title: The subunit composition of human extracellular superoxide dismutase (EC-SOD) regulates enzymatic activity.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Center for Insoluble Protein Structures (inSPIN) and Interdisciplinary Nanoscience Center (iNANO) Department of Molecular Biology, Aarhus University, Gustav Wieds Vej 10C, DK-8000 Aarhus C, Denmark. svp@mb.au.dk
authors.forename: Steen V
authors.initials: SV
authors.lastname: Petersen
authors.forename: Zuzana
authors.initials: Z
authors.lastname: Valnickova
authors.forename: Tim D
authors.initials: TD
authors.lastname: Oury
authors.forename: James D
authors.initials: JD
authors.lastname: Crapo
authors.forename: Niels
authors.initials: N
authors.lastname: Chr Nielsen
authors.forename: Jan J
authors.initials: JJ
authors.lastname: Enghild
mesh_terms: D000818:Animals; D000975:Antioxidants / Q000378:metabolism; D019281:Dimerization; D004789:Enzyme Activation / Q000502:physiology; D005110:Extracellular Space / Q000201:enzymology / Q000378:metabolism; D006801:Humans; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D013482:Superoxide Dismutase / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D013552:Swine
chemical_list: D000975:Antioxidants; D021122:Protein Subunits; C441642:SOD3 protein, human; D013482:Superoxide Dismutase