Progesterone-binding components of chick oviduct. IX. The kinetics of nuclear binding.

A cell-free system was used to study the kinetics of progesterone-receptor interaction with purified nuclei prepared from estrogen-primed chick oviducts. The binding process was a saturable phenomenon in both target and nontarget tissues. More nuclear acceptor sites were available to target tissue (similar to 9000 sites per oviduct nucleus) than in nontarget tissues (similar to 1000 to 3000 sites per nucleus), but the binding constant was essentially the same (Kd similar to 10-8 M). A second much smaller class of higher affinity sites (Kd similar to 10-11M) may exist. Its presence was detected by Scatchard plot nonlinearity at very low concentrations of added receptor-hormone complex (similar to 10-10 to 10-12M). The current study focused on the prevalent class of acceptor sites which was more readily detectable. Receptor binding to these sites was highly sensitive to salt. More sites were exposed at 25 degrees than at 0 degrees. Binding to these sites was inhibited in a nonselective fashion by the addition of protein. Although receptors may be activated by temperature or conditions of high ionic strength, these conditions could not capacitate more than 30 to 40% of the progesterone-receptor proteins for binding. Rate studies suggested that temperature plays a minimal role in nuclear uptake of activated receptors. Such a finding is consistent with a diffusion-limited uptake process.