Positive cooperativity of [3H]dexamethasone binding to chick corticosteroid-binding globulin

1. Dilution of chick plasma with Tris-EDTA buffer containing dithiothreitol (TED buffer) induced a positive cooperativity in the interaction between corticosteroids and CBG. 2. Positive cooperativity binding is probably induced by the polymerization of chick CBG in the presence of DTT. 3. As the increase of the value of B/F in Scatchard plot analysis was dependent on the concentration of [3H]dexamethasone, it is possible that the polymerization of CBG is steroid-dependent. 4. Positive cooperativity was also observed in TED buffer containing 0.2 M KCl and 0.5 M KCl. Addition of KCl seemed to enhance the polymerization of chick CBG.