Isolation of human S-protein, an inhibitor of the membrane attack complex of complement

E. R. Podack, H. J. Muller-Eberhard

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128 Scopus citations


A procedure is described for purifying the S-protein, a control protein of the membrane attack complex (MAC) of human complement. S-protein's apparent function in the complement sequence is to restrict cytolysis to the target cell and to inhibit lysis of bystander cells. The final product of the purification procedure is homogeneous as judged by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, immunoelectrophoresis, cellulose acetate strip electrophoresis, isoelectrofocusing, gel filtration, and sucrose density gradient ultracentrifugation. S-protein is an acidic glycoprotein with an electrophoretic mobility at pH 8.6 of -4.8x105cm2/(inter-α) and is isoelectric at pH 3.9. The molecular weight of S-protein is approximately 80,000 as determined by rate zonal ultracentrifugation, gel filtration, and SDS-polyacrylamide gel electrophoresis. When not reduced, the S-protein in the presence of SDS migrates electrophoretically as a single band with an apparent molecular weight of 89,000. Upon reduction, three components are detectable with approximate molar ratios of unity and apparent molecular weights of 84,000, 69,000, and 15,000. The M(r)=84,000 species is cleaved by trypsin into fragments with molecular weights of 69,000 and 15,000. Apparently, the S-protein occurs in two forms, one a single-chain molecule and the other a two-chain molecule in which the subunits are linked by disulfide bonds. Both forms of the S-protein inhibit the MAC by binding to the forming MAC thus interfering with its attachment to target cells. Although not previously recognized, the S-protein is present in plasma at concentrations ranging from 0.35 to 0.5 mg/ml. Whereas the S-protein is distributed symmetrically upon two-dimensional immunoelectrophoresis of plasma, the distribution in serum is bimodal suggesting a function of S-protein in the coagulation system.

Original languageEnglish (US)
Pages (from-to)9908-9914
Number of pages7
JournalJournal of Biological Chemistry
Issue number19
StatePublished - 1979

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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