Evidence for testosterone binding macromolecule in human placental cytosol

Specific binding of [3H]-testosterone was found in the human placental cytosol. At a ['HI-testosterone concentration of 3 x IO-* M, the binding was maximal after 1 h and remained at that level for at least 24 h. [3H]-testosterone binding was linearly related to the amount of cytosol protein concentration up to 4.0 mg/ml. Kinetic analysis revealed the presence of specific binding sites with a KD of 11.7 x 10m9 M and a concentration of binding sites of 306fmol/mg protein. The [3H]-testosterone macromolecule complex exhibited a sedimentation coefficient of 4.6 S under low and high salt conditions. The 4.6 S radioactivity peak disappeared after treatment at 45°C for I h and was inhibited or suppressed by a IO or 1000 fold excess of cold testosterone. In competitive binding studies cold testosterone, Sa-dihydrotestosterone, androstenedione and dehydroepiandrosterone were the most active competitors. The fact that estradiol also competes for [3H]-testosterone binding, but binds to this cytoplasmic component only to a small extent, indicates that the macromolecule studied is not SHBG. The possible role of this macromolecule as an androgen receptor in the human placenta is suggested.