Restriction of heterogeneity of goat antibodies specific for human hemoglobin S

The authors previously reported the isolation of an antibody population (termed antiVal antibody) specific for the site of difference between human hemoglobin S (HbS) (β⁶Val) and hemoglobin A₁ (HbA) (β⁶ glu). This population has a stoichiometry of reaction of unity in combining with HbS (αβ dimer) and shows no reaction with HbA. The combination of antiVal Fab fragments with HbS was found to be kinetically homogeneous and had a second order rate constant of 0.58 x 10⁶M^-1 sec^-1 at 20°C. Here, they studied other properties of the antiVal population. These antibodies are restricted to only one of the two heavy chain subclasses of goat IgG. Electrophoresis experiments indicated that the antiVal population is much less polydisperse than the total antiHbS response. A method was developed to measure the dissociation kinetics of antiVal HbS complexes. This dissociation was also found to be kinetically homogeneous and could be described by a single first order rate constant of 2.67 x 10^-5 sec^-1 at 20°C. With homogeneous association and dissociation rate constants, an affinity constant of 2.1 x 10¹⁰M^-1 at 20°C was calculated. It appears, then, that this population of antibodies, which are directed toward a single antigenic determinant on a globular protein, exhibit limited structural heterogeneity associated with great functional homogeneity.