Hapten binding studies on mouse IgA myeloma proteins with antibody activity.

Five mouse IgA myeloma proteins (S23, S63, S107, S129, J539) with antibody specificities for either phosphorylcholine, ε-DNP-caproic acid or galactoside derivatives were studied in terms of their binding of these ligands. The immunoglobulins appeared to be homogeneous with respect to their affinity for hapten and had association constants between 2 × 103 and 105 liters/mol as determined from equilibrium dialysis measurements. Binding activity was associated with the Fab fragment of each IgA. By means of a membrane filter assay, unfractionated S107 protein in serum as well as the reduced and alkylated monomer purified by immunoadsorption was shown to have two sites per 156,000 unit molecular weight. Digestion of the monomer with trypsin gave a Fab fraction in 64% yield with one site per 53,000 molecular weight. However, protein purified by procedures involving chromatography, gel filtration and repeated dialysis steps gave fractional numbers of binding sites, a finding which suggests that mouse IgA's are unusually sensitive to denaturation.