Characterization of the prolactin receptor in human prostate

The uptake of 125I-labelled human prolactin by subcellular fractions obtained from the human hyperplastic prostate was investigated and the specific binding sites were characterized. The binding was both time- and temperature-dependent with maximum specific uptake achieved after incubation for 20 h at 4 degrees C (dissociation constant = 2.4 x 10(-10) mol/l). The present studies also indicate that the bulk of the specific prolactin binding was confined to the 105 000 and 15 000 g membrane fractions whilst the cytosol and nuclear pellet exhibited a lower capacity for the peptide hormone. Attempts to optimize the binding revealed that pretreatment of the subcellular fractions with 4 mol MgCl2/l quadrupled the binding sites; a similar effect was produced after pretreatment with dextran-coated charcoal, but the changes were less pronounced. Furthermore, our studies suggest that the binding was specific for the human prolactin, with other hormones such as ovine prolactin, human LH, human FSH and human GH showing little or no competition. The addition of magnesium and copper ions to the incubation medium also markedly increased the specific binding, whereas calcium, manganese and EDTA inhibited the prolactin uptake. Freezing and storage of tissue at -70 degrees C did not greatly affect the binding sites. The results suggest that we are clearly dealing with a specific prolactin-binding protein.