Studies on the androgen receptor of the rat seminal vesicle: comparison of the binding characteristics of dihydrotestosterone and testosterone.

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The postmitochondrial supernatant (PMS) prepared from homogenized seminal vesicles of castrated rats contains the same number of specific binding sites (105–120 fmol/mg protein) for testosterone and dihydrotestosterone (DHT). The identical rate of inactivation of these binding sites in the absence of ligand and the identical velocity of sedimentation of the bound steroids through sucrose density gradients suggest that they bind to the same androgen receptor. Furthermore, competition studies favor the view that the same binding site of the receptor is occupied by these androgens. The affinity of testosterone to the receptor is lower than that of DHT. Our results indicate that a higher dissociation rate of the testosterone-receptor complex is primarily responsible for this difference in affinities. In contrast with the association reaction which attains an apparent equilibrium within 60 min, the dissociation of these steroid-receptor complexes is a relatively long process with half times of several hours. Moreover, with respect to the androgenicity of a steroid, the rate of dissociation from the receptor may be decisive, since only the results of those competition experiments in which long incubation times (days) have been employed reflect the relative androgenic potency of various steroids. Using a reconstituted in vitro system a PMS dependent and tissue specific uptake of testosterone by vesicular nuclei has been demonstrated. Under identical conditions about half as much testosterone as DHT is translocated into the nuclei. On the basis of this finding a direct role of testosterone-receptor complex in the regulation of nuclear functions is postulated.