Two synthetic analogues of bovine parathyroid hormone (PTH) with NH2-terminal modifications, PTH-(3-34) and [desamino-Ala-1]PTH-(1-34), were found to lack agonist activity but to demonstrate antagonist properties when tested in the rat renal cortical adenylyl cyclase assay in vitro against the native hormone or against PTH-(1-34), the active synthetic NH2-terminal tetratriacontapeptide. The inhibition exhibited by these analogues was proportional in degree to the dose of inhibitor, abolished by oxidation of the analogue, reversible by addition of an excess of active hormone, and specific for parathyroid hormone-stimulated renal adenylyl cyclase. No inhibition of basal or sodium fluoride-stimulated renal adenylyl cyclase could be demonstrated. Two other synthetic bovine analogues, PTH-(13-34) and PTH-(1-26), were devoid of agonist and antagonist properties. The over-all results suggest that the requirements for receptor binding of parathyroid hormone are rather broad. Conformational factors or binding interactions involving specific residues, or both seem to require the entire sequence from residue 3 to residue 27 for receptor binding to occur. A dichotomy between receptor binding and adenylyl cyclase activation was demonstrated only by alterations or deletions involving the first 2 NH2-terminal residues of the hormone and emphasizes the importance of these residues in eliciting the biological activity of parathyroid hormone. The two antagonists, [desamino-Ala-1]PTH-(1-34) and PTH-(3-34), should be useful in further analysis of the initial steps in hormone action.
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