The folding of human active and inactive extracellular superoxide dismutases is an intracellular event

J Biol Chem. 2008 May 30;283(22):15031-6. doi: 10.1074/jbc.M801548200. Epub 2008 Apr 2.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Substitution
  • Base Sequence
  • Cell Line
  • Disulfides
  • Enzyme Activation / physiology
  • Extracellular Space / enzymology*
  • Humans
  • Molecular Sequence Data
  • Protein Biosynthesis / physiology
  • Protein Folding*
  • Superoxide Dismutase / genetics
  • Superoxide Dismutase / metabolism*
  • Superoxides / metabolism


  • Disulfides
  • Superoxides
  • SOD3 protein, human
  • Superoxide Dismutase

Associated data

  • SWISSPROT/P08294