The Effect of Metallic Ions on Steroid Hormone Receptors in the Rabbit Uterus

Abstract

The copper-IUD has been proved to be more effective for contraception than the popular IUD. To understand this mechanism of action, the effects of metallic ions on estrogen and progesterone receptors were examined in the cytosol of estrogen primed white female rabbits. The 274,200×G supernatant of the uterine homogenate was used as the cytosol.The cytosol and 3H-progesterone (3. 12×10-9M) or 3H-estradiol-17β (3.46 x 10-9M), with or without various metallic ions of different concentrations had been incubated for 2 hours. Bindings of steroids were estimated by the dextran coated charcoal assay (0. 001% dextran and 0.1% Norite A) and were evaluated by 5-20% sucrose linear gradient centrifugation.1) Effect of various metallic ions on steroid hormone-receptor binding (determined by dextran coated charcoal assay. Fig. 1) : The steroid hormone-receptor interactions were markedly inhibited by Cu++, Fe++, and Zn++ ions and moderately by Mn++, but K+ and Ca++ increased or slightly affected the binding at concentrations between 10-2M and 10-4M. There were some differences between estrogen and progesterone receptors in their sensitivities to various metallic ions.2) The effect of copper ion on the binding of steroid to the receptor : The dextran coated charcoal assay (Fig. 2) demonstrated that the inhibitory effect of Cu++ appeared at 10-6M and the steroidhormone-receptor bound decreased down to 10% at 10-2M Cu++. The estrogen receptor was less affected by copper than the progesterone receptor.It was demonstrated by 5-20% sucrose linear gradient centrifugation (Fig. 3) that estrogen and progesterone receptors, which both sedimented at 8 S, were changed to more sedimenting forms in the presence of 10-4M Cu++, and were dissociated to 6. 5 S form with moderate loss of steroid hormone binding affinity in 10-2M Cu++.The kinetic study (Fig. 4), determined by dextran coated charcoal, showed that Cu++ was a competitive inhibitor against steroid hormone receptor bindings with Ki≅2.7.×10-5M to estrogen receptor (Kd≅1.4 × 10-9M), and with Ki≅5.1 × 10-6M to progesterone receptor (Kd≅8.1 × 10-10M).These results indicate the inhibiting factor of copper is the direct interference at the steroid binding site of the receptor, resulting in the increase in the effectiveness of the IUD.While progesterone receptor is more affected by copper ion than estrogen receptor, it is suggested that estrogen receptor survives longer than progesterone receptor and thus the biological effect of copper seems to be somewhat estrogenic.