Demonstration and Partial Characterization of a Rabbit Serum Protein Which Binds Testosterone and Dihydrotestosterone

A protein in the serum of the adult male rabbit which binds testosterone and dihydrotestosterone is demonstrated. It is salted out by 50% ammonium sulfate and has a greater R_f than the analogous human protein at pH 9.5 in polyacrylarride gel electrophoresis. It can be separated from rabbit corticosteroid-binding globulin on Sephadex G200 and polyacrylamide gel electrophoresis. The concentration of specific binding sites in pooled serum is 1.8 X 10-⁷M and the association constant for dihydrotestosterone at 4C is 1.4 X l-⁹M-¹. A number of steroids were examined for their ability to compete with testosterone for binding sites. In contrast to the human protein, estradiol was an extremely poor competitor. (Endocrinology92: 1700, 1973)