A common mechanism of hapten binding to immunoglobulins and their heterologous chain recombinants

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Abstract
Kinetics and thermodynamics of binding of the hapten .beta.-D-(1-6)-galactotriose to the homogeneous Ig[immunoglobulin]A T-601 and to heterologous recombinants of H and L chains prepared from mouse IgA myeloma cells X-24, J-539 and T-601, which all have the same galactan specificity, were studied by the chemical relaxation method. All the Ig-hapten systems investigated exhibited 2 relaxation times. The reciprocal value of the fast time increased linearly; that of the slow time leveled off with increasing hapten concentration. This behavior indicates the presence of a fast bimolecular association and a slower monomolecular step. The data obtained for homologous and hybrid Ig fit a mechanism where the proteins exist in 2 conformations and hapten binding shifts their equilibrium to the higher affinity conformer. The kinetic and thermodynamic parameters for the hapten binding by the hybrids were similar to those of their parent proteins. This conformational transition probably is an inherent property of the tertiary domain structure of the antibody, probably involving changes in the interactions between H and L chain domains.