CONFORMATIONAL ISOMERISM OF IGG ANTIBODIES

The purpose of this study was to determine why apparently homogeneous IgG antibodies were, in some cases, fractionated into at least two com ponents by liquid-liquid partition chromatography (LLPC) in an aqueous two-phase system. Four mouse monoclonal IgG antibodies, two against a lbumin, one against IgG and one against thyroxine, were shown to adopt different conformational isomeric forms. The four antibodies existed in an equilibrium between two or three conformational forms, the propo rtion of which could also be estimated by LLPC. Since LLPC detects mai nly conformational differences within the antigen-binding sites of IgG antibodies, it could be concluded that the conformational forms diffe red with respect to their combining sites. Moreover, the isomeric form s of an antibody directed against a protein antigen, formed antigen-an tibody complexes with almost identical surface properties. In contrast , complexes with different surface properties were formed when the hap ten or hapten conjugated to BSA was bound. Thus, both the conformation al isomers could bind antigen, at least when the antigen was a small h apten or a hapten conjugated to a carrier protein. Our results suggest that six out of 57 monoclonal Ige antibodies exist in equilibrium bet ween at least two conformational forms and the biological significance of this isomerism is discussed.