CORTICOSTEROID-BINDING GLOBULIN RECEPTOR OF THE RAT HEPATIC MEMBRANE - SOLUBILIZATION, PARTIAL CHARACTERIZATION, AND THE EFFECT OF STEROIDS ON BINDING

The corticosteroid-binding globulin (CBG) receptor of rat hepatic membranes was solubilized using 1.5% Triton X-100. An assay for its activity was developed that was dependent upon the fact that the [I-125] CBG-receptor complex adsorbs to hydroxylapatite, whereas [I-125]CBG does not. Scatchard analysis of the soluble receptor at 37 C showed a single set of high affinity binding sites, with a K(d) of 44 nm and a binding capacity of 7.3 pmol/mg protein. The association rate constant (k1) was 0.92 x 10(5) M-1 Min-1 at 37 C, and the dissociation rate constant (k2) was 1.0 x 10(-3) min-1. Only unliganded CBG could bind to the receptor. Steroids that bound to CBG, eg. corticosterone and cortisol, noncompetitively inhibited CBG's binding to the receptor. Steroids that did not bind to CBG, e.g. dexamethasone, were without effect on the interaction of CBG with its receptor,