Liver cytosol corticosteroid binder IB, a new binding protein.

Abstract

A new binding protein named corticosteroid Binder IB elutes just after ligandin in DEAE-Sephadex chromatograms. It has been partially purified to about 2500-fold over cytosol proteins. Calculation of the number of steroid binding sites, assuming one site per molecule of Binder IB fraction after DEAE-Sephadex chromatography, suggests a concentration of the binding protein of about 0.0004% of cytosol proteins. Its pI value is judged to be 7.5 to 8 from it elution position on DEAE-Sephadex chromatograms. IB has an apparent molecular weight of 30,500 +/- 10% by gel filtration and a Stokes radius of 20 A. Binder IB binds radioactive dexamethasone, cortisol, and corticosterone in vitro with estimated KD values of 1, 13, and 25 nM, respectively. Saturation curves are abnormal, showing two phases. The saturation curves within the physiological range of concentrations of steroid are abnormal and suggestive of cooperativity. The second phase, at concentrations of glucocortidoids above saturation and physiological levels, shows extensive binding. After fractionation from other steroid binding proteins, the specificity of binding from competition studies in vitro is dexamethasone greater than or equal to cortisol = corticosterone = estradiol-17beta greater than or equal to deoxycorticosterone = dihydrotestosterone greater than aldosterone = cortexolone greater than testosterone. Other steroids tested are less efficient ligands. The binding is probably noncovalent, but strong; and the complex becomes more dissociable as purification proceeds, suggesting a conformational change in the protein. Storage and rebinding with steroid are possible throughout the purification process, although extensive ligand dissociation and denaturation of the protein occur after the final purification step. Binding in vitro is temperature-sensitive and binding is sharply pH dependent with an optimum at 7.5. The ligand is the unmetabolized steroid as judged by extraction of steroid-IB complex with methylene chloride and subsequent thin layer chromatography. The physiological function of this protein is unknown at present and purification fo the major corticosteroid hormone receptor to homogeneity may be required before the function of Binder IB is fully understood.