The heparin-binding domain of extracellular superoxide dismutase is proteolytically processed intracellularly during biosynthesis

J Biol Chem. 1999 May 21;274(21):14818-22. doi: 10.1074/jbc.274.21.14818.


Extracellular superoxide dismutase (EC-SOD) is the only known extracellular enzyme designed to scavenge the superoxide anion. The purified enzyme exists in two forms when visualized by reduced SDS-polyacrylamide gel electrophoresis: (i) intact EC-SOD (Trp1-Ala222) containing the C-terminal heparin-binding domain and (ii) cleaved EC-SOD (Trp1-Glu209) without the C-terminal heparin-binding domain. The proteolytic event(s) leading to proteolysis at Glu209-Arg210 and removal of the heparin-binding domain are not known, but may represent an important regulatory mechanism. Removal of the heparin-binding domain affects both the affinity of EC-SOD for and its distribution to the extracellular matrix, in which it is secreted. During the purification of human EC-SOD, the intact/cleaved ratio remains constant, suggesting that proteolytic removal of the heparin-binding domain does not occur during purification (Oury, T. D., Crapo, J. D., Valnickova, Z., and Enghild, J. J. (1996) Biochem. J. 317, 51-57). This was supported by the finding that fresh mouse tissue contains both intact and cleaved EC-SOD. To study other possible mechanisms leading to the formation of cleaved EC-SOD, we examined biosynthesis in cultured rat L2 epithelial-like cells using a pulse-chase protocol. The results of these studies suggest that the heparin-binding domain is removed intracellularly just prior to secretion. In addition, the intact/cleaved EC-SOD ratio appears to be tissue-dependent, implying that the intracellular processing event is regulated in a tissue-specific manner. The existence of this intracellular processing pathway may thus represent a novel regulatory pathway for affecting the distribution and effect of EC-SOD.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Culture Techniques
  • Extracellular Space / metabolism
  • Heparin / pharmacokinetics*
  • Humans
  • Mice
  • Protein Splicing*
  • Rats
  • Superoxide Dismutase / biosynthesis*
  • Superoxide Dismutase / metabolism*


  • Heparin
  • Superoxide Dismutase