title: Formation of covalent complexes between the fourth component of human complement and IgG immune aggregates.
abstract: The binding properties of activated C4 to immune complexes (ovalbumin-rabbit IgG antiovalbumin) were studied by using 125I-IgG in the immune complexes or performing the C4 binding assays in the presence of 14C-iodoacetamide. High molecular weight complexes formed between C4 and IgG could be detected by the incorporation of 14C-iodoacetamide in the -SH group generated in the nascent C4b during the activation process. The same complexes with an apparent molecular weight of 180,000 daltons were detected when the immune aggregates contained 125I-IgG. Two-dimensional SDS-PAGE analysis of the C4b-IgG covalent complexes indicated: In the absence of control proteins, the complexes are formed by the alpha'-chain of C4b and the H chain of the antibody. The alpha'-H complexes are 36% sensitive to hydroxylamine and 64% resistant. This is consistent with the presence of two populations of C4, which are not equivalent in their covalent binding with immune complexes. Covalent complexes C4-C4b or C4b(like)-C4b(like) are generated during the C4 activation and they are detected as alpha-alpha' or alpha-alpha complexes, respectively. Interaction of C4b with the L chain of the antibody molecule also seems to occur, but to a lesser extent than with the H chain.
mesh_terms: D000936:Antigen-Antibody Complex / Q000378:metabolism*; D003181:Complement C4 / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006461:Hemolysis; D006801:Humans; D019811:Hydroxylamine; D006898:Hydroxylamines; D007074:Immunoglobulin G / Q000378:metabolism*; D007460:Iodoacetamide; D008970:Molecular Weight
chemical_list: D000936:Antigen-Antibody Complex; D003181:Complement C4; D006898:Hydroxylamines; D007074:Immunoglobulin G; D019811:Hydroxylamine; D007460:Iodoacetamide