Cytoplasmic glucocorticoid binding proteins in bone cells.

The binding of 3H-dexamethasone (3H-DM) was studied in the cytoplasmic fraction of bone cells isolated from fetal rat calvaria by enzymatic digestion. An high-affinity glucocorticoid binding protein resembling those described in other target tissues was demonstrated. Scatchard analysis revealed a single class of binding sites with an apparent dissociation constant for 3H-DM (0C) of 7 x 10-9M and a concentration of binding sites of 0.11 pmoles/mg cytosol protein. The number of cytoplasmic binding sites per cell was calculated at 6,000 which is probably an underestimate due to occupancy of some sites by endogenous steroids. The binding sites appeared protein in nature since incubation with pronase destroyed 100% of the binding. Nuclear transfer was demonstrated in a reconstituted system utilizing bone cytosol as donor and liver nuclei as the acceptor. Competitive binding analysis revealed corticosterone to be equivalent to DM in binding affinity; progesterone was 75% as potent as DM. Aldosterone and SC-26304 (a spirolactone analogue) had, respectively, 1