The presence of an estradiol binding component in cytosol from immature rat epididymides

Cytosol prepared from epididymides of sexually immature (21-23-day-old) rats contains a macromolecular binding component for estradiol-17 beta. This binding moiety sediments as an 8-S species on 5-20% sucrose gradients containing 0.01 M KCl. Under conditions of high ionic strength (0.4 M KCl) the 8-S peak of estradiol binding is shifted to the 4-S region, suggesting dissociation of receptor aggregates. Time-course studies indicated that binding equilibrium was essentially achieved after 2 hours incubation at 0 degrees C. Although unlabeled estrone and estriol are capable of inhibiting [3H]estradiol binding to epididymal cytosol, they are less effective than unlabeled estradiol. Unlabeled 5 alpha-dihydrotestosterone (5 alpha-DHT) at a 100-fold molar excess did not cause a statistically significant inhibition of [3H]estradiol binding. Unlabeled estrogens, but not unlabeled 5 alpha-DHT or cortisol (at the concentrations used), were capable of displacint [3H]estradiol from its binding sites. The dissociation of [3H]estradiol from the binding component is very slow, with half-time of dissociation being greater than 16 hours. The epididymal estrogen binder is saturable at low concentrations of ligand. The dissociation constant was of the order of 10(-11)M and the concentration of binding sites was approximately 10(-14) mol/mg protein. This estrogen binder has the characteristics which are usually attributed to steroid receptors and is clearly different from the testicular androgen-binding protein and the epididymal androgen receptor.