Reactivation of thymocyte glucocorticoid receptors in a cell-free system.

The specific glucocorticoid binding capacity of the 100,000 X g supernatant from thymocytes becomes rapidly inactivated with a half-life of approximately 4 h at 0 degrees. Following this inactivation, the receptor can be reactivated by a factor (or factors) present in the high speed supernatant of L929 mouse fibroblasts. The L cell activating factor is excluded from Sephadex G-25. It can be separated from the glucocorticoid receptor of L cells by chromatography of Sephadex G-150 where it elutes with the small molecular weight material. The probable molecular weight range for the activating factor is 5,000 to 20,000. Approximately 50% of the activating activity is retained after heating at 90 degrees for 15 min.