Binding of Human IgA Fragments to Protein A-Sepharose Studied with an ELISA Method

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Abstract
An enzyme‐linked immunosorbent assay method was developed to investigate the binding of IgA fragments in protein A. The method proved to be specific and highly sensitive. Contamination with IgA did not interfere with the detection of IgA binding to protein A. and less than 10 mg 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(abe)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 doth Fab′and IgAl‐protease‐produced Fab fragments did bind to protein A. These results demonstrate that the Fabh fragment has a binding site tor protein A. It is, suggested that the protein A binding site is located on the CHI domain of the IgAI molecule. On Fabc fragments this binding site may be blocked because of structural alterations.

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