Kinetics of the hormone-receptor interaction. Competition experiments with slowly equilibrating ligands.

Kinetics of formation of protein-tracer complex in the presence of competitor were calculated. The parameters were chosen so that they should realistically describe the in vitro association of steroid hormones with their receptors. Time necessary for equilibration depends on four rate constants in addition to initial concentrations and may be more than 1000 min. Competitors forming complexes with the protein that dissociate faster than the protein tracer complex have relative binding affinities apparently decreasing with incubation time. Conversely, relative binding affinity apparently increases with time if the protein-competitor complex dissociates more slowly than the protein-tracer complex. Moreover, lack of equilibration is not easily detected. It is suggested that kinetic analyses, more detailed than usual, precede competition experiments.