The Structure of Rabbit Extracellular Superoxide Dismutase Differs from the Human Protein†

Redox Regulation of the Superoxide Dismutases SOD3 and SOD2 in the Pulmonary Circulation

When evaluating the role of redox-regulating signaling in pulmonary vascular diseases, it is intriguing to consider the modulation of key antioxidant enzymes like superoxide dismutase (SOD) because SOD isoforms are regulated by redox reactions, and, in turn, modulate downstream redox sensitive processes. The emerging field of redox biology is built upon understanding the regulation and consequences of tightly controlled and specific reduction-oxidation reactions that are critical for diverse cellular processes including cell signaling. Of relevance, both the site of production of specific reactive oxygen and nitrogen species and the site of the antioxidant defenses are highly compartmentalized within the cell. For example, superoxide is generated during oxidative phosphorylation in the mitochondria as well as by a number of enzymatic sources within the cytosol and at the cell membrane. In the pulmonary circulation, these sources include the mitochondrial electron transport chain, NADPH oxidases (NOX1-4, Duox1,2), nitric oxide synthases, and xanthine oxidase; this important topic has been thoroughly reviewed recently [1]. In parallel with these different cellular sites of superoxide production, the three SOD isoforms are also specifically localized to the cytosol (SOD1), mitochondria (SOD2) or extracellular compartment (SOD3). This chapter focuses on the role of redox mechanisms regulating SOD2 and SOD3, with an emphasis on these processes in the setting of pulmonary hypertension.

Disulfide Bond Pattern of Transforming Growth Factor β-Induced Protein

Transforming growth factor β-induced protein (TGFBIp) is an extracellular matrix protein composed of an NH₂-terminal cysteine-rich domain (CRD) annotated as an emilin (EMI) domain and four fasciclin-1 (FAS1-1-FAS1-4) domains. Mutations in the gene cause corneal dystrophies, a group of debilitating protein misfolding diseases that lead to severe visual impairment. Previous studies have shown that TGFBIp in the cornea is cross-linked to type XII collagen through a reducible bond. TGFBIp contains 11 cysteine residues and is thus able to form five intramolecule disulfide bonds, leaving a single cysteine residue available for the collagen cross-link. The structures of TGFBIp and its homologues are unknown. We here present the disulfide bridge pattern of TGFBIp, which was determined by generating specific peptides. These were separated by ion exchange followed by reversed-phase high-performance liquid chromatography and analyzed by mass spectrometry and Edman degradation. The NH₂-terminal CRD contains six cysteine residues, and one of these (Cys65) was identified as the candidate for the reducible cross-link between TGFBIp and type XII collagen. In addition, the CRD contained two intradomain disulfide bridges (Cys49-Cys85 and Cys84-Cys97) and one interdomain disulfide bridge to FAS1-2 (Cys74-Cys339). Significantly, this arrangement violates the predicted disulfide bridge pattern of an EMI domain. The cysteine residues in FAS1-3 (Cys473 and Cys478) were shown to form an intradomain disulfide bridge. Finally, an interdomain disulfide bridge between FAS1-1 and FAS1-2 (Cys214-Cys317) was identified. The interdomain disulfide bonds indicate that the NH₂ terminus of TGFBIp (CRD, FAS1-1, and FAS1-2) adopts a compact globular fold, leaving FAS1-3 and FAS1-4 exposed.

N-Glycosylation is essential for the secretion of extracellular superoxide dismutase

Extracellular superoxide dismutase (EC-SOD or SOD3) protects against various oxidative stress-related diseases by scavenging reactive superoxides in the extracellular space. It is the only SOD isozyme that is secreted and glycosylated (at asparagine 89). However, the physiological roles of its glycosylation are poorly understood. In this study, we found that the glycosylation site on EC-SOD is well conserved and that a glycosylation-deficient EC-SOD mutant retains its enzymatic activity, but is not secreted. This impairment in secretion may, in part, be due to the ability of the mutants to form unusual higher order oligomers. Our findings reveal that the glycan modification is a key regulator of EC-SOD secretion and contributes to the understanding of the roles of glycans in EC-SOD-related diseases.

Involvement of superoxide dismutase isoenzymes and their genetic variants in progression of and higher susceptibility to vitiligo

Oxidative stress has been implicated as the initial triggering event in vitiligo pathogenesis leading to melanocyte destruction. Here, we report a significant increase in oxidative stress in vitiligo patients as evidenced by high lipid peroxidation levels suggesting an imbalance in the antioxidant enzyme system as reported in our previous studies. This study examined the role of the enzymatic antioxidant SOD, which converts the pro-oxidant superoxide into H2O2, in vitiligo pathogenesis. The activity of three isoforms of SOD, i.e., SOD1, SOD2, and SOD3, was significantly higher in vitiligo patients. To identify the underlying mechanism for the increase in activities of SOD isoforms, we explored the SOD1, SOD2, and SOD3 genes for their genetic variations and transcript levels. The SOD2 Thr58Ile (rs35289490) and Leu84Phe (rs11575993) polymorphisms were significantly associated with vitiligo patients, and the Val16Ala (rs4880) polymorphism was associated with active vitiligo patients. Interestingly, SOD2 activity was contributed by these polymorphisms along with its increase in transcript levels in patients. SOD3 activity was associated with the Arg213Gly (rs8192291) polymorphism. The SOD3 transcript levels were also increased in patients, which might contribute to the increased SOD3 activity. However, we could not establish the genotype-phenotype correlation for SOD1 as we could not detect any novel or reported SNPs in SOD1. In addition, both transcript and protein levels of SOD1 were unchanged between patients and controls, though SOD1 activity was increased in patients. Activities of SOD isoforms also correlated with progression of the disease as the activity was higher in active cases of vitiligo compared to stable cases. Here, we report that SOD2 and SOD3 polymorphisms may be genetic risk factors for susceptibility and progression of vitiligo and hence the genetic makeup of an individual may form a basis for the effective treatment of the disease. Overall, our results suggest that increased activity of SOD isoforms under the influence of genetic factors may lead to accumulation of H2O2 in cytoplasmic, mitochondrial, and extracellular compartments resulting in oxidative damage to the melanocytes.

Murine extracellular superoxide dismutase is converted into the inactive fold by the Ser195Cys mutation

We have previously shown that human extracellular superoxide dismutase (EC-SOD) exists as two variants with differences in their disulfide bridge patterns: one form is the active enzyme (aEC-SOD), and the other is inactive (iEC-SOD). The availability of both active and inactive folding variants significantly reduces the specific activity of EC-SOD in vivo. Both forms are produced during biosynthesis, but the underlying folding mechanisms remain unclear. To address this issue, we expressed EC-SOD in heterologous systems that do not endogenously express iEC-SOD. Rodents express only aEC-SOD because they lack Cys195 (human EC-SOD sequence numbering), which is essential for the formation of iEC-SOD. However, cultured hamster cells and transgenic mice expressing human EC-SOD were able to produce both human a- and iEC-SOD variants, which led us to hypothesize that the folding was sequence-dependent rather than a property of the expression system. To substantiate this hypothesis, we expressed murine EC-SOD in a human cell line, and as expected, only aEC-SOD was produced. Significantly, when Cys195 was introduced, both murine aEC-SOD and a novel murine iEC-SOD were generated, and the specific activity of the murine EC-SOD was significantly reduced by the mutation. Collectively, these data suggest that Cys195 actuates the formation of iEC-SOD, independent of the expression system or host. In addition, the dual-folding pathway most likely requires biosynthesis factors that are common to both humans and rodents.

Significant association between SNPs in the superoxide dismutase 3, extracellular (SOD3) gene and resistance to Aeromonas hydrophila in the freshwater mussel Hyriopsis cumingii

Extracellular superoxide dismutase (SOD3) is a major antioxidant enzyme that protects organs from damage by reactive oxygen species (ROS). In this study, the SOD3 gene was identified and characterized from the freshwater mussel Hyriopsis cumingii (Hc-SOD3). The cDNA sequence consists of 763 bp, encoding a protein of 208 amino acids. The amino acid sequence possesses two CuZnSOD signature sequences, and amino acids required for binding of Cu (His-93, -95, -110 and -169) and Zn (His-110, -118, -129 and Asp-132) were conserved in Hc-SOD3. The Hc-SOD3 genomic sequence was 9165 bp in length, containing four exons and three introns. Eighteen single nucleotide polymorphisms were detected in the Hc-SOD3 gene from resistant stock (RS) and susceptible stock (SS) of H. cumingii to Aeromonas hydrophila. The genotype and allele distribution were examined in resistant and susceptible stocks. Among them, a C/G substitution at the g.7994C>G locus and G/C substitution at the g.8087G>C locus were significantly associated with resistance/susceptibility of H. cumingii to A. hydrophila, both in genotype (P = 0.017, P = 0.004 respectively) and allele frequency (P = 0.021, P = 0.006 respectively). Linkage disequilibrium analysis revealed that g.7994C>G, g.8001A>G, g.8035G>A, g.8087G>C and g.8191T>A were in linkage disequilibrium. The results suggest that the two polymorphic loci, g.7994C>G and g.8087G>C, could be potential genetic markers for future molecular selection of strains that are resistant to diseases.

Sex differences in oxidant/antioxidant balance under a chronic mild stress regime

The deterioration of homeostasis between oxidant/antioxidant species may represent an important mechanism linking psychological stress to cardiovascular risk despite the many sex differences in stress responsiveness. The goal of the present study was to investigate the influence of chronic mild stress (CMS), a widely accepted animal model of depression, on oxidative homeostasis-allostasis markers and sICAM-1, a marker of endothelial injury, in the serum of Wistar rats, by taking into account the effect of sex. After six weeks of exposure to mild unpredictable environmental stressors, both male and female rat groups displayed typical changes in hedonic status (anhedonia), which is a core symptom of human depression. Control female rats had higher (nitrite and nitrate) NOx, lower malondealdehyde (MDA) levels with lower activity of antioxidant enzymes and sICAM-1 levels than did control males. CMS induced oxidant/antioxidant responses in both sexes. Females tended to increase their nitric oxide (NO) levels further, while MDA levels did not reach those of males, thus retaining significantly higher NO bioavailability than in males. Concerning the antioxidant enzymes, CMS-females exhibited significantly higher glutathione peroxidase (GPx) activity and lower glutathione reductase (GR) and superoxide dismutase (SOD) activity compared to CMS-males. The CMS response in females was accompanied by lower sICAM-1 levels than in males, suggesting lower endothelial injury. In conclusion, the results of the present study showed that CMS induces different oxidative stress and compensatory responses in both sexes probably due to differences in the mechanisms regulating oxidant/antioxidant pathways.

The folding of human active and inactive extracellular superoxide dismutases is an intracellular event

Human extracellular superoxide dismutase (EC-SOD) is a tetrameric glycoprotein responsible for the removal of superoxide generated in the extracellular space. Two different folding variants of EC-SOD exist based on the disulfide bridge connectivity, resulting in enzymatically active (aEC-SOD) and inactive (iEC-SOD) subunits. As a consequence of this, the assembly of the EC-SOD tetramers produces molecules with variable activity and may represent a way to regulate the antioxidant level in the extracellular space. To determine whether the formation of these two folding variants is an intra- or extracellular event, we analyzed the biosynthesis in human embryonic kidney 293 cells expressing wild-type EC-SOD. These analyses revealed that both folding variants were present in the intra- and extracellular spaces, suggesting that the formation is an intracellular event. To further analyze the biosynthesis, we constructed mutants with the capacity to generate only aEC-SOD (C195S) or iEC-SOD (C45S). The expression of these suggested that the cellular biosynthetic machinery supported the secretion of aEC-SOD but not iEC-SOD. The coexpression of these two mutants did not affect the expression pattern. This study shows that generation of the EC-SOD folding variants is an intracellular event that depends on a free cysteine residue not involved in disulfide bonding.

The subunit composition of human extracellular superoxide dismutase (EC-SOD) regulates enzymatic activity

BACKGROUND

Human extracellular superoxide dismutase (EC-SOD) is a tetrameric metalloenzyme responsible for the removal of superoxide anions from the extracellular space. We have previously shown that the EC-SOD subunit exists in two distinct folding variants based on differences in the disulfide bridge pattern (Petersen SV, Oury TD, Valnickova Z, Thøgersen IB, Højrup P, Crapo JD, Enghild JJ. Proc Natl Acad Sci USA. 2003;100(24):13875-80). One variant is enzymatically active (aEC-SOD) while the other is inactive (iEC-SOD). The EC-SOD subunits are associated into covalently linked dimers through an inter-subunit disulfide bridge creating the theoretical possibility of 3 dimers (aa, ai or ii) with different antioxidant potentials. We have analyzed the quaternary structure of the endogenous EC-SOD disulfide-linked dimer to investigate if these dimers in fact exist.

RESULTS

The analyses of EC-SOD purified from human tissue show that all three dimer combinations exist including two homo-dimers (aa and ii) and a hetero-dimer (ai). Because EC-SOD is a tetramer the dimers may combine to generate 5 different mature EC-SOD molecules where the specific activity of each molecule is determined by the ratio of aEC-SOD and iEC-SOD subunits.

CONCLUSION

This finding shows that the aEC-SOD and iEC-SOD subunits combine in all 3 possible ways supporting the presence of tetrameric enzymes with variable enzymatic activity. This variation in enzymatic potency may regulate the antioxidant level in the extracellular space and represent a novel way of modulating enzymatic activity.

Purification, identification, characterization, and cDNA cloning of a high molecular weight extracellular superoxide dismutase of hamster that transiently increases in plasma during arousal from hibernation

We previously studied antioxidant profiles in the plasma of hibernating Syrian hamsters and found a transient increase of a superoxide radical-scavenging activity during the arousal phase. In this report, we purified and identified the high molecular weight superoxide dismutase (SOD)-like factor from the plasma of arousing hamsters. The cyanide-sensitive 240 kDa SOD-like factor showed a significant homology to mammalian extracellular SOD (EC-SOD) reported, although the molecular mass of EC-SOD was 135 kDa. The cDNA cloning revealed that the 240 kDa SOD-like factor was identical to the hamster ortholog of EC-SOD. It consisted of 245 amino acid residues including a signal sequence of 20 amino acid residues. Five cysteine residues that would participate in inner- and inter-subunit bonds were well conserved among species. Interestingly, there were four potential N-glycosylation sites in hamster EC-SOD, whereas there is only one site in other species. The amino acid sequence analysis indicated that three of the four sites were modified. These results suggest that the anomalistically high molecular weight of hamster EC-SOD is ascribed, at least in part, to the addition of extra sugar chains. Furthermore, results obtained here also propose the involvement of EC-SOD in the antioxidative defense of hibernating hamsters.

Extracellular superoxide dismutase exists as an octamer

Human extracellular superoxide dismutase (EC-SOD) is involved in the defence against oxidative stress induced by the superoxide radical. The protein is a homotetramer stabilised by hydrophobic interactions within the N-terminal region. During the purification of EC-SOD from human aorta, we noticed that material with high affinity for heparin-Sepharose formed not only a tetramer but also an octamer. Analysis of the thermodynamic stability of the octamer suggested that the C-terminal region is involved in formation of the quaternary structure. In addition, we show that the octamer is composed of both aEC-SOD and iEC-SOD folding variants. The presence of the EC-SOD octamer with high affinity may represent a way to influence the local concentration of EC-SOD to protect tissues specifically sensitive to oxidative damage.

Oxidative stress and adhesion molecules in children with type 1 diabetes mellitus: a possible link

OBJECTIVE

To examine whether oxidative stress parameters were correlated with adhesion molecules derived from endothelial/platelet activation in a group of juveniles with type 1 diabetes mellitus (T1DM).

SUBJECTS AND METHODS

Indicative parameters of patient oxidant/antioxidant capacity were measured and associated with P-selectin and tetranectin (TN), markers of endothelial/platelet activation, in the plasma of 45 diabetic children and adolescents and 20 healthy age-matched subjects (HS).

RESULTS

Significantly, higher nitrate/nitrite (NOx) and lipid hydroperoxide (LPO) levels (p=0.049 and p=0.0011, respectively), lower glutathione peroxidase activity (GPx; p=0.038), and elevated TN and P-selectin plasma levels (p=0.0046 and p=0.042, respectively) were found in T1DM children compared with HS. Well-controlled T1DM children (HbA1c <or= 7%) showed significantly lower GPx (p=0.0259), higher NOx and LPO (p=0.01093 and p=0.0092, respectively) compared with HS, while poorly controlled patients (HbA1c >7%) showed significantly higher TN, sP-selectin and LPO (p=0.0064, p=0.0234 and p=0.0121, respectively), a tendency to higher NOx (p=0.063) compared with HS and only TN higher (p=0.0123) compared with well-controlled patients. Patients with shorter diabetes duration (<or=3 yr) showed significantly higher LPO and TN (p=0.034 and 0.017, respectively), a tendency to higher NOx and lower GPx and higher P-selectin, while those with longer duration (>3 yr) differed significantly in all the examined parameters (TN, p=0.0015; GPx, p=0.0420; NOx, p=0.0196; LPO, p=0.0054; sP-selectin, p=0.0187) compared with HS.

CONCLUSIONS

Decreased antioxidative protection from simultaneous LPO and NOx overproduction is evident in T1DM juveniles with a parallel endothelial/platelet activation even in the first years of the disease, being more pronounced later in diabetes progression, contributing to the vascular complications of the disease.

Modulation of the expression of superoxide dismutase gene in lung injury by 2-chloroethyl ethyl sulfide, a mustard analog

Mustard gas exposure causes inflammatory lung diseases. Many inflammatory lung diseases are associated with oxidative stress. Reactive oxygen species (ROS) are involved in the maintenance of physiological functions. In tissues, it is therefore essential to maintain a steady-state level of antioxidant activity to allow both for the physiological functions of ROS to proceed and at the same time preventing tissue damage. We have recently reported that mustard gas exposure decreases the overall activity of superoxide dismutase (SOD). In the present study, we investigated the effects of mustard gas on each of the three isozymes: SOD-1 (Cu/Zn), SOD-2 (Mn), and SOD-3 (extracellular). Adult guinea pigs were intratracheally injected single doses of 2-chloroethyl ethyl sulfide (CEES) (2 mg/kg body weight) in ethanol. Control animals were injected with vehicle in the same way. The animals were sacrificed after 7 days, and lungs were removed after perfusion with physiological saline. Lung injury was established by measuring the leakage of iodinated-BSA into lung tissue. Mustard gas exposure caused a significant increase in the activity of SOD-1 (35%). However, the SOD-3 activity which is the predominant type in lung was significantly decreased (62%), whereas no change was observed in SOD-2 activity. Thus the decrease in the total activity of SOD was primarily due to the SOD-3 isozyme. Northern blot analysis indicated 3.5-fold increased expression of SOD-1 in mustard gas exposed lung, but no significant change in the expression of SOD-2 and SOD-3 was observed. Mustard gas exposure did not cause mutation in the coding region of SOD-1 gene while causing modulation in expression levels. The protein levels of SOD-1, SOD-2, and SOD-3 were not altered significantly in the mustard gas exposed lung. Our results indicate that the overall decrease in the activity of SOD by mustard gas exposure is probably mediated by direct inactivation of the SOD-3 gene or the enzyme itself. This decrease in the activity of SOD-3 may be due to the cleavage of active form of the protein to an inactive form. The existence of active and inactive forms of SOD-3 as a result of shifts in Cys-Cys disulfide bonding has been described in human, recently. Studies are underway in our laboratory to investigate whether mustard gas induced inactivation of SOD-3 in lung is similarly mediated by a change in Cys-Cys disulfide bonding.

Extracellular superoxide dismutase: structural and functional considerations of a protein shaped by two different disulfide bridge patterns

The effects of reactive oxygen species are detrimental and can cause damage to DNA, protein, and lipids. Hence, the etiology of a large range of diseases resides in the generation of excess reactive oxygen species. However, these species are also involved in the maintenance of physiological functions. In tissues, it is therefore essential to maintain a steady-state level of antioxidant activity to allow both for the physiological functions of reactive oxygen species to proceed and at the same time preventing tissue damage. Extracellular superoxide dismutase (EC-SOD) is the only extracellular scavenger of the superoxide radical. The reactivity of superoxide is promiscuous and it is crucial that EC-SOD is positioned at the site of superoxide production to prevent adventitious reactions. It is therefore likely beneficial to have mechanisms for regulating the EC-SOD tissue distribution and enzymatic activity. The modular architecture of EC-SOD, encompassing three functional regions, is an ideal construction to generate diversity. By intracellular proteolytic processing and generation of active and inactive molecules, EC-SOD represents a flexible protein with the capacity to fine-tune the tissue localization and the antioxidant level in the extracellular space. The present review will address the function and activity of the separate regions of EC-SOD.