Mechanism of adenylate cyclase activation through the beta-adrenergic receptor: catecholamine-induced displacement of bound GDP by GTP.

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Abstract
The fate of the guanyl nucleotide bound to the regulatory site of adenylate cyclase was studied on a preparation of turkey erythrocyte membranes that was incubated with [3H]GTP plus isoproterenol and subsequently washed to remove hormone and free guanyl nucleotide. Further incubation of this preparation with .beta.-adrenergic agonists resulted in the membrane release of tritiated nucleotide, identified as [3H]GDP. The catecholamine-induced release of [3H]GDP was increased 2-3 times with the unlabeled guanyl nucleotides GTP, guanosine 5''-(.beta.,.gamma.-imino)triphosphate [Gpp(NH)p], GDP and GMP, whereas adenine nucleotides had little effect. With Gpp(NH)p, isoproterenol induced the release of [3H]GDP and the activation of adenylate cyclase, both effects following similar time courses. The inactive adenylate cyclase possesses tightly bound GDP, produced by the hydrolysis of GTP at the regulatory site. The hormone stimulates adenylate cyclase activity by inducing an opening of the guanyl nucleotide site, resulting in dissociation of the bound GDP and binding of the activating GTP.