Evidence for an androgen receptor in human testis

The present study identified and characterized an androgen-binding protein in human testicular tissue. Human testes were homogenized in dilute Tris buffer containing thioglycerol, phenylmethylsulfonylfluoride, and molybdate. The supernatant (termed cytosol) was incubated with radiolabeled androgen methyltrienolone (tritium-labeled R1881), and nonspecific binding was determined by adding 100-fold excess of unlabeled R1881 together with [3H]R1881 to cytosol. Specific binding with saturation at 24 hours was observed. Scatchard analysis of the specific binding with the use of increasing concentrations of [3H]R1881 alone and [3H]R1881 plus 200-fold excess unlabeled R1881 demonstrated a high-affinity (dissociation constant = 2.18 X 10(-10) mol/L), low-capacity (2924 molecules per cell) class of binding sites. A second class of lower-affinity sites was identified with a dissociation constant equaling 1.2 X 10(-8) and 26,300 molecules per cell. The bulk of the higher-affinity class of sites was precipitated at 35% ammonium sulfate. In competitive binding assays, dihydrotestosterone and testosterone greatly diminished binding to this high-affinity class of sites. Progesterone also diminished binding but to a lesser degree. Estradiol, estriol, and estrone failed to compete for these sites. Analysis of the receptor, using sucrose gradients, revealed a major peak in the 4S region and a small peak at 8S. A similar high-affinity (dissociation constant = 4.28 X 10(-9), low-capacity (4860 molecules per cell) binding protein was identified in purified nuclei. Binding to nuclear chromatin was demonstrated in the cell-free binding assay, and nuclear binding was further illustrated in the biopsy assay of intact tissue, suggesting translocation in vivo. These properties are characteristic of the androgen receptor and suggest that human testis is a target tissue for androgen, as has been found in animal tissue.