Kinetics of the glucocorticoid hormone-receptor interaction. False association constants determined in slowly equilibrating systems.

The usual way of in vitro determination of association constants for hormone-receptor complexes is criticized. It is shown that if incubation time is short, relative to the half-life of the hormone-receptor complex, the value of the apparent Ka is proportional to the time of incubation. No sign of lack of equilibrium is apparent from the Scatchard plots. The case of rapidly denaturing receptor molecule is also discussed, with similar conclusions. Although terminology and examples are taken from the field of the glucocorticoid receptor research, all deductions are valid for other systems with similar association (and denaturation or monomolecular transformation) mechanisms and kinetic parameters.