The nature of heterogeneous components of extracellular-superoxide dismutase purified from human umbilical cords

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.

The intracellular proteolytic processing of extracellular superoxide dismutase (EC-SOD) is a two-step event

Extracellular superoxide dismutase (EC-SOD) is a tetramer composed of either intact (Trp(1)-Ala(222)) or proteolytically cleaved (Trp(1)-Glu(209)) subunits. The latter form is processed intracellularly before secretion and lacks the C-terminal extracellular matrix (ECM)-binding region ((210)RKKRRRESECKAA(222)-COOH). We have previously suggested that the C-terminal processing of EC-SOD is either a one-step mechanism accomplished by a single intracellular endoproteolytic event cleaving the Glu(209)-Arg(210) peptide bond or a two-step mechanism involving two proteinases (Enghild, J. J., Thogersen, I. B., Oury, T. D., Valnickova, Z., Hojrup, P., and Crapo, J. D. (1999) J. Biol. Chem. 274, 14818-14822). In the latter case, an initial endoproteinase cleavage occurs somewhere in the region between Glu(209) and Glu(216). A carboxypeptidase specific for basic amino acid residues subsequently trims the remaining basic amino acid residues to Glu(209). A naturally occurring mutation of EC-SOD substituting Arg(213) for Gly enabled us to test these hypotheses. The mutation does not prevent proteolysis of the ECM-binding region but prevents a carboxypeptidase B-like enzyme from trimming residues beyond Gly(213). The R213G mutation is located in the ECM-binding region, and individuals carrying this mutation have an increased concentration of EC-SOD in the circulatory system. In this study, we purified the R213G EC-SOD variant from heterozygous or homozygous individuals and determined the C-terminal residue of the processed subunit to be Gly(213). This finding supports the two-step processing mechanism and indicates that the R213G mutation does not disturb the initial endoproteinase cleavage event but perturbs the subsequent trimming of the C terminus.

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.

Adenovirus-mediated overexpression of extracellular superoxide dismutase improves endothelial dysfunction in a rat model of hypertension

Gene transfer may be appropriate for therapeutic protocols targeted at the vascular endothelium. Endothelial dysfunction is the principal phenotype associated with atherosclerosis and hypertension. Oxidative stress has been implicated in the development of endothelial dysfunction. We have explored the ability of overexpressing anti-oxidant genes (superoxide dismutases; SODs) in vitro and in vivo to assess their potential for reversing endothelial dysfunction in a rat model, the stroke-prone spontaneously hypertensive rat (SHRSP). Western blotting and immunofluorescence assays in vitro showed efficient overexpression of MnSOD and ECSOD with respect to localisation to the mitochondria and extracellular surface, respectively. Transgene functional activity was quantified with SOD activity assays. MnSOD and ECSOD overexpression in intact SHRSP vessels in vivo led to endothelial and adventitial overexpression. Pharmacological assessment of transduced vessels following in vivo delivery by basal NO availability quantification demonstrated that the "null" adenovirus and MnSOD adenovirus did not significantly increase NO availability. However, AdECSOD-treated carotid arteries showed a significant increase in NO availability (1.91 +/- 0.04 versus 0.75 +/- 0.08 g/g, n = 6, P = 0.029). In summary, efficient overexpression of ECSOD, but not MnSOD in vivo, results in improved endothelial function in a rat model of hypertension and has important implications for the development of endothelial-based vascular gene therapy.

Tissue distribution of immunoreactive mouse extracellular superoxide dismutase

Protein content and mRNA expression of extracellular superoxide dismutase (EC-SOD) were investigated in 16 mouse tissues. We developed a double-antibody sandwich ELISA using the affinity-purified IgG against native mouse EC-SOD. EC-SOD could be detected in all of the tissues examined (lung, kidney, testis, brown fat, liver, adrenal gland, pancreas, colon, white fat, thymus, stomach, spleen, heart, skeletal muscle, ileum, and brain, in decreasing order of content measured as microg/g wet tissue). Lung showed a markedly higher value of EC-SOD than other tissues. Interestingly, white fat had a high content of EC-SOD in terms of micrograms per milligram protein, which corresponded to that of lung. Kidney showed the strongest expression of EC-SOD mRNA. Relatively strong expression of the mRNA was observed in lung, white fat, adrenal gland, brown fat, and testis. Heart and brain showed only weak signals, and no such expression could be detected in either digestive organs or skeletal muscle. Immunohistochemically, EC-SOD was localized mainly to connective tissues and vascular walls in the tissues examined. Deep staining in the cytosol was observed in the cortical tubular cells of kidney. These results suggest that EC-SOD is distributed systemically in mice and that the physiological importance of this enzyme may be a compensatory adaptation to oxidative stress, particularly in lung and kidney.

Purification and subunit structure of extracellular superoxide dismutase from mouse lung tissue

The first purification of mouse extracellular superoxide dismutase (EC-SOD) and the analysis of the native enzyme are described. Mouse EC-SOD was purified from lung tissues with a high recovery (41%) and a specific polyclonal antibody against the purified enzyme was obtained. The purified enzyme had a strong affinity for, heparin and a molecular mass of 150 kDa (estimated by a gel filtration chromatography). The native mouse EC-SOD was composed of two different sizes of subunits, a M(r) of 33 and 35 kDa (determined by SDS-PAGE). The 35-kDa subunit had an interchain disulfide bond at the C-terminus and existed as a covalent dimer in the molecule, whereas the 33-kDa subunit resulted from the 35-kDa subunit by truncating its C-terminus as a posttranslational modification, with resultant loss of the interchain disulfide bond. These results suggest that the native mouse EC-SOD is a heterotetramer composed of two different dimers, with or without a covalent bond.

Human extracellular superoxide dismutase is a tetramer composed of two disulphide-linked dimers: a simplified, high-yield purification of extracellular superoxide dismutase

Studies examining the biochemical characteristics and pharmacological properties of extracellular superoxide dismutase (EC SOD) have been severely limited because of difficulties in purifying the enzyme. Recently EC SOD was found to exist in high concentrations in the arteries of most mammals examined and it is the predominant form of SOD activity in many arteries. We now describe a three-step, high-yield protocol for the purification of EC SOD from human aorta. In the first step, the high affinity of EC SOD for heparin is utilized to obtain a fraction in which EC SOD constitutes roughly 13% of the total protein compared with only 0.3% of that of the starting material. In addition, over 80% of the original EC SOD activity present in the aortic homogenate was retained after the first step of purification. EC SOD was further purified using a combination of cation- and anion-exchange chromatography. The overall yield of EC SOD from this purification procedure was 46%, with over 4 mg of EC SOD obtained from 230 g of aorta. Purified EC SOD was found to exist predominantly as a homotetramer composed of two disulphide-linked dimers. However, EC SOD was also found to form larger multimers when analysed by native PAGE. It was shown by urea denaturation that the formation of multimers increased the thermodynamic stability of the protein. Limited proteolysis of EC SOD suggested that there is one interchain disulphide bond covalently linking two subunits. This disulphide bond involves cysteine-219 and appears to link the heparin-binding domains of the two subunits.

Substitution of glycine for arginine-213 in extracellular-superoxide dismutase impairs affinity for heparin and endothelial cell surface

Extracellular-superoxide dismutase (EC-SOD) levels in sera divide into two discontinuous groups: a low-level group below 400 ng/ml and a high-level group above 400 ng/ml [Adachi, Nakamura, Yamada, Futenma, Kato and Hirano (1994) Clin. Chim. Acta 229, 123-131]. Molecular genetic studies have shown that the donors in the high-level group have a single base substitution generating the exchange of glycine for arginine-213 (R213G) in the heparin-binding domain of EC-SOD [Sandström, Nilsson, Karlsson and Marklund (1994) J. Biol. Chem. 269, 19163-19166; Yamada, Yamada, Adachi, Goto, Ogasawara, Futenma, Kitano, Hirano and Kato (1995) Jpn. J. Hum. Genet. 40, 177-184]. The serum EC-SOD level in homozygote subjects was significantly higher than that in heterozygotes and in normal subjects. Serum EC-SOD from heterozygotes and homozygotes had equally decreased affinity for heparin, as judged by heparin-HPLC, as compared with that from normal donors. This result suggests that the serum EC-SOD in heterozygotes was mainly composed of the mutant form which has reduced heparin affinity. On the other hand, fibroblast cells derived from heterozygote subjects generated mRNA of both normal and mutant EC-SOD (m-EC-SOD), and expressed the corresponding proteins. EC-SOD is a tetrameric enzyme, and in heterozygote donors would be heterogeneous with regard to the constitution of normal and mutant subunits. The enzyme form consisting of only mutant subunits, the form with the weakest heparin affinity, can be preferentially driven out to the plasma phase, because EC-SOD in the vasculature exists in equilibrium between plasma and the endothelial cell surface. The binding of m-EC-SOD to bovine aortic endothelial cells was about 50-fold less than that of normal EC-SOD. This result suggests that the binding of m-EC-SOD to vascular endothelial cells is much decreased in vivo, which causes a high level of serum EC-SOD.

Quantitative and qualitative changes of extracellular-superoxide dismutase in patients with various diseases

Extracellular-superoxide dismutase (EC-SOD) is a secretory glycoprotein that is the major SOD isozyme in extracellular fluids. It has previously been shown that EC-SOD levels in sera from healthy persons are clearly divided into two discontinuous groups: a lower group (named Group I, below 120 ng/ml) and a higher group (Group II, above 400 ng/ml). The family studies have shown that the high EC-SOD level in healthy persons is genetically transmitted. We report here on the EC-SOD levels in the sera of patients with various diseases. The EC-SOD levels were distinctly higher in patients with renal diseases and moderately higher in liver diseases and diabetes than those in normal healthy persons. In cerebrovascular diseases, heart diseases and acute digestive diseases, significant differences of EC-SOD were not observed. In patients with renal diseases, the increase of EC-SOD was accompanied by the lack of renal function. Serum EC-SOD in Group I healthy persons is known to be heterogeneous with regard to heparin affinity and can be separated into three fractions: A without affinity, B with weak affinity and C with relatively strong heparin affinity, whereas the EC-SOD in Group II is mainly one fraction of C-type. Also in the case of hemodialysis patients, serum EC-SOD in Group I or Group I' (approximately 120-400 ng/ml) was divided into three fractions. EC-SOD in Group II showed two different profiles on heparin-Sepharose column chromatographies: one consisted mainly of EC-SOD C and the other consisted of EC-SOD A and C. It is probable that the high serum EC-SOD level in hemodialysis patients was due to two possible factors: the genetic transmitted factor and unknown pathophysiological factor(s).