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

The corticosteroid-binding globulin (CBG) receptor of rat hepatic memb ranes was solubilized using 1.5% Triton X-100. An assay for its activi ty 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 singl e set of high affinity binding sites, with a K(d) of 44 nm and a bindi ng 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 constan t (k2) was 1.0 x 10(-3) min-1. Only unliganded CBG could bind to the r eceptor. Steroids that bound to CBG, eg. corticosterone and cortisol, noncompetitively inhibited CBG's binding to the receptor. Steroids tha t did not bind to CBG, e.g. dexamethasone, were without effect on the interaction of CBG with its receptor,