Characterization of the cytoplasmic androgen receptor of rat seminal vesicle.

Postmitochondrial supernatant (PMS) (1) has been prepared from the homogenate of rat seminal vesicles and the characteristics of the binding reaction of 5alpha-dihydrotestosterone (DHT) to the cytoplasmic androgen receptor have been studied using a charcoal adsorption procedure. At 0 degrees C apparent equilibrium of binding is reached between 60 and 90 min of incubation but no exchange of bound (3H)DHT can be observed in the presence of a 100-fold excess of unlabelled DHT. Saturation analysis shows a single class of independent binding sites for DHT with an apparent dissociation constant of 1 nM at 0 degrees C and 2 nM at 25 degrees C. Concentration of binding sites is in the range of 25-80 fmoles/mg protein. When not occupied by DHT the receptor molecules are inactivated spontaneously following first order reaction kinetics. A rate constant of 0.27 hours-1 at 0 degrees C was determined for the inactivation reaction. In the (3H)DHT-binding reaction testosterone and 19-nortestosterone are even more efficient competitors than unlabelled DHT, while hydrocortisone does not compete at all. On the other hand significant binding of (3H) testosterone could not be demonstrated. The (3H)DHT-receptor complex is precipitated from the cytosol by 0 to 33% saturation of ammonium sulphate and sediments as a single, 3.1 S peak in sucrose gradients prepared in 0.4 M NaCl.