Hormone-induced guanyl nucleotide binding and activation of adenylate cyclase in the Leydig cell

The adenylate cyclase activity of Leydig cell homogenates and membrane fractions is highly dependent on guanyl nucleotides, and enzyme responses to luteinizing hormone or human chorionic gonadotropin are small in the absence of guanyl nucleotides. However, in the presence of 10 microM guanosine 5'-[beta, gamma-imido]triphosphate Gpp[NH]p, both hormones consistently stimulated testicular adenylate cyclase activity by up to 200%. Leydig cell membranes bound [3H]Gpp[NH]p at 30 degrees C with high affinity (Ka = 1.5 X 10(7) M-1) and binding capacity of 60 pmol/mg of protein. During kinetic studies, the association rate constant was 1.7 X 10(6) M-1 min-1, and the dissociation constant was 0.038 min-1. In the presence of gonadotropin (10 pM to 10 nM), concentration-dependent increases of 40% to 100% in Gpp[NH]p binding were observed in Leydig cell membranes. Kinetic studies showed that gonadotropin decreased the association rate constant to 0.73 X 10(6) M-1 min-1 and the dissociation rate constant to 0.017 min-1, with no effect on the equilibrium binding constant. Thus, the increase in Gpp[NH]p binding was not due to a change in receptor affinity but was attributable to increased availability of nucleotide binding sites. The 50% effective dose for adenylate cyclase activation by gonadotropin in the presence of Gpp[NH]p was identical with that observed for gonadotropin-induced binding of the GTP analog (50 nM). Gonadotropin-induced binding of Gpp[NH]p in Leydig cell membranes may represent interaction with the guanyl nucleotide regulatory site during hormonal activation of adenylate cyclase.