Spectral and physical properties of human extracellular superoxide dismutase: a comparison with CuZn superoxide dismutase.
Comparison of amino acid sequences have suggested similarities between the active site portions of the tetrameric, Cu- and Zn-containing glycoprotein, extracellular superoxide dismutase (EC-SOD) and the dimeric CuZn-SOD. In the present study spectral and physical properties of EC-SOD were analyzed to further knowledge about the enzyme and to extend the comparison with CuZn-SOD. EC-SOD displays an absorbance peak at 652 nm, blue-shifted some 30 nm compared with CuZn-SOD. The molar extinction coefficient, 167 M-1 cm-1 per copper atom, is similar to those of CuZn-SODs. In contrast to bovine and human CuZn-SOD, EC-SOD has a strong uv absorption originating from the rich content of aromatic amino acids. The extinction coefficient at 280 nm is 1.73 ml mg-1 cm-1. The electron paramagnetic resonance spectrum is also similar to those of bovine and human CuZn-SOD, but shows a slightly higher Az and a smaller difference between gx and gy. These spectral findings support the notion that the active sites of the SOD iso-enzymes are similar, but with a higher tetragonal symmetry of the copper ligands in EC-SOD. The circular dichroism spectrum of EC-SOD in the ultraviolet indicates a high alpha-helix content of the amino-terminal and carboxy-terminal parts flanking the central homologous active site portion. Similar to bovine CuZn-SOD, EC-SOD displays a marked physical resistance toward high temperature, pH extremes, and high urea and guanidinium chloride concentrations. This similarity in physical resistance suggests a central role of the homologous active site portion for the structural integrity of the SOD iso-enzymes.
Symbicom AB, Umeå, Sweden.
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Aasa
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Marklund
D000818:Animals; D016466:CHO Cells / Q000201:enzymology; D002942:Circular Dichroism; D006224:Cricetinae; D003486:Cyanides; D004578:Electron Spin Resonance Spectroscopy; D004795:Enzyme Stability; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007527:Isoenzymes / Q000737:chemistry* / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry; D013053:Spectrophotometry; D013482:Superoxide Dismutase / Q000737:chemistry* / Q000235:genetics; D014470:Umbilical Cord / Q000201:enzymology
D003486:Cyanides; D007527:Isoenzymes; D011994:Recombinant Proteins; D013482:Superoxide Dismutase