Aprotinin Antagonizes the Binding of Human Chorionic Gonadotropih to Its Receptor

Aprotinin caused a dose-dependent inhibition of [125I]hCG binding to its receptor in plasma membranes prepared from luteinized rat ovaries. Displacement of [125I]hCG from its receptor by aprotinin was temperature dependent, with an ID66 of 300 μM at 4 C and an ID50 of 3700 μM at 22 C. Equilibrium binding data showed a decrease in the Ka for hCG in the presence of increasing concentrations of aprotinin; aprotinin had no effect on the number of binding sites. The rate of association of hCG with its receptor was markedly reduced in the presence of aprotinin, but aprotinin had little effect on the rate of dissociation. Control experiments established that aprotinin did not inhibit the binding of [125I]hCG to its receptor due to its lectin properties, an ionic effect, or some irreversible impairment of the receptor or hormone. Aprotinin did not inhibit hCG binding when the receptor was solubilized out of ovarian membranes with Triton X-100. Aprotinin inhibited the hCG-mediated activation of adenylate cyclase i...