title: Action of the C3b-inactivator on the cell-bound C3b.
abstract: The action of C3bINA and beta 1H on cell-bound C3b is described in this paper. The alpha-polypeptide of C3b that binds covalently to cell surfaces is cleaved by the C3bINA and beta 1H into two fragments: one of 60,000 (C3b alpha-60) and another of 40,000 (C3b alpha-40) daltons. The beta-chain of C3b is unaffected by the C3bINA and beta 1H. The three polypeptides, C3b alpha-60, C3b alpha-40, and C3 beta, are held together as a single unit by disulfide bonds. This unit, referred to as C3b' is covalently bound to cell surfaces via the C3b alpha-60 polypeptide. The conversion of C3b to C3b' by C3bINA and beta 1H abolishes the ability of the C3b-bearing cells to adhere to human erythrocytes as well as the ability to form, on the cell surface, the B, D, and properdin-dependent amplification C3-convertase. However, the agglutinability of the cells with either anti-C3c or anti-C3d is not affected. Treatment of the C3b'-bearing cells with trypsin releases fragments of C3b' into solution, leaving a polypeptide of 32,000 daltons covalently linked to the membrane. Since the trypsinized cells are agglutinable by anti-C3d but not by anti-C3c, the 32,000 dalton polypeptide appears to correspond antigenically to C3d.
mesh_terms: D000818:Animals; D001665:Binding Sites; D003176:Complement C3 / Q000276:immunology; D003179:Complement C3b / Q000276:immunology*; D003180:Complement C3b Inactivator Proteins / Q000494:pharmacology*; D003181:Complement C4 / Q000276:immunology; D004591:Electrophoresis, Polyacrylamide Gel; D006168:Guinea Pigs; D006801:Humans; D008565:Membrane Proteins / Q000276:immunology; D008970:Molecular Weight; D010455:Peptides / Q000276:immunology; D011817:Rabbits; D014357:Trypsin / Q000494:pharmacology
chemical_list: D003176:Complement C3; D003180:Complement C3b Inactivator Proteins; D003181:Complement C4; D008565:Membrane Proteins; D010455:Peptides; D003179:Complement C3b; D014357:Trypsin