Olsen DA, Petersen SV, Oury TD, Valnickova Z, Thøgersen IB, Kristensen T, Bowler RP, Crapo JD, Enghild JJ. The intracellular proteolytic processing of extracellular superoxide dismutase (EC-SOD) is a two-step event.
J Biol Chem 2004;
279:22152-7. [PMID:
15044467 DOI:
10.1074/jbc.m401180200]
[Citation(s) in RCA: 43] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
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.
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