Activation of adenylate cyclase by beta-adrenergic receptors: investigation of rate limiting steps by simultaneous assay of high affinity agonist binding and GDP release.

We report the development and application of a novel assay for high affinity binding of the agonist [3H]hydrozybenzyl-isoproterenol simultaneously with the agonist-promoted release of membrane bound [32P]GDP in the frog erythrocyte beta-adrenergic receptor system. We find that under various assay conditions both events occur with the same rate, ranging from 0.05 to 0.5 min-1. Addition of the non-hydrolyzable guanine nucleotide, guanylyl-imidodiphosphate simultaneously increases the rate of high affinity agonist binding and agonist promoted GDP release. In addition, the guanine nucleotide analog decreases the steady state level of high affinity agonist binding and increases the steady state level of agonist promoted GDP release with comparable potencies of 0.5 microM and 0.1 microM, respectively. The decrement in the steady state level of high affinity agonist binding (180 fmol/mg protein) due to the guanine nucleotide analog is in the same range as the reciprocal increment in the extent of agonist-induced [32P]GDP release (180 fmol/mg protein). The concommittant activation of adenylate cyclase, by submaximal concentrations of the agonist [3H]hydroxybenzylisoproterenol and guanylylimido-diphosphate under similar assay conditions proceeds with the same rate as for the two other measured functions of the system, i.e. 3H-agonist binding and agonist-promoted [32P]GDP release. This represents the first attempt at comparing the time course of adenylate cyclase activation with that of agonist binding and GDP release under similar assay conditions. The results indicate that GDP is not released prior to but rather coincident with formation of the complex of the hormone receptor with the regulatory protein and that enzyme activation proceeds with the same time course as agonist binds to the receptor. It is concluded that both high affinity agonist binding and GDP release represent integral aspects of the rate limiting step in the enzyme activation mechanism.