Interaction of C3b(2)--IgG complexes with complement proteins properdin, factor B and factor H: implications for amplification

Biochem J. 2000 Jul 1;349(Pt 1):217-23. doi: 10.1042/0264-6021:3490217.

Abstract

Nascent C3b can form ester bonds with various target molecules on the cell surface and in the fluid phase. Previously, we showed that C3b(2)--IgG complexes represent the major covalent product of C3 activation in serum [Lutz, Stammler, Jelezarova, Nater and Späth (1996) Blood 88, 184--193]. In the present report, binding of alternative pathway proteins to purified C3b(2)--IgG complexes was studied in the fluid phase by using biotinylated IgG for C3b(2)--IgG generation and avidin-coated plates to capture complexes. Up to seven moles of properdin 'monomer' bound per mole of C3b(2)--IgG at physiological conditions in the absence of any other complement protein. At low properdin/C3b(2)--IgG ratios bivalent binding was preferred. Neither factor H nor factor B affected properdin binding. On the other hand, properdin strongly stimulated factor B binding. Interactions of all three proteins with C3b(2)--IgG exhibited pH optima. An ionic strength optimum was most pronounced for properdin, while factor B binding was largely independent of the salt concentration. C3b(2)--IgG complexes were powerful precursors of the alternative pathway C3 convertase. In the presence of properdin, C3 convertase generated from C3b(2)--IgG cleaved about sevenfold more C3 than the enzyme generated on C3b. C3b(2)--IgG complexes could therefore maintain the amplification loop of complement longer than free C3b.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Complement C3-C5 Convertases / metabolism
  • Complement C3b / chemistry
  • Complement C3b / metabolism*
  • Complement Factor B / chemistry
  • Complement Factor B / metabolism*
  • Complement Factor H / chemistry
  • Complement Factor H / metabolism*
  • Dimerization
  • Dose-Response Relationship, Drug
  • Electrophoresis, Polyacrylamide Gel
  • Humans
  • Hydrogen-Ion Concentration
  • Immunoglobulin G / chemistry
  • Immunoglobulin G / metabolism*
  • Ions
  • Kinetics
  • Properdin / chemistry
  • Properdin / metabolism*
  • Protein Binding
  • Radioimmunoassay
  • Sodium Chloride / pharmacology

Substances

  • Immunoglobulin G
  • Ions
  • complement factor H, human
  • Properdin
  • Sodium Chloride
  • Complement C3b
  • Complement Factor H
  • Complement C3-C5 Convertases
  • Complement Factor B