KINETIC AND THERMODYNAMIC STUDIES OF ANTIGEN-ANTIBODY INTERACTIONS IN HETEROGENEOUS REACTION PHASES .1. L-THYROXINE (T4) WITH SPECIFIC ANTIBODY IMMOBILIZED ON CONTROLLED PORE GLASS

The binding reaction of thyroxine (T4) to its specific immobilized antibody (IMA) was treated as a general heterogeneous reaction and kinetic experiments were performed in order to elucidate the reaction mechanism. Except in the complete absence of serum protein, liquid phase diffusion is not the rate determining step. The T4 binding reaction follows second-order kinetics, in which the two concentration dependent terms are the antibody concentration and the instantaneously free (unbound) T4. The rate of this binding reaction can be described by: rate = k[FT4] [IMA]. The [FT4] is a function of the total T4 and protein concentrations. In the presence of serum protein the rate is much faster when a chemical deblocking agent (merthiolate) is used. The extent of binding does not change much with temperature indicating that the entropic change is the major contribution to the free energy change. The rate of binding, however, changes by an order of magnitude over the range 12-38°C. The apparent activation energies are 9.6 and 16.9 kcal mole in the presence and absence of merthiolate respectively, the difference (7 kcal mole) being consistent with the binding enthalpy of T4 to thyroxine binding globulin. © 1979.