Evidence for involvement of tyrosine in estradiol binding by rat uterus estrogen receptor

The possibility of tyrosine involvement in steroid binding by rat uterus estrogen receptor (rER) was investigated. Chemical modification of rER with reagents such as tetranitromethane (TNM) and N-acetylimidazole (NAcI) inhibited estradiol binding. Steroid binding was inhibited to a greater extent at pH 8 than at pH 6, indicating the participation of tyrosine (TNM has increasing affinity for cysteine ove tyrosine at pH 6). Inhibition patterns remained similar for incubations at 0-4 degrees C or 37 degrees C. NAcI inhibited rER steroid binding at 37 degrees C, but not at 0-4 degrees C. Hydroxylamine incubated in the presence of rER and NAcI appeared to reverse this inhibition. Thus, these findings indicate that the phenyl ring and possibly the phenolic hydroxyl of tyrosine are involved in steroid binding of the receptor.