Identity of Corticosteroid Binder I with the M acro molecule Binding 3-Methylcholanthrene in Liver Cytosol in Vivo1

forms of radioactivity is apparent when the corticosteroid binder is purified to homogeneity by a series of column Chromatographie fractionation procedures. The binder for 3-methylcholanthrene also has a molecular weight identical to that of the corticosteroid binder as determined by calibration on Sephadex G-75 columns and by sedimentation velocity experiments in the analytical ultracentrifuge. Furthermore, isoelectrofocusing assigns to the homogeneous 3-methylcholanthrene binder a pH, of 8.65 to 8.83 which is the same range as for the corticosteroid binder. Although cortisol radioactivity does not remain bound after the electrofocusing step, aligning with the behavior of Binder I, the 3-methylcholanthrene binder after electrofocusing can be shown to rebind corticosterone to form a complex which has the anticipated molecular weight.