9S binding protein for androgens and progesterone.

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Abstract
A steroid binding protein fraction with a sedimentation coefficient of approximately 9 s (MW .simeq. 200,000) was identified in 105,000 .times. g supernatants of several [rat] androgen-responsive organs. Highest concentrations were found in epididymis and testis, but small amounts were detected in prostate, seminal vesicle, kidney, submandibular gland and lung. The 9S protein binds [3H]dihydrotestosterone (17.beta.-hydroxy-5.alpha.-androstan-3-one) and [3H]progesterone (4-pregnene-3,20-dione) with equilibrium binding constants of approximately 105 M-1 and 106 M-1, respectively. The concentration of 9S binding sites in epididymis is approximately 10-11 mol/mg of supernatant protein, which is at least 105 times greater than the concentration of androgen receptor. 9S binding protein appears to be a nonsecretory, intracellular protein and has properties different from the androgen receptor. It is unretarded on DEAE-Sephadex chromatography at pH 8.0, and its sedimentation rate on sucrose gradients is not altered at high ionic strength (0.4 M KCl). Like the androgen receptor, its binding activity, which is maximal between pH 7-9.5, is heat labile, decreased by sulfhydryl reagents, and enhanced by 2-mercaptoethanol. Because of its high concentration and low affinity, 9S binding protein may function in the intracellular accumulation or compartmentalization of androgens or progesterone.