Determination of the structural role of the N-terminal domain of human extracellular superoxide dismutase by use of protein fusions

Superoxide Dismutase Multigene Family from a Primitive Chondrostean Sturgeon, Acipenser baerii: Molecular Characterization, Evolution, and Antioxidant Defense during Development and Pathogen Infection

Three distinct superoxide dismutases (SODs)-copper/zinc-SOD (SOD1), manganese-SOD (SOD2), and extracellular copper/zinc-SOD (SOD3)-were identified from a primitive chondrostean fish, Acipenser baerii, enabling the comparison of their transcriptional regulation patterns during development, prelarval ontogeny, and immune stimulation. Each A. baerii SOD isoform (AbSOD) shared conserved structural features with its vertebrate orthologs; however, phylogenetic analyses hypothesized a different evolutionary history for AbSOD3 relative to AbSOD1 and AbSOD2 in the vertebrate lineage. The AbSOD isoforms showed different tissue distribution patterns; AbSOD1 was predominantly expressed in most tissues. The expression of the AbSOD isoforms showed isoform-dependent dynamic modulation according to embryonic development and prelarval ontogenic behaviors. Prelarval microinjections revealed that lipopolysaccharide only induced AbSOD3 expression, while Aeromonas hydrophila induced the expression of AbSOD2 and AbSOD3. In fingerlings, the transcriptional response of each AbSOD isoform to bacterial infection was highly tissue-specific, and the three isoforms exhibited different response patterns within a given tissue type; AbSOD3 was induced the most sensitively, and its induction was the most pronounced in the kidneys and skin. Collectively, these findings suggest isoform-dependent roles for the multigene SOD family in antioxidant defenses against the oxidative stress associated with development and immune responses in these endangered sturgeon fish.

Differential expression of CuZnSOD gene under low temperature stress in noble scallop Chlamys nobilis with different carotenoid content

The noble scallop Chlamys nobilis belongs to a warm-water mollusk and has been cultured in the sea of southern China since 1980s'. However, accidents of massive mortality have often occurred during the winter, and one of the reasons could be accumulation of harmful reactive oxygen species caused by lower temperature. Carotenoids are well known for their anti-oxidant function. To investigate whether carotenoids do play a role in mollusks' antioxidant defense system under lower temperature stress, an acute lower temperature experiment was conducted by using two types of scallops: the orange with higher carotenoids content and the brown with lower carotenoids content. Their CuZnSOD gene was cloned, mRNA expression levels were determined, and SOD activity and carotenoids content were measured. The complete CuZnSOD cDNA consists of 1078 nucleotides with an open reading frame encoding 154 amino acid residues, which has high identity with that of its sister species Chlamys farreri. The mRNA expression levels in both the mantle and gill from the orange scallops were significantly higher (P < 0.05) than that of the brown ones, but the result was the opposite in the blood. SOD activity in the mantle and gill from the orange scallops was significantly higher than (P < 0.05) that from the brown ones. Further, significantly positive correlations were found among CuZnSOD gene transcript levels, SOD activity and total carotenoids content in the orange scallops. The present results suggested that carotenoids could play roles in antioxidant defense system by upregulating gene expression under lower temperature stress in the noble scallop.

Oxidative stress induced by chlorine dioxide as an insecticidal factor to the Indian meal moth, Plodia interpunctella

A novel fumigant, chlorine dioxide (ClO2) is a commercial bleaching and disinfection agent. Recent study indicates its insecticidal activity. However, its mode of action to kill insects is yet to be understood. This study set up a hypothesis that an oxidative stress induced by ClO2 is a main factor to kill insects. The Indian meal moth, Plodia interpunctella, is a lepidopteran insect pest infesting various stored grains. Larvae of P. interpunctella were highly susceptible to ClO2 gas, which exhibited an acute toxicity. Physiological damages by ClO2 were observed in hemocytes. At high doses, the larvae of P. interpunctella suffered significant reduction of total hemocytes. At low doses, ClO2 impaired hemocyte behaviors. The cytotoxicity of ClO2 was further analyzed using two insect cell lines, where Sf9 cells were more susceptible to ClO2 than High Five cells. The cells treated with ClO2 produced reactive oxygen species (ROS). The produced ROS amounts increased with an increase of the treated ClO2 amount. However, the addition of an antioxidant, vitamin E, significantly attenuated the cytotoxicity of ClO2 in a dose-dependent manner. To support the oxidative stress induced by ClO2, two antioxidant genes (superoxide dismutase (SOD) and thioredoxin-peroxidase (Tpx)) were identified from P. interpunctella EST library using ortholog sequences of Bombyx mori. Both SOD and Tpx were expressed in larvae of P. interpunctella especially under oxidative stress induced by bacterial challenge. Exposure to ClO2 gas significantly induced the gene expression of both SOD and Tpx. RNA interference of SOD or Tpx using specific double stranded RNAs significantly enhanced the lethality of P. interpunctella to ClO2 gas treatment as well as to the bacterial challenge. These results suggest that ClO2 induces the production of insecticidal ROS, which results in a fatal oxidative stress in P. interpunctella.

The effects of hypochlorous acid and neutrophil proteases on the structure and function of extracellular superoxide dismutase

Extracellular superoxide dismutase (EC-SOD) is expressed by both macrophages and neutrophils and is known to influence the inflammatory response. Upon activation, neutrophils generate hypochlorous acid (HOCl) and secrete proteases to combat invading microorganisms. This produces a hostile environment in which enzymatic activity in general is challenged. In this study, we show that EC-SOD exposed to physiologically relevant concentrations of HOCl remains enzymatically active and retains the heparin-binding capacity, although HOCl exposure established oxidative modification of the N-terminal region (Met32) and the formation of an intermolecular cross-link in a fraction of the molecules. The cross-linking was also induced by activated neutrophils. Moreover, we show that the neutrophil-derived proteases human neutrophil elastase and cathepsin G cleaved the N-terminal region of EC-SOD irrespective of HOCl oxidation. Although the cleavage by elastase did not affect the quaternary structure, the cleavage by cathepsin G dissociated the molecule to produce EC-SOD monomers. The present data suggest that EC-SOD is stable and active at the site of inflammation and that neutrophils have the capacity to modulate the biodistribution of the protein by generating EC-SOD monomers that can diffuse into tissue.

Molecular cloning, characterization and expression analysis of cytoplasmic Cu/Zn-superoxide dismutase (SOD) from pearl oyster Pinctada fucata

Because of its capacity to rapidly convert superoxide to hydrogen peroxide, superoxide dismutase (SOD) is crucial in both intracellular signalling and regulation of oxidative stress. In this paper we report the cloning of a Cu/Zn SOD (designated as pfSOD) from the pearl oyster (Pinctada fucata) using rapid amplification of cDNA ends (RACE) PCR. The full-length cDNA of this Cu/Zn SOD contains an open reading frame (ORF) of 471 bp coding for 156 amino acids. No signal peptide was identified at the N-terminal amino acid sequence of Cu/Zn SOD indicating that this pfSOD encodes a cytoplasmic Cu/Zn SOD. This is supported by the presence of conserved amino acids required for binding copper and zinc. Semi-quantitative analysis in adult tissues showed that the pfSOD mRNA was abundantly expressed in haemocytes and gill and scarcely expressed in other tissues tested. After challenge with lipopolysaccharide (LPS), expression of pfSOD mRNA in haemocytes was increased, reaching the highest level at 8 h, then dropping to basal levels at 36 h. These results suggest that Cu/Zn SOD might be used as a bioindicator of the aquatic environmental pollution and cellular stress in pearl oyster.

The C-terminal proteolytic processing of extracellular superoxide dismutase is redox regulated

The antioxidant protein extracellular superoxide dismutase (EC-SOD) encompasses a C-terminal region that mediates interactions with a number of ligands in the extracellular matrix (ECM). This ECM-binding region can be removed by limited proteolysis before secretion, thus supporting the formation of EC-SOD tetramers with variable binding capacity. The ECM-binding region contains a cysteine residue (Cys219) that is known to be involved in an intersubunit disulfide bridge. We have determined the redox potential of this disulfide bridge and show that both EC-SOD dimers and EC-SOD monomers are present within the intracellular space. The proteolytic processing of the ECM-binding region in vitro was modulated by the redox status of Cys219, allowing cleavage under reducing conditions only. When wild-type EC-SOD or the monomeric variant Cys219Ser was expressed in mammalian cells proteolysis did not occur. However, when cells were exposed to oxidative stress conditions, proteolytic processing was observed for wild-type EC-SOD but not for the Cys219Ser variant. Although the cellular response to oxidative stress is complex, our data suggest that proteolytic removal of the ECM-binding region is regulated by the intracellular generation of an EC-SOD monomer and that Cys219 plays an important role as a redox switch allowing the cellular machinery to secrete cleaved EC-SOD.

Expression, high cell density culture and purification of recombinant EC-SOD in Escherichia coli

Superoxide dismutase (SOD) catalyzes the dismutation of the biologically toxic superoxide anion into oxygen and hydrogen peroxide and is deployed by the immune system to kill invading microorganisms. Extracellular SOD (EC-SOD) is a copper- and zinc-containing glycoprotein found predominantly in the soluble extracellular compartment that consists of approximately 30-kDa subunits. Here, we purified recombinant EC-SOD3 (rEC-SOD) from Escherichia coli BL21(DE3) expressing a pET-SOD3-1 construct. Cells were cultured by high-density fed-batch fermentation to a final OD(600) of 51.8, yielding a final dry cell weight of 17.6 g/L. rEC-SOD, which was expressed as an inclusion body, comprised 48.7% of total protein. rEC-SOD was refolded by a simple dilution refolding method and purified by cation-exchange and reverse-phase chromatography. The highly purified rEC-SOD thus obtained was a mixture of monomers and dimers, both of which were active. The molecular weights of monomeric and dimeric rEC-SOD were 25,255 and 50,514 Da, respectively. The purified rEC-SOD had 4.3 EU/mg of endotoxin and the solubility of rEC-SOD was more than 80% between pH 7 and 10. In 2 L of fed-batch fermentation, 60 mg of EC-SOD (99.9% purity) could be produced and total activity was 330.24 U. The process established in this report, involving high-cell-density fermentation, simple dilution refolding, and purification with ion-exchange and reverse-phase chromatography, represents a commercially viable process for producing rEC-SOD.

Thermodynamic characterization of the interaction between the C-terminal domain of extracellular superoxide dismutase and heparin by isothermal titration calorimetry

Extracellular superoxide dismutase (ECSOD) interacts with heparin through its C-terminal domain. In this study we used isothermal titration calorimetry (ITC) to get detailed thermodynamic information about the interaction. We have shown that the interaction between ECSOD and intestinal mucosal heparin (M(w) 6000-30000 Da) is exothermic and driven by enthalpy at physiological salt concentration. However, the contribution from entropy is favorable for binding of small isolated heparin fragments. By studying different size-defined heparin fragments, we also concluded that a hexasaccharide moiety is sufficient for strong binding to ECSOD. The binding involves proton transfer from the buffer to the ECSOD-heparin complex, and the results indicate that the number of ionic interactions made between ECSOD and heparin upon binding varies from three to five for heparin and an octasaccharide fragment, respectively. Surprisingly and despite the many charges found on both the protein and the polysaccharide, our results indicate that the nonionic contribution to the binding is large. From the temperature dependence we have calculated the constant pressure heat capacity change (DeltaC(p)) of the interaction to -644 J K(-1) mol(-1) and -306 J K(-1) mol(-1) for heparin and an octasaccharide, respectively.

Spectrofluorimetric determination of superoxide dismutase using a Europium-tetracycline probe

Superoxide dismutase (SOD) can enhance the characteristic fluorescence of europium in europium (Eu(3+))-tetracycline (TC) system. According to this, a new spectrofluorimetric determination of SOD was developed. Under the optimum conditions, Eu(3+)-TC formed a ternary complex in close proximity with SOD and then intra-molecular energy transfer from TC-SOD complex to Eu(3+), which resulted in the enhancement of characteristic peak of Eu(3+) at 612 nm. The enhanced fluorescence intensity is in proportion to the concentration of SOD, and the linear range was 0.0553-38.71 microg mL(-1) with the limit of detection of 5.53 ng mL(-1). The developed method was practical, simple, sensitive and relatively free from interference coexisting substances and has been successfully applied to the determination of SOD in the plant and blood samples. The mechanism of fluorescence enhancement between Eu(3+)-TC complex and SOD was also studied.

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 cDNA cloning and mRNA expression of cytoplasmic Cu, Zn superoxide dismutase (SOD) gene in scallop Chlamys farreri

Cu, Zn superoxide dismutases (SODs) are metalloenzymes that represent one important line of defence against reactive oxygen species (ROS). A cytoplasmic Cu, Zn SOD cDNA sequence was cloned from scallop Chlamys farreri by the homology-based cloning technique. The full-length cDNA of scallop cytoplasmic Cu, Zn SOD (designated CfSOD) was 1022 bp with a 459 bp open reading frame encoding a polypeptide of 153 amino acids. The predicted amino acid sequence of CfSOD shared high identity with cytoplasmic Cu, Zn SOD in molluscs, insects, mammals and other animals, such as cytoplasmic Cu, Zn SOD in oyster Crassostrea gigas (CAD42722), mosquito Aedes aegypti (ABF18094), and cow Bos taurus (XP_584414). A quantitative reverse transcriptase real-time PCR (qRT-PCR) assay was developed to assess the mRNA expression of CfSOD in different tissues and the temporal expression of CfSOD in scallop challenged with Listonella anguillarum, Micrococcus luteus and Candida lipolytica respectively. Higher-level mRNA expression of CfSOD was detected in the tissues of haemocytes, gill filaments and kidney. The expression of CfSOD dropped in the first 8-16 h and then recovered after challenge with L. anguillarum and M. luteus, but no change was induced by the C. lipolytica challenge. The results indicated that CfSOD was a constitutive and inducible acute-phase protein, and could play an important role in the immune responses against L. anguillarum and M. luteus infection.

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.

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.

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.

Coexpression of yeast copper chaperone (yCCS) and CuZn-superoxide dismutases in Escherichia coli yields protein with high copper contents

To fully understand the function of the Cu- and Zn-containing superoxide dismutases in normal and disordered cells, it is essential to study protein variants with full metal contents. We describe the use of an Escherichia coli-based expression system for the overproduction of human intracellular wild type CuZn-superoxide dismutase (SOD), the CuZnSOD variant F50E/G51E (monomeric), two amyotrophic lateral sclerosis-related mutant CuZnSOD variants (D90A and G93A), and PseudoEC-SOD, all with high Cu contents. This system is based on coexpression of the SOD variants with the yeast copper chaperone yCCS during growth in a medium supplemented with Cu(2+) and Zn(2+). The recombinant SOD enzymes were all found in the cytosol and represented 30-50% of the total bacterial protein. The enzymes were purified to homogeneity and active enzymes were obtained in high yield. The resulting proteins were characterized through immunochemical reactivity and specific activity analyses, in conjunction with mass-, photo-, and atomic absorption-spectroscopy.

Molecular phylogenetic evidence for an extracellular Cu Zn superoxide dismutase gene in insects

Representatives of three ancient gene families of the antioxidant enzyme superoxide dismutase (SOD) can be found in most metazoans. In mammals and Caenorhabditis elegans, there is at least one gene each of the cytoplasmic, mitochondrial and extracellular lineages of SOD genes. The cytoplasmic SOD was one of the first enzymes to be implicated in ageing due to its protection against damaging oxygen free radicals. In contrast to other metazoans, insects were thought to lack a gene for the extracellular SOD. We have cloned and sequenced an SOD mRNA in the ant Lasius niger that appears to belong to this extracellular family. Subsequent searches and analyses of SOD gene sequences in insect databases revealed that insects do indeed express all three SOD genes including the extracellular form. We conclude that insects as well as other metazoans appear to have the full repertoire of the three families of SOD.

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

The cDNA sequence encoding rabbit, mouse, and rat extracellular superoxide dismutase (EC-SOD) predicts that the protein contains five cysteine residues. Human EC-SOD contains an additional cysteine residue and folds into two forms with distinct disulfide bridge patterns. One form is enzymatically active (aEC-SOD), while the other is inactive (iEC-SOD). Due to the lack of the additional cysteine residue rabbit, mouse, and rat EC-SOD are unable to generate an inactive fold identical to human iEC-SOD. The amino acid sequences predict the formation of aEC-SOD only, but other folding variants cannot be ruled out based on the heterogeneity observed for human EC-SOD. To test this, we purified EC-SOD from rabbit plasma and determined the disulfide bridge pattern. The results revealed that the disulfide bridges are homogeneous and identical to human aEC-SOD. Four cysteine residues are involved in two intra-disulfide bonds while the C-terminal cysteine residue forms an intersubunit disulfide bond. No evidence for other folding variants was detected. These findings show that rabbit EC-SOD exists as an enzymatically active form only. The absence of iEC-SOD in rabbits suggests that the structure and aspects of the physiological function of EC-SOD differs significantly between rabbit and humans. This is an important notion to take when using these animals as model systems for oxidative stress.

The dual nature of human extracellular superoxide dismutase: one sequence and two structures

Human extracellular superoxide dismutase (EC-SOD; EC 1.15.1.1) is a scavenger of superoxide anions in the extracellular space. The amino acid sequence is homologous to the intracellular counterpart, Cu/Zn superoxide dismutase (Cu/Zn-SOD), apart from N- and C-terminal extensions. Cu/Zn-SOD is a homodimer containing four cysteine residues within each subunit, and EC-SOD is a tetramer composed of two disulfide-bonded dimers in which each subunit contains six cysteines. The amino acid sequences of all EC-SOD subunits are identical. It is known that Cys-219 is involved in an interchain disulfide. To account for the remaining five cysteine residues we purified human EC-SOD and determined the disulfide bridge pattern. The results show that human EC-SOD exists in two forms, each with a unique disulfide bridge pattern. One form (active EC-SOD) is enzymatically active and contains a disulfide bridge pattern similar to Cu/Zn-SOD. The other form (inactive EC-SOD) has a different disulfide bridge pattern and is enzymatically inactive. The EC-SOD polypeptide chain apparently folds in two different ways, most likely resulting in different three-dimensional structures. Our study shows that one gene may produce proteins with different disulfide bridge arrangements and, thus, by definition, different primary structures. This observation adds another dimension to the functional annotation of the proteome.

Extracellular superoxide dismutase in biology and medicine

Accumulated evidence has shown that reactive oxygen species (ROS) are important mediators of cell signaling events such as inflammatory reactions (superoxide) and the maintenance of vascular tone (nitric oxide). However, overproduction of ROS such as superoxide has been associated with the pathogenesis of a variety of diseases including cardiovascular diseases, neurological disorders, and pulmonary diseases. Antioxidant enzymes are, in part, responsible for maintaining low levels of these oxygen metabolites in tissues and may play key roles in controlling or preventing these conditions. One key antioxidant enzyme implicated in the regulation of ROS-mediated tissue damage is extracellular superoxide dismutase (EC-SOD). EC-SOD is found in the extracellular matrix of tissues and is ideally situated to prevent cell and tissue damage initiated by extracellularly produced ROS. In addition, EC-SOD is likely to play an important role in mediating nitric oxide-induced signaling events, since the reaction of superoxide and nitric oxide can interfere with nitric oxide signaling. This review will discuss the regulation of EC-SOD and its role in a variety of oxidant-mediated diseases.

Structural requirements for high-affinity heparin binding: alanine scanning analysis of charged residues in the C-terminal domain of human extracellular superoxide dismutase

An essential property of human extracellular superoxide dismutase (hEC-SOD) is its affinity for heparin and heparan sulfate proteoglycans located on cell surfaces and in the connective tissue matrix. The C-terminal domain of hEC-SOD plays the major role in this interaction. This domain has an unusually high content of charged amino acids: six arginine, three lysine, and five glutamic acid residues. In this study, we used alanine scanning mutagenesis of charged amino acids in the C-terminal domain to elucidate the requirements for the heparin/heparan sulfate interaction. As a tool in this study, we used a fusion protein comprising the C-terminal domain of hEC-SOD fused to human carbonic anhydrase II (HCAII). The interaction studies were performed using the surface plasmon resonance technique and heparin-Sepharose chromatography. Replacement of the glutamic acid residues by alanine resulted, in all cases, in tighter binding. All alanine substitutions of basic amino acid residues, except one (R205A), reduced heparin affinity. The arginine and lysine residues in the cluster of basic amino acid residues (residues 210-215), the RK-cluster, are of critical importance for the binding to heparin, and arginine residues promote stronger interactions than lysine residues.

High-level expression of human extracellular superoxide dismutase in Escherichia coli and insect cells

Much is known about the physical properties of the Cu,Zn- and Mn-superoxide dismutases (SODs). However, the biochemical characteristics and pharmacological properties of extracellular (EC)-SOD have been severely limited due to difficulties in obtaining and purifying the enzyme. The EC-SOD cDNA was inserted into the Escherichia coli expression plasmid pET-28a(+) which contains the T7 promoter and transformed into the E. coli BL21(DE3). After induction with 1 mmol/L isopropyl beta-D-thiogalactoside, the recombinant human EC-SOD was highly expressed as inclusion bodies. SDS-PAGE analysis revealed that recombinant EC-SOD accumulated up to 26% of the total soluble protein of E. coli cells. The expression product was purified by a Ni(2+)-IDA-Sepharose 6B column. After the denaturing and refolding processes, the recombinant human EC-SOD retains the specific enzymatic activity of 920 U/mg of the purified product. The gene encoding human EC-SOD mature peptide was also inserted into the donor plasmid pFastBacHTb. After transposition, transfection, and amplification were performed, the recombinant baculoviruses infected the Tn-5B1-4 cells and EC-SOD was highly expressed in Tn-5B1-4 cells. SDS-PAGE and Western blot analysis revealed that the subunit molecular weight of the expression product is 28 kDa. Furthermore, recombinant human EC-SOD retains the enzymatic specific activity of 200 U/mg of the Tn-5B1-4 cell lysates.

Cloning, expression and some properties of alpha-carbonic anhydrase from Helicobacter pylori

The alpha-carbonic anhydrase gene from Helicobacter pylori strain 26695 has been cloned and sequenced. The full-length protein appears to be toxic to Escherichia coli, so we prepared a modified form of the gene lacking a part that presumably encodes a cleavable signal peptide. This truncated gene could be expressed in E. coli yielding an active enzyme comprising 229 amino acid residues. The amino acid sequence shows 36% identity with that of the enzyme from Neisseria gonorrhoeae and 28% with that of human carbonic anhydrase II. The H. pylori enzyme was purified by sulfonamide affinity chromatography and its circular dichroism spectrum and denaturation profile in guanidine hydrochloride have been measured. Kinetic parameters for CO2 hydration catalyzed by the H. pylori enzyme at pH 8.9 and 25 degrees C are kcat=2.4x10(5) s(-1), KM=17 mM and kcat/KM=1.4x10(7) M(-1) x s(-1). The pH dependence of kcat/KM fits with a simple titration curve with pK(a)=7.5. Thiocyanate yields an uncompetitive inhibition pattern at pH 9 indicating that the maximal rate of CO2 hydration is limited by proton transfer between a zinc-bound water molecule and the reaction medium in analogy to other forms of the enzyme. The 4-nitrophenyl acetate hydrolase activity of the H. pylori enzyme is quite low with an apparent catalytic second-order rate constant, k(enz), of 24 M(-1) x s(-1) at pH 8.8 and 25 degrees C. However, with 2-nitrophenyl acetate as substrate a k(enz) value of 665 M(-1) x s(-1) was obtained under similar conditions.

Chimeras of human extracellular and intracellular superoxide dismutases. Analysis of structure and function of the individual domains

Human extracellular superoxide dismutase (hEC-SOD) is a secreted tetrameric protein involved in protection against oxygen free radicals. Since EC-SOD is too large a protein for structural determination by multi-dimensional NMR and attempts to crystallize the protein for X-ray structural determination have failed, the three-dimensional structure of hEC-SOD is unknown. By fusion protein techniques we have previously shown that an amphipathic alpha-helix in the N-terminal domain of hEC-SOD is essential for the tetramer interaction. However, the central domain, which is homologous to intracellular hCuZnSOD, has also been proposed to be involved in the tetramer formation. Despite great efforts, the production of recombinant hEC-SOD in prokaryotic systems or simple eukaryotes (such as yeast) has failed. This lack of success has greatly complicated large-scale production and genetic engineering of the protein. In the study reported here, we constructed two chimeras comprising the N- or the N- and C-terminal domains from hEC-SOD fused to hCuZnSOD, called FusNCZ and PseudoEC-SOD, respectively. We show that these proteins can be produced in large quantities in Escherichia coli, that they can be purified with high yields and that the characteristics of PseudoEC-SOD closely resemble those of hEC-SOD. Further, we extended our studies of the nature of the subunit interaction by investigating the involvement of the central domain.

Subunit interaction in extracellular superoxide dismutase: effects of mutations in the N-terminal domain

Human extracellular superoxide dismutase (hEC-SOD) is a secreted tetrameric protein involved in protection against oxygen free radicals. Because EC-SOD is too large a protein for structural determination by multidimensional NMR, and attempts to crystallize the protein for X-ray structural determination have failed, the three-dimensional structure of hEC-SOD is unknown. This means that alternative strategies for structural studies are needed. The N-terminal domain of EC-SOD has already been studied using the fusion protein FusNN, comprised of the 49 N-terminal amino acids from hEC-SOD fused to human carbonic anhydrase (HCAII). The N-terminal domain in this fusion protein forms a well-defined three-dimensional structure, which probably contains alpha-helical elements and is responsible for the tetramerization of the protein. In this work, we have extended the studies, using site-directed mutagenesis in combination with size-exclusion chromatography, CD, and fluorescence spectroscopy, to investigate the nature of the tetrameric interaction. Our results show that the hydrophobic side of a predicted amphiphatic alpha-helix (formed by residues 14-32) in the N-terminal domain is essential for the subunit interaction.

Characterization of the heparin-binding domain of human extracellular superoxide dismutase

The C-terminal, heparin-binding domain of human extracellular superoxide dismutase (hEC-SOD) has been studied as a fusion to human carbonic anhydrase II (HCAII). This technique allows the properties of the EC-SOD domain to be characterized. At the same time, it allows us to differentiate the contributions from the domain, from those properties originating from other parts of EC-SOD. The fusion of the 27 C-terminal amino acids of hEC-SOD to the C-terminal of HCAII (FusCC) resulted in the formation of a monomeric protein, which binds to heparin-Sepharose with approximately the same affinity as the tetrameric hEC-SOD. The structure of the fused C-terminal was characterized by CD and NMR spectroscopy and the data were compatible with the presence of alpha-helical structures as suggested by secondary structure predictions. The NMR data show that the C-terminal of FusCC moves independently from the rest of the protein and that its central part is involved in conformational exchange. The NOESY spectra demonstrate that the C-terminal in both FusCC and hEC-SOD binds to heparin, and that arginine side chains take part in the binding.

The complete sequence, expression in Escherichia coli, purification and some properties of carbonic anhydrase from Neisseria gonorrhoeae

The complete nucleotide sequence of the carbonic anhydrase gene from Neisseria gonorrhoeae has been determined. The gene encodes a 252-residue polypeptide with a molecular mass of 28085 Da. The gene has been cloned and overexpressed in Escherichia coli, and the enzyme has been purified. A 26-residue signal peptide is cleaved off by the E. coli processing machinery. Thus, the isolated enzyme contains 226 amino acid residues with a molecular mass of 25314 Da. Most of the enzyme seems to be produced as a soluble protein located in the periplasm of E. coli. The enzyme is homologous to carbonic anhydrases from the animal kingdom; it is an alpha-carbonic anhydrase. A comparison with the amino acid sequences of human carbonic anhydrases I and II suggests that the secondary structures are essentially intact in the bacterial enzyme but that several loops are much shorter than in the human forms. Most of the active-site residues are identical to those found in the high-activity human isozyme II. The bacterial enzyme has a high CO2 hydration activity with a k(cat) of 1.1 x 10(6) s(-1) and Km of 20 mM at pH 9 and 25 degrees C. The enzyme also catalyzes the hydrolysis of 4-nitrophenyl acetate. The pH/rate profile can be described as a titration curve with pKa of 6.7 and a maximal value of the catalytic second-order rate constant, k(enz), of 130 M(-1) x s(-1).