Antibody hapten reaction kinetics: a comparison of hapten interactions with IgG and Fab preparations

Association and dissociation kinetics for the interaction of hapten with both IgG and Fab preparations from a high affinity rabbit antiserum were determined by recently described techniques by using dextran-coated charcoal for rapid separation of free and antibody-bound hapten. The association and dissociation rate constant were determined for the interactions of ³H-ouabain with an IgG preparation, and Fab preparation obtained by treatment of IgG with papain followed by Sephadex G-100 gel permeation chromatography, and a mixture containing approximately equal concentrations of Fab and IgG-binding sites. These data were then analyzed for evidence of cooperativity of the two binding sites of IgG. Association rate constants for interactions of IgG and Fab with ³H-ouabain were not significantly different [3.43 ± 0.29 (S.D.) x 10⁶ M^-1 sec ^-1 and 3.36 ± 0.26 x 10⁶ M^-1 sec^-1, respectively]. Dissociation data revealed no significant difference in dissociation kinetics between preparations of IgG, Fab fragments, or a mixture of the two containing approximately equal concentrations of binding sites. The ratios of association and dissociation rate constants were in satisfactory agreement with average intrinsic association constants determined under equilibrium conditions. The results are interpreted as showing no demonstrable cooperativity between the two binding sites of the IgG preparation under study in its interaction with the hapten ³H-ouabain.