Extracellular superoxide dismutase exists as an octamer

Role of Enzymic Antioxidants in Mediating Oxidative Stress and Contrasting Wound Healing Capabilities in Oral Mucosal/Skin Fibroblasts and Tissues

Unlike skin, oral mucosal wounds are characterized by rapid healing and minimal scarring, attributable to the "enhanced" healing properties of oral mucosal fibroblasts (OMFs). As oxidative stress is increasingly implicated in regulating wound healing outcomes, this study compared oxidative stress biomarker and enzymic antioxidant profiles between patient-matched oral mucosal/skin tissues and OMFs/skin fibroblasts (SFs) to determine whether superior oral mucosal antioxidant capabilities and reduced oxidative stress contributed to these preferential healing properties. Oral mucosa and skin exhibited similar patterns of oxidative protein damage and lipid peroxidation, localized within the lamina propria/dermis and oral/skin epithelia, respectively. SOD1, SOD2, SOD3 and catalase were primarily localized within epithelial tissues overall. However, SOD3 was also widespread within the lamina propria localized to OMFs, vasculature and the extracellular matrix. OMFs were further identified as being more resistant to reactive oxygen species (ROS) generation and oxidative DNA/protein damage than SFs. Despite histological evaluation suggesting that oral mucosa possessed higher SOD3 expression, this was not fully substantiated for all OMFs examined due to inter-patient donor variability. Such findings suggest that enzymic antioxidants have limited roles in mediating privileged wound healing responses in OMFs, implying that other non-enzymic antioxidants could be involved in protecting OMFs from oxidative stress overall.

β-Barrels and Amyloids: Structural Transitions, Biological Functions, and Pathogenesis

Insoluble protein aggregates with fibrillar morphology called amyloids and β-barrel proteins both share a β-sheet-rich structure. Correctly folded β-barrel proteins can not only function in monomeric (dimeric) form, but also tend to interact with one another-followed, in several cases, by formation of higher order oligomers or even aggregates. In recent years, findings proving that β-barrel proteins can adopt cross-β amyloid folds have emerged. Different β-barrel proteins were shown to form amyloid fibrils in vitro. The formation of functional amyloids in vivo by β-barrel proteins for which the amyloid state is native was also discovered. In particular, several prokaryotic and eukaryotic proteins with β-barrel domains were demonstrated to form amyloids in vivo, where they participate in interspecies interactions and nutrient storage, respectively. According to recent observations, despite the variety of primary structures of amyloid-forming proteins, most of them can adopt a conformational state with the β-barrel topology. This state can be intermediate on the pathway of fibrillogenesis ("on-pathway state"), or can be formed as a result of an alternative assembly of partially unfolded monomers ("off-pathway state"). The β-barrel oligomers formed by amyloid proteins possess toxicity, and are likely to be involved in the development of amyloidoses, thus representing promising targets for potential therapy of these incurable diseases. Considering rapidly growing discoveries of the amyloid-forming β-barrels, we may suggest that their real number and diversity of functions are significantly higher than identified to date, and represent only "the tip of the iceberg". Here, we summarize the data on the amyloid-forming β-barrel proteins, their physicochemical properties, and their biological functions, and discuss probable means and consequences of the amyloidogenesis of these proteins, along with structural relationships between these two widespread types of β-folds.

Extracellular superoxide dismutase and its role in cancer

Reactive oxygen species (ROS) are increasingly recognized as critical determinants of cellular signaling and a strict balance of ROS levels must be maintained to ensure proper cellular function and survival. Notably, ROS is increased in cancer cells. The superoxide dismutase family plays an essential physiological role in mitigating deleterious effects of ROS. Due to the compartmentalization of ROS signaling, EcSOD, the only superoxide dismutase in the extracellular space, has unique characteristics and functions in cellular signal transduction. In comparison to the other two intracellular SODs, EcSOD is a relatively new comer in terms of its tumor suppressive role in cancer and the mechanisms involved are less well understood. Nevertheless, the degree of differential expression of this extracellular antioxidant in cancer versus normal cells/tissues is more pronounced and prevalent than the other SODs. A significant association of low EcSOD expression with reduced cancer patient survival further suggests that loss of extracellular redox regulation promotes a conducive microenvironment that favors cancer progression. The vast array of mechanisms reported in mediating deregulation of EcSOD expression, function, and cellular distribution also supports that loss of this extracellular antioxidant provides a selective advantage to cancer cells. Moreover, overexpression of EcSOD inhibits tumor growth and metastasis, indicating a role as a tumor suppressor. This review focuses on the current understanding of the mechanisms of deregulation and tumor suppressive function of EcSOD in cancer.

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.

Cloning, Purification, and Characterization of Recombinant Human Extracellular Superoxide Dismutase in SF9 Insect Cells

A balance between production and degradation of reactive oxygen species (ROS) is critical for maintaining cellular homeostasis. Increased levels of ROS during oxidative stress are associated with disease conditions. Antioxidant enzymes, such as extracellular superoxide dismutase (EC-SOD), in the extracellular matrix (ECM) neutralize the toxicity of superoxide. Recent studies have emphasized the importance of EC-SOD in protecting the brain, lungs, and other tissues from oxidative stress. Therefore, EC-SOD would be an excellent therapeutic drug for treatment of diseases caused by oxidative stress. We cloned both the full length (residues 1-240) and truncated (residues 19-240) forms of human EC-SOD (hEC-SOD) into the donor plasmid pFastBacHTb. After transposition, the bacmid was transfected into the Sf9-baculovirus expression system and the expressed hEC-SOD purified using FLAG-tag. Western blot analysis revealed that hEC-SOD is present both as a monomer (33 kDa) and a dimer (66 kDa), as detected by the FLAG antibody. A water-soluble tetrazolium (WST-1) assay showed that both full length and truncated hEC-SOD proteins were enzymatically active. We showed that a potent superoxide dismutase inhibitor, diethyldithiocarbamate (DDC), inhibits hEC-SOD activity.

Cu/Zn incorporation during purification of soluble human EC-SOD from E. coli stabilizes proper disulfide bond formation

Extracellular superoxide dismutase (EC-SOD) is the only enzyme that removes superoxide radical in the extracellular space. The reduction of EC-SOD is linked to many diseases, suggesting that the protein may have therapeutic value. EC-SOD is reported to be insoluble and to make inclusion bodies when overexpressed in the cytoplasm of Escherichia coli. The refolding process has the advantage of high yield, but has the disadvantage of frequent aggregation or misfolding during purification. For the first time, this study shows that fusion with maltose-binding protein (MBP), N-utilization substance protein A, and protein disulfide isomerase enabled the soluble overexpression of EC-SOD in the cytoplasm of E. coli. MBP-tagged human EC-SOD (hEC-SOD) was purified by MBP affinity and anion exchange chromatography, and its identity was confirmed by MALDI-TOF MS analysis. The purified protein showed good enzyme activity in vitro; however, there was a difference in metal binding. When copper and zinc were incorporated into hEC-SOD before MBP tag cleavage, the enzymatic activity was higher than when the metal ions were bound to the purified protein after MBP tag cleavage. Therefore, the enzymatic activity of hEC-SOD is associated with metal incorporation and protein folding via disulfide bond.

Functional characterization and immune recognition of the extracellular superoxide dismutase from the human pathogenic parasite Onchocerca volvulus (OvEC-SOD)

Onchocerca volvulus is a human pathogenic filarial nematode causing chronic onchocerciasis, a disease characterized by chronic skin and eye lesions. Despite attempts to control this infection from many perspectives, it still remains a threat to public health because of adverse effects of available drugs and recent reports of drug resistance. Under control of an intact immune system, O. volvulus survives for a long time in the host by employing a variety of strategies including the utility of antioxidant enzymes. In the present study, we focus on the extracellular superoxide dismutase from O. volvulus (OvEC-SOD) found in the excretory/secretory products of adult worms. Contrary to previous studies, the OvEC-SOD was found to have a 19 amino acid long signal peptide that is cleaved off during the process of maturation. To validate this result, we designed a novel method based on Caenorhabditis elegans cup5(ar465) mutants to specifically evaluate signal peptide-mediated secretion of nematodal proteins. Following purification, the recombinant OvEC-SOD was active as a dimer. Site-directed mutagenesis of the three cysteines present in the OvEC-SOD shows that enzyme activity is markedly reduced in the Cys-192 mutant. A homology model of the OvEC-SOD underlines the importance of Cys-192 for the stabilization of the adjacent active site channel. The generation of a humoral immune response to secretory OvEC-SOD was indicated by demonstrating IgG reactivity in sera from patients infected with O. volvulus while the cross-reactivity of IgG in plasma samples from cows, infected with the most closely related parasite Onchocerca ochengi, occurred only marginally. High IgG1 and IgM titres were recorded in sera from mice infected with the filaria Litomosoides sigmodontis, however, low or no cellular proliferative responses were observed. Thus, the present data suggest that secretory OvEC-SOD is a target of the humoral immune response in human onchocerciasis and induced strongest IgG responses in hyperreactive onchocerciasis. Furthermore, humoral response during murine infection induced SOD-specific IgG that cross-reacted with OvEC-SOD.

N-Glycosylation profiling of recombinant mouse extracellular superoxide dismutase produced in Chinese hamster ovary cells

Extracellular superoxide dismutase (EC-SOD), the major SOD isoenzyme in biological fluids, is known to be N-glycosylated and heterogeneous as was detected in most glycoproteins. However, only one N-glycan structure has been reported in recombinant human EC-SOD produced in Chinese hamster ovary (CHO) cells. Thus, a precise N-glycan profile of the recombinant EC-SOD is not available. In this study, we report profiling of the N-glycan in the recombinant mouse EC-SOD produced in CHO cells using high-resolution techniques, including the liberation of N-glycans by treatment with PNGase F, fluorescence labeling by pyridylamination, characterization by anion-exchange, normal and reversed phase-HPLC separation, and mass spectrometry. We succeeded in identifying 26 different types of N-glycans in the recombinant enzyme. The EC-SOD N-glycans were basically core-fucosylated (98.3% of the total N-glycan content), and were high mannose sugar chain, and mono-, bi-, tri-, and tetra-antennary complex sugar chains exhibiting varying degrees of sialylation. Four of the identified N-glycans were uniquely modified with a sulfate group, a Lewis(x) structure, or an α-Gal epitope. The findings will shed new light on the structure-function relationships of EC-SOD N-glycans.

Extracellular/microenvironmental redox state

Extracellular redox (reduction-oxidation) state is a factor that serves as an important regulator of cell-microenvironmental interactions and is determined by several known variables; including redox-modulating proteins that are located on the plasma membrane or outside of cells, extracellular thiol/disulfide couples, and reactive oxygen species (ROS)/reactive nitrogen species (RNS) that are capable of traveling across plasma membranes into the extracellular space. The extracellular redox state works in concert with the intracellular redox state to control both the influx and efflux of ROS/RNS that may serve to modulate redox signaling or to perturb normal cellular processes or both. Under physiologic conditions, the extracellular space is known to have a relatively more-oxidized redox state than the interior of the cell. During pathologic conditions, such as cancer, the extracellular redox state may be altered, causing specific proteins such as proteases, soluble factors, or the extracellular matrix to have altered functions or activities. Recent studies have strongly supported an important relation between the extracellular redox state and cancer cell aggressiveness. The purpose of this review is to identify redox buffer networks in extracellular spaces and to emphasize the possible roles of the extracellular redox state in cancer, knowledge that may contribute to potential therapeutic interventions.

Antioxidant enzymatic defenses and oxidative damage in Dentex dentex fed on different dietary macronutrient levels

A wide range of antioxidant mechanisms are present in fish maintaining an adequate "oxidative balance". When this balance tilts in favor of the oxidant agents "oxidative stress" arises with detrimental effects in molecules of great biological importance. Little has been reported about the influence of different dietary energy sources on antioxidant defenses in fish. The influence of different dietary macronutrient combinations on the key antioxidant enzyme activity, the oxidative damage to lipids and proteins and the possible modifications in the SOD isoenzymatic pattern were evaluated in liver, white muscle, heart and erythrocytes of common dentex (Dentex dentex). Four experimental diets with different protein:lipid:carbohydrate ratios (43/16/28; 43/24/4; 38/19/28 and 38/24/13) were formulated. In general, neither different dietary macronutrient levels nor the interaction among them induces substantial modifications in enzymatic antioxidant defense mechanisms. Two constitutive SOD isoforms, CuZn-SOD I and Mn-SOD, were detected in the tissues analyzed in all experimental groups, independently of diet formulation, but, a third SOD isoenzyme, CuZn-SOD II seems to be induced in white muscle by higher dietary protein levels. Densitometric analyses of western blotting membranes revealed higher CuZn-SOD expression in the heart of dentex fed on lower dietary protein levels, although these differences did not correlate with the SOD activity. Finally, a direct relation exists between the lipid or protein intake level and occurrence of oxidative damage in different tissue components.

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.

Redox regulation in the extracellular environment

The oxidizing nature of the extracellular environment is vastly different from the highly reducing nature of the intracellular compartment. The redox potential of the cytosolic compartment of the intracellular environment limits disulfide bond formation, whereas the oxidizing extracellular environment contains proteins rich in disulfide bonds. If not for an extracellular antioxidant system to eliminate reactive oxygen and nitrogen species, lipid peroxidation and protein oxidation would become excessive, resulting in cellular damage. Many reviews have focused on the role of intracellular antioxidants in the elimination of oxidative stress, but this one will focus on the coordinated action of both intracellular and extracellular antioxidants in limiting cellular oxidant stress.

A musculoskeletal model of low grade connective tissue inflammation in patients with thyroid associated ophthalmopathy (TAO): the WOMED concept of lateral tension and its general implications in disease

Background

Low level connective tissue inflammation has been proposed to play a role in thyroid associated ophthalmopathy (TAO). The aim of this study was to investigate this postulate by a musculoskeletal approach together with biochemical parameters.

Methods

13 patients with TAO and 16 controls were examined. Erythrocyte levels of Zn, Cu, Ca²⁺, Mg, and Fe were determined. The musculoskeletal evaluation included observational data on body posture with emphasis on the orbit-head region. The angular foot position in the frontal plane was quantified following gait observation. The axial orientation of the legs and feet was evaluated in an unloaded supine position. Functional propioceptive tests based on stretch stimuli were done by using foot inversion and foot rotation.

Results

Alterations in the control group included neck tilt in 3 cases, asymmetrical foot angle during gait in 2, and a reaction to foot inversion in 5 cases. TAO patients presented facial asymmetry with displaced eye fissure inclination (mean 9.1°) as well as tilted head-on-neck position (mean 5.7°). A further asymmetry feature was external rotation of the legs and feet (mean 27°). Both foot inversion as well as foot rotation induced a condition of neuromuscular deficit. This condition could be regulated by gentle acupressure either on the lateral abdomen or the lateral ankle at the acupuncture points gall bladder 26 or bladder 62, respectively. In 5 patients, foot rotation produced a phenomenon of moving toes in the contra lateral foot. In addition foot rotation was accompanied by an audible tendon snapping. Lower erythrocyte Zn levels and altered correlations between Ca²⁺, Mg, and Fe were found in TAO.

Conclusion

This whole body observational study has revealed axial deviations and body asymmetry as well as the phenomenon of moving toes in TAO. The most common finding was an arch-like displacement of the body, i.e. eccentric position, with foot inversion and head tilt to the contra lateral side and tendon snapping. We propose that eccentric muscle action over time can be the basis for a low grade inflammatory condition. The general implications of this model and its relations to Zn and Se will be discussed.

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.