FUNCTIONAL ACTIVITIES OF IGG AND IGM ANTI-A AND ANTI-B

Abstract

The rate constants for association and dissociation, and the equilibrium constants, were determined for 125I-labelled anti-A and anti-B of both IgG and IgM molecular types. The following results and conclusions were obtained The equilibrium constants were within the range 0.6 x 108-13.0 x 1081/mole, and were of the same order for both IgG and IgM antibodies. The initial rate constants for association were in the range 2.1 x 105-4.8x1051/mole/sec, and the energy of activation (Ea) 6700-9000 cal/mole [calorie/mole]. These results indicate that the rate of association is approaching the limit set by the rate of diffusion of the reactants. The initial rate constants for dissociation were 1 x 10-4- 5 x 10-4/sec and Ea = 20,000-36,000 cal/mole. These latter values suggest that more than 1 bond must be broken simultaneously during dissociation. Ionic strength and pH changes have only a minor effect on the constants; this indicates absence of ionic groups on A and B antigen sites. The changes in enthalpy were -5400 to [long dash]21,800 cal/mole; the reactions are mainly enthalpy driven and this accounts for the fact that anti-A and anti-B agglutination titers increase as the temperature is decreased. There was heterogeneity of the values of the standard change in free energy, enthalpy and entropy within each example of antibody. The approximate concentrations of antibody at the end-points of the agglutination titers were: for IgG antibody, 0.2 [mu]g/ml; for IgM antibody, 0.01 [mu]g/ml.