Effects of urea and molybdate on the chick oviduct progesterone receptor

The chick oviduct cytosol progesterone receptor, when complexed with ligand, can be exposed to urea concentrations as high as 3 M (at 0 degrees C) without loss of steroid binding capacity. The ligand dissociation rate is increased greater than or equal to 10 fold under these conditions. The "native" 8 S form of the receptor is progressively converted to a 4 S species by urea (greater than 2 M) as seen in ultracentrifugation analysis. This conversion is inhibited by Na2MoO4 (5-50 mM) suggesting that molybdate stabilizes the 8 S molecule by direct interaction. At urea concentrations above 2 M, the ligand-free receptor looses progressively its binding capacity. The "transformed", 4 S receptor was less stable than the 8 S species, and could not be protected by molybdate.