Characteristics of the binding of corticosteroid-binding globulin to rat cell membranes

Specific binding sites for corticosteroid-binding globulin (CBG) were detected on membranes prepared from rat spleen. The binding sites are typical of membrane receptors; they are saturable, specific, have high affinity, and require Mg2+ or Ca2+ for binding. There was little specific binding at 4 C, and maximal binding was obtained at 37 C. Scatchard analysis revealed a single set of binding sites with an apparent Kd of 0.84 microM, and a binding capacity of 39 pmol/mg membrane protein. The sites were specific for CBG; binding of [125I]CBG was not inhibited by a 10,000-fold excess of either rat albumin or rat transferrin. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate of the membrane-bound [125I]CBG revealed the presence of lower mol wt iodinated products which were undetectable in the unbound [125I]CBG fraction. Further, whereas the electrophoretic patterns from uterine, pulmonary, and renal membranes showed that the less mobile band (mol wt, 60K) of the normal CBG doublet appeared to be metabolized to a greater extent than the more mobile band (mol wt, 52K), those from splenic membranes showed equal metabolism of both parts of the doublet.