Peptide-receptor protein relationships: steroid feedback in GnRH stimulation of the anterior pituitary

Gonadotropin releasing hormone receptor protein (GnRH.RP) purified to homogeneity has several binding sites for its effector hormone, as evidenced from the concentration dependence binding curve. Binding of a fully biologically active iodinated GnRH decreased (P less than 0.001) by pretreatment of immobilized receptor protein with physiological concentrations of both main sex steroids, testosterone and estradiol 17-beta. With increasing concentrations of the steroids the binding of 125I-GnRH decreased biphasically for testosterone and multiphasically for estradiol 17-beta. The findings are conducive to an allosteric receptor model.