Receptor properties of corticosteroid binder IB

Corticosteroid binder IB, present in liver and kidney, is pronounced in liver cytosol after injection of ³H]triamcinolone acetonide. Following injection of the radioactive ligand, livers homogenized in the presence of 20 mm molybdate, 2 mm leupeptin hemisulfate, 2 mm antipain, or 2 mm phenylmethylsufonyl fluoride produce cytosols with Chromatographie profiles of binders II and IB identical to controls, as determined by DEAE-Sephadex chromatography, suggesting that IB is a cellular constituent rather than a product of protease action (sensitive to the above inhibitors) after cell breakage. Generation of IB in kidney cytosols in vitro appears to be unrelated to protease activity. Liver binder IB has an S value of 5–6 and a Stokes radius of about 26 Å producing a calculated range of molecular weight from 40,000 to 50,000 with frictional coefficient and axial ratio close to spherical values. As expected of a steroid receptor, IB, like II, binds to DNA and to liver cell nuclei but IB binds more tightly as evidenced by the fact that KCl is more effective in eluting II than IB from nuclei. Because recovery of bound radioactivity from acceptors is sometimes difficult to achieve, indirect experiments have been used frequently to determine the binding. Pyridoxal phosphate extracts liver IB and II equally from nuclei but spermidine is ineffective. While IB and II can be extracted partially from nuclei by pancreatic DNase I, more binder II is extracted by this method than IB. Micrococcal nuclease is poorly effective in either case. Binder II is extracted to a greater degree from DNA-cellulose than is IB by spermidine, MgCl₂, pyridoxal phosphate, and NaCl. IB binds more extensively to homodeoxypolymers than II. The extent of binding of liver IB to homodeoxypolymers is in the order: poly(dC) ≥ poly(dG) > poly(dA) ? poly(dT), whereas the order for liver binder II is: poly(dG) ≥ poly(dT) > poly(dC) ? poly(dA). Binders IB and II may be separate gene products or IB may arise in the cell from post-translational action. In the latter case, the activity of a protease cannot be ruled out.