifies superoxide, a ROS that has been shown, when its normal physiological levels are disrupted, to lead to oncogenicity and therapy resistance. Here, we will also discuss how the acetylation of MnSOD leads to a change in detoxification function that leads to a cellular environment permissive for the development of lineage plasticity-like properties that may be one mechanism leading to tumorigenic and therapy-resistant phenotypes.

A Single Mutation is Sufficient to Modify the Metal Selectivity and Specificity of a Eukaryotic Manganese Superoxide Dismutase to Encompass Iron

We have generated a site-directed mutant of the manganese superoxide dismutase SOD-3 of C.elegans (MnSOD-3) which modifies the metal specificity of the enzyme. While wild-type MnSOD-3 functions with manganese in the active site (3600 U mg-1 of protein) it has little or no activity when iron is incorporated. However, when histidine replaces glutamine 142 in the active site, the enzyme retains 50 % of its activity and becomes cambialistic for its metal cofactor exhibiting very similar specific activity with either manganese or iron.

Insights into the Dichotomous Regulation of SOD2 in Cancer

While loss of antioxidant expression and the resultant oxidant-dependent damage to cellular macromolecules is key to tumorigenesis, it has become evident that effective oxidant scavenging is conversely necessary for successful metastatic spread. This dichotomous role of antioxidant enzymes in cancer highlights their context-dependent regulation during different stages of tumor development. A prominent example of an antioxidant enzyme with such a dichotomous role and regulation is the mitochondria-localized manganese superoxide dismutase SOD2 (MnSOD). SOD2 has both tumor suppressive and promoting functions, which are primarily related to its role as a mitochondrial superoxide scavenger and H₂O₂ regulator. However, unlike true tumor suppressor- or onco-genes, the SOD2 gene is not frequently lost, or rarely mutated or amplified in cancer. This allows SOD2 to be either repressed or activated contingent on context-dependent stimuli, leading to its dichotomous function in cancer. Here, we describe some of the mechanisms that underlie SOD2 regulation in tumor cells. While much is known about the transcriptional regulation of the SOD2 gene, including downregulation by epigenetics and activation by stress response transcription factors, further research is required to understand the post-translational modifications that regulate SOD2 activity in cancer cells. Moreover, future work examining the spatio-temporal nature of SOD2 regulation in the context of changing tumor microenvironments is necessary to allows us to better design oxidant- or antioxidant-based therapeutic strategies that target the adaptable antioxidant repertoire of tumor cells.

A Single Outer-Sphere Mutation Stabilizes apo-Mn Superoxide Dismutase by 35 °C and Disfavors Mn Binding

The catalytic active site of Mn-specific superoxide dismutase (MnSOD) is organized around a redox-active Mn ion. The most highly conserved difference between MnSODs and the homologous FeSODs is the origin of a Gln in the second coordination sphere. In MnSODs it derives from the C-terminal domain whereas in FeSODs it derives from the N-terminal domain, yet its side chain occupies almost superimposable positions in the active sites of these two types of SODs. Mutation of this Gln69 to Glu in Escherichia coli FeSOD increased the Fe3+/2+ reduction midpoint potential by >0.6 V without disrupting the structure or Fe binding [ Yikilmaz, E., Rodgers, D. W., and Miller, A.-F. ( 2006 ) Biochemistry 45 ( 4 ), 1151 - 1161 ]. We now describe the analogous Q146E mutant of MnSOD, explaining its low Mn content in terms increased stability of the apo-Mn protein. In 0.8 M guanidinium HCl, Q146E-apoMnSOD displays an apparent melting midpoint temperature (Tm) 35 °C higher that of wild-type (WT) apoMnSOD, whereas the Tm of WT-holoMnSOD is only 20 °C higher than that of WT-apoMnSOD. In contrast, the Tm attributed to Q146E-holoMnSOD is 40 °C lower than that of Q146E-apoMnSOD. Thus, our data refute the notion that the WT residues optimize the structural stability of the protein and instead are consistent with conservation on the basis of enzyme function and therefore ability to bind metal ion. We propose that the WT-MnSOD protein conserves a destabilizing amino acid at position 146 as part of a strategy to favor metal ion binding.

Gene variants encoding proteins involved in antioxidant defense system and the clinical expression of Wilson disease

BACKGROUND & AIMS

Wilson disease (WD) is an autosomal recessive disorder of copper metabolism resulting from pathogenic mutations of the ATP7B gene. The basis of phenotypic variability of the disease is not understood. The main mechanism of copper toxicity is probably related to generation of intracellular oxidative stress. To evaluate whether interindividual variability within genes encoding proteins involved in antioxidant defense system may modulate phenotypic expressions of WD.

METHODS

Variability within genes encoding the cytosolic enzymes: glutathione peroxidase (GPX1 rs1050450) and manganese superoxide dismutase (SOD2 rs4880), and peroxisomal enzyme: catalase (CAT rs1001179) were analysed in 435 patients. Individual genotypes were tested for their relationship with phenotypic features of WD.

RESULTS

GPX1 genotypes were not related to phenotypic manifestations of WD. Among males homozygocity for the SOD2 rs4880 T allele was related to earlier onset of WD. Patients homozygous for the CAT rs1001179 T allele characterized with later onset of WD [median (interquartile range) age: 29.0 (14.0) years vs. 22.0 (12.0) years, respectively, P < 0.004], later manifestation of hepatic symptoms [34.5 (14.0) years vs. 22.0 (12.0) years, P < 0.0009], and later presentation of neurological symptoms [37.0 (16.0) years vs. 28.0 (13.0) years, P < 0.03] than those having one or two C alleles.

CONCLUSION

Variability within the CAT gene may be an important modifier of the clinical course of WD. SOD2 genotype may influence WD phenotype among males. These observations indirectly confirm a role of oxidative stress in the pathogenesis of WD, as well as indirectly suggest that peroxisomes impairment may be involved in WD pathophysiology.

MnSOD in oxidative stress response-potential regulation via mitochondrial protein influx

SIGNIFICANCE

The mitochondrial antioxidant manganese superoxide dismutase (MnSOD) is encoded by genomic DNA and its dismutase function is fully activated in the mitochondria to detoxify free radical O2(•-) generated by mitochondrial respiration. Accumulating evidence shows an extensive communication between the mitochondria and cytoplasm under oxidative stress. Not only is the MnSOD gene upregulated by oxidative stress, but MnSOD activity can be enhanced via the mitochondrial protein influx (MPI).

RECENT ADVANCES

A cluster of MPI containing cytoplasmic/nuclear proteins, such as cyclins, cyclin-dependent kinases, and p53 interact with and alter MnSOD activity. These proteins modulate MnSOD superoxide scavenging activity via post-translational modifications in the mitochondria. In addition to well-established pathways in gene expression, recent findings suggest that MnSOD enzymatic activity can also be enhanced by phosphorylation of specific motifs in mitochondria. This review attempts to discuss the pre- and post-translational regulation of MnSOD, and how these modifications alter MnSOD activity, which induces a cell adaptive response to oxidative stress.

CRITICAL ISSUES

MnSOD is biologically significant to aerobic cells. Its role in protecting the cells against the deleterious effects of reactive oxygen species is evident. However, the exact network of MnSOD-associated cellular adaptive reaction to oxidative stress and its post-translational modifications, especially its enzymatic enhancement via phosphorylation, is not yet fully understood.

FUTURE DIRECTIONS

The broad discussion of the multiple aspects of MnSOD regulation, including gene expression, protein modifications, and enzymatic activity, will shed light onto the unknown mechanisms that govern the prosurvival networks involved in cellular and mitochondrial adaptive response to genotoxic environment.

Structural, spectroscopic and functional investigation into Fe-substituted MnSOD from human pathogen Clostridium difficile

SOD_cd could modulate the Fe and Mn dependent activity through its active site microenvironment.

A manganese-rich environment supports superoxide dismutase activity in a Lyme disease pathogen, Borrelia burgdorferi

The Lyme disease pathogen Borrelia burgdorferi represents a novel organism in which to study metalloprotein biology in that this spirochete has uniquely evolved with no requirement for iron. Not only is iron low, but we show here that B. burgdorferi has the capacity to accumulate remarkably high levels of manganese. This high manganese is necessary to activate the SodA superoxide dismutase (SOD) essential for virulence. Using a metalloproteomic approach, we demonstrate that a bulk of B. burgdorferi SodA directly associates with manganese, and a smaller pool of inactive enzyme accumulates as apoprotein. Other metalloproteins may have similarly adapted to using manganese as co-factor, including the BB0366 aminopeptidase. Whereas B. burgdorferi SodA has evolved in a manganese-rich, iron-poor environment, the opposite is true for Mn-SODs of organisms such as Escherichia coli and bakers' yeast. These Mn-SODs still capture manganese in an iron-rich cell, and we tested whether the same is true for Borrelia SodA. When expressed in the iron-rich mitochondria of Saccharomyces cerevisiae, B. burgdorferi SodA was inactive. Activity was only possible when cells accumulated extremely high levels of manganese that exceeded cellular iron. Moreover, there was no evidence for iron inactivation of the SOD. B. burgdorferi SodA shows strong overall homology with other members of the Mn-SOD family, but computer-assisted modeling revealed some unusual features of the hydrogen bonding network near the enzyme's active site. The unique properties of B. burgdorferi SodA may represent adaptation to expression in the manganese-rich and iron-poor environment of the spirochete.

Battles with iron: manganese in oxidative stress protection

The redox-active metal manganese plays a key role in cellular adaptation to oxidative stress. As a cofactor for manganese superoxide dismutase or through formation of non-proteinaceous manganese antioxidants, this metal can combat oxidative damage without deleterious side effects of Fenton chemistry. In either case, the antioxidant properties of manganese are vulnerable to iron. Cellular pools of iron can outcompete manganese for binding to manganese superoxide dismutase, and through Fenton chemistry, iron may counteract the benefits of non-proteinaceous manganese antioxidants. In this minireview, we highlight ways in which cells maximize the efficacy of manganese as an antioxidant in the midst of pro-oxidant iron.

Do genetic variations in antioxidant enzymes influence the course of hereditary hemochromatosis?

Iron-induced oxidative stress promotes hepatic injury in hereditary hemochromatosis, which can be influenced by genetic traits affecting antioxidant enzymes. We assessed the influence of Ala16Val-superoxide dismutase 2, Pro198Leu-glutathione peroxidase 1, and -463G/A-myeloperoxidase genotypes (high activity for the Ala, Pro, and G alleles, respectively) on the risks of cirrhosis and hepatocellular carcinoma (HCC) in patients homozygous for the C282Y-hemochromatosis (HFE) gene mutation. Both the 2G-myeloperoxidase genotype and carriage of one or two copies of the Ala-superoxide dismutase 2 allele were more frequent in patients with cirrhosis or HCC. Patients cumulating these two genetic traits had higher rates of cirrhosis and HCC than other patients.

MnSOD overexpression prevents liver mitochondrial DNA depletion after an alcohol binge but worsens this effect after prolonged alcohol consumption in mice

Both acute and chronic alcohol consumption increase reactive oxygen species (ROS) formation and lipid peroxidation, whose products damage hepatic mitochondrial DNA (mtDNA). To test whether manganese superoxide dismutase (MnSOD) overexpression modulates acute and chronic alcohol-induced mtDNA lesions, transgenic MnSOD-overexpressing (TgMnSOD(+++)) mice and wild-type (WT) mice were treated by alcohol, either chronically (7 weeks in drinking water) or acutely (single intragastric dose of 5 g/kg). Acute alcohol administration increased mitochondrial ROS formation, decreased mitochondrial glutathione, depleted and damaged mtDNA, durably increased inducible nitric oxide synthase (NOS) expression, plasma nitrites/nitrates and the nitration of tyrosine residues in complex V proteins and decreased complex V activity in WT mice. These effects were prevented in TgMnSOD(+++) mice. In acutely alcoholized WT mice, mtDNA depletion was prevented by tempol, a superoxide scavenger, L-NAME and 1400W, two NOS inhibitors, or uric acid, a peroxynitrite scavenger. In contrast, chronic alcohol consumption decreased cytosolic glutathione and increased hepatic iron, lipid peroxidation products and respiratory complex I protein carbonyls only in ethanol-treated TgMnSOD(+++) mice but not in WT mice. In chronic ethanol-fed TgMnSOD(+++) mice, but not WT mice, mtDNA was damaged and depleted, and the iron chelator, deferoxamine (DFO), prevented this effect. In conclusion, MnSOD overexpression prevents mtDNA depletion after an acute alcohol binge but aggravates this effect after prolonged alcohol consumption, which selectively triggers iron accumulation in TgMnSOD(+++) mice but not in WT mice. In the model of acute alcohol binge, the protective effects of MnSOD, tempol, NOS inhibitors and uric acid suggested a role of the superoxide anion reacting with NO to form mtDNA-damaging peroxynitrite. In the model of prolonged ethanol consumption, the protective effects of DFO suggested the role of iron reacting with hydrogen peroxide to form mtDNA-damaging hydroxyl radical.

Myeloperoxidase and superoxide dismutase 2 polymorphisms comodulate the risk of hepatocellular carcinoma and death in alcoholic cirrhosis

Alcohol increases reactive oxygen species (ROS) formation in hepatocyte mitochondria and by reduced nicotinamide adenine dinucleotide phosphate oxidases and myeloperoxidase (MPO) in Kupffer cells and liver-infiltrating neutrophils. Manganese superoxide dismutase (MnSOD) converts superoxide anion into hydrogen peroxide, which, unless detoxified by glutathione peroxidase or catalase (CAT), can form the hydroxyl radical with iron. Our aim was to determine whether Ala16Val-superoxide dismutase 2 (SOD2), G-463A-MPO, or T-262C-CAT dimorphisms modulate the risks of hepatocellular carcinoma (HCC) and death in alcoholic cirrhosis. Genotypes and the hepatic iron score were assessed in 190 prospectively followed patients with alcoholic cirrhosis. During follow-up (61.1 +/- 2.7 months), 51 patients developed HCC, and 71 died. The T-262C-CAT dimorphism did not modify hepatic iron, HCC, or death. The GG-MPO genotype did not modify iron but increased the risks of HCC and death. The hazard ratio (HR) was 4.7 (2.1-10.1) for HCC and 3.6 (1.9-6.7) for death. Carriage of one or two Ala-SOD2 allele(s) was associated with higher liver iron scores and higher risks of HCC and death. The 5-year incidence of HCC was 34.4% in patients with both the GG-MPO genotype and one or two Ala-SOD2 alleles, 5.1% in patients with only one of these two traits, and 0% in patients with none of these traits. Corresponding 5-year death rates were 37.6%, 11.6%, and 5%.

CONCLUSION

The combination of the GG-MPO genotype (leading to high MPO expression) and at least one Ala-SOD2 allele (associated with high liver iron score) markedly increased the risks of HCC occurrence and death in patients with alcoholic cirrhosis.

Post-translational modifications of superoxide dismutase

Post-translational modifications of proteins control many biological processes through the activation, inactivation, or gain-of-function of the proteins. Recent developments in mass spectrometry have enabled detailed structural analyses of covalent modifications of proteins and also have shed light on the post-translational modification of superoxide dismutase. In this review, we introduce some covalent modifications of superoxide dismutase, nitration, phosphorylation, glutathionylaion, and glycation. Nitration has been the most extensively analyzed modification both in vitro and in vivo. Reaction of human Cu,Zn superoxide dismutase (SOD) with reactive nitrogen species resulted in nitration of a single tryptophan residue to 6-nitrotryptophan, which could be a new biomarker of a formation of reactive nitrogen species. On the other hand, tyrosine 34 of human MnSOD was exclusively nitrated to 3-nitrotyrosine and almost completely inactivated by the reaction with peroxynitrite. The nitrated MnSOD has been found in many diseases caused by ischemia/reperfusion, inflammation, and others and may have a pivotal role in the pathology of the diseases. Most of the post-translational modifications have given rise to a reduced activity of SOD. Since phosphorylation and nitration of SOD have been shown to have a possible reversible process, these modifications may be related to a redox signaling process in cells. Finally we briefly introduce a metal insertion system of SOD, focusing particularly on the iron misincorporation of nSOD, as a part of post-translational modifications.

Glutaredoxin 5 regulates osteoblast apoptosis by protecting against oxidative stress

There is now increasing evidence which suggests an important role for reactive oxygen species (ROS) in the pathogenesis of osteoporosis. However, little is known on the molecular components of the oxidative stress pathway or their functions in bone. In this study, we evaluated the role and mechanism of action of glutaredoxin (Grx) 5, a protein that is highly expressed in bone. Osteoblasts were transfected with Grx5 siRNA and treated with hydrogen peroxide (H(2)O(2)). Grx5 siRNA treatment increased apoptosis while Grx5 overexpression protected MC3T3-E1 cells against H(2)O(2) induced apoptosis and ROS formation. Grx5 deficiency results in impaired biogenesis of Fe-S cluster in yeast. Accordingly, activity of mitochondrial aconitase, whose activity is dependent on Fe-S cluster, decreased in Grx5 siRNA treated cells. Since reduced formation of Fe-S cluster would lead to increased level of free iron, a competitive inhibitor of manganese superoxide dismutase (MnSOD), we measured MnSOD activity in Grx5 deficient osteoblasts and found MnSOD activity was significantly reduced. The consequence of long term inhibition of Grx5 on osteoblast apoptosis was evaluated using lentiviral shRNA technology. Grx5 shRNA cells exhibited higher caspase activity and cardiolipin oxidation in the presence of H(2)O(2). MnSOD activity was rescued by the addition of MnCl(2) to Grx5 shRNA osteoblasts in the presence of H(2)O(2). Our findings are consistent with the hypothesis that Grx5 is an important determinant of osteoblast apoptosis and acts via a molecular pathway that involves regulation of ROS production, cardiolipin oxidation, caspase activity, Fe-S cluster formation, and MnSOD activity.

Conformationally gated metal uptake by apomanganese superoxide dismutase

Metal uptake by apomanganese superoxide dismutase in vitro is a complex process exhibiting multiphase "gated" reaction kinetics and a striking sigmoidal temperature profile that has led to a model of conformationally gated metal binding, requiring conversion between "closed" and "open" forms. This work systematically explores the structural determinants of metal binding in both wild-type (WT) apoprotein and mutational variants as a test of mechanistic models. The pH dependence of metalation under physiological conditions (37 degrees C) shows it is linked to ionization of a single proton with a p K a of 7.7. Size exclusion chromatography demonstrates that the apoprotein is dimeric even when it is fully converted to the open form. The role of molecular motions in metal binding has been probed by using disulfide engineering to introduce covalent constraints into the protein. While restricting motion at domain interfaces has no effect, constraining the subunit interface significantly perturbs metal uptake but does not prevent the process. Mutagenesis of residues in the active site environment results in a dramatic shift in the transition temperature by as much as 20 degrees C or a loss of pH sensitivity. On the basis of these results, a mechanism for metal uptake by manganese superoxide dismutase involving reorientation of active site residues to form a metal entry channel is proposed.