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Identity of Corticosteroid Binder I with the Macromolecule Binding 3-Methylcholanthrene in Liver Cytosol in Vivo¹

Fels Research Institute and Department of Biochemistry, Temple University School of Medicine, Philadelphia, Pennsylvania 19140

Four macromolecules in liver cytosol bind corticosteroids and their anionic metabolites. One of these, a basic protein, designated "corticosteroid Binder I" appears to be identical to the major protein that binds 3-methylcholanthrene. This conclusion is based on double label studies with 3-methylcholanthrene-¹⁴C and cortisol-³H. Alignment of both forms of radioactivity is apparent when the corticosteroid binder is purified to homogeneity by a series of column chromatographic 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_I 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.

¹ This work was supported by Research Grants GB-8784 from the National Science Foundation, AM-13531 and AM-08350 from the National Institute of Arthritis and Metabolic Diseases, and CA-10439 from the National Cancer Institute.

² To whom requests for reprints should be addressed.

Received 2/12/71. Accepted 5/17/71.