Binding of human IgA fragments to protein A-Sepharose studied with an ELISA method.

An enzyme-linked immunosorbent assay method was developed to investigate the binding of IgA fragments to protein A. The method proved to be specific and highly sensitive. Contamination with IgG did not interfere with the detection of IgA binding to protein A, and less than 10 ng of IgA could be detected. Four of nine IgA proteins tested bound to protein A to different extents. The binding was not disturbed by reduction and alkylation of the IgA proteins. Four-chain F(abc)2 and F(ab')2 fragments of the protein A-reactive IgA proteins also bound to protein A. On reduction and alkylation these fragments formed two-chain Fabc and Fab' fragments. Of these, Fabc did not bind, whereas both Fab' and IgA1-protease-produced Fab fragments did bind to protein A. These results demonstrate that the Fab fragment has a binding site for protein A. It is suggested that the protein A binding site is located on the CH1 domain of the IgA1 molecule. On Fabc fragments this binding site may be blocked because of structural alterations.