Differential Binding Affinities of Rat Testis Luteinizing Hormone (LH) Receptors for Human Chorionic Gonadotropin, Human LH, and Ovine LH

Specific binding of hCG, human LH (hLH), and ovine LH (oLH) to LH receptors was analyzed in rat testis homogenate. Equilibration studies with each of the gonadotropins indicated that the three hormones bind to a common receptor with similar binding capacity for each hormone. Scatchard analysis of the binding data revealed a single set of binding sites for hCG and hLH, with an equilibrium association constant (Ka) of 7.6 × 1010 M-1 for hCG and 2.5 × 1010 M-1 for hLH. In contrast, two sets of binding sites were found for oLH, about 20% with a mean Ka of 7.0 × 1010 M-1 and the remainder with a Ka of 0.08 × 1010 M-1. Kinetic binding experiments gave similar association rate constants (k1) of 3.4 × 108, 4.0 × 108 and 4.0 × 108 M-1 min-1 for hCG, hLH, and oLH, respectively. In dissociation experiments, the half-times of bound hCG, hLH, and oLH were 25, 9.2, and 2.1 h, respectively, and two dissociation components were seen with each of the three hormones. The rate constants of the fast components were 0.19, 0.1...