Partial agonism in the glucagon receptor system is a consequence of the two-state rat hepatic receptor

We have previously demonstrated that the glucagon receptor binds hormone to form a low affinity complex which, by a time- and temperature-dependent mechanism, is converted to a high affinity complex (Horwitz, E.M., Jenkins, W.T., Hoosein, N.M., and Gurd, R.S. (1985) J. Biol. Chem. 260, 9307-9315). In this report we have investigated the effects of agonist concentration, potency, and intrinsic activity on the characteristics of the two, interconvertible states of the glucagon receptor. As the glucagon concentration is increased from 0.02 to 0.50 nM, the initial velocity of binding increases. The conversion of a low affinity to a high affinity complex is the rate-limiting step in the overall binding reaction and approaches its maximal velocity as the hormone concentration exceeds 0.20 nM. At equilibrium, 87-90% of the hormone-receptor complexes are in the high affinity state at all hormone concentrations examined. [S-methyl-Met27]glucagon, a full agonist with reduced potency, binds to the two-state system in a manner analogous to that of native glucagon. The binding of N alpha-biotinyl-N epsilon-acetimidoglucagon, a partial agonist with reduced potency, effects a two-state system where the high affinity state accounts for only 35% of the total hormone-receptor complexes at equilibrium. We conclude that the formation of the high affinity complex is the rate-limiting step involved in glucagon binding; reduction in binding potency with full agonism is due to a reduction in the affinity of the ligand for the unoccupied receptor and not to an alteration of the interconversion of the two states, and decreased intrinsic activity is due to a quantitative decrease in conversion of the low to high affinity state.