Photoaffinity label for the alpha 1-adrenergic receptor: synthesis and effects on membrane and affinity-purified receptors.

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Abstract
An azide analog, 2-[4-(4-azidobenzoyl)piperazin-1-yl]-4-amino-6,7-dimethoxyquinazoline (CP59,430), of the highly selective .alpha.1-adrenergic receptor antagonist prazosin was synthesized and its effects on rat hepatic membrane and affinity-purified .alpha.1-adrenergic receptor preparations were examined. CP59,430 behaved as a competitive antagonist before photolysis. When the membrane or purified preparations pretreated with CP59,430 were irradiated with UV light, CP59,430 behaved as a noncompetitive antagonist. Labeling of membrane .alpha.1-adrenergic receptors was irreversible; repeated dialysis or washing could not reverse the photolysis-induced inactivation by CP59,430, whereas dialysis completely reversed the antagonism by the same concentration of the label prior to photolysis. Photolabeling of purified receptors was resistant to Sephadex G-50 chromatography, whereas in the absence of photolysis the same concentration of CP59,430 or prazosin (10 .mu.M) could be readily removed by this procedure. CP59,430 appears to label specifically only .alpha.1-adrenergic receptors because prazosin protected the membrane and purified receptors from photolysis-induced inactivation by CP59,430. Specific [3H]dihydroalprenolol and [3H]yohimbine binding to membrane .beta.- and .alpha.2-adrenergic receptors, respectively, was unchanged by CP59,430 at 1 .mu.M, a concentration that decreased specific [3H]prazosin binding to .alpha.1-adrenergic receptors by 72%. The photolysis-induced receptor inactivation by CP59,430 remained unchanged in the presence of the scavenger p-aminobenzoic acid. Receptor labeling by CP59,430 may occur via a true photoaffinity mechanism. CP59,430, which specifically and irreversibly labels the .alpha.1-adrenergic receptor after photolysis, should thus be uniquely valuable for the molecular characterization of this receptor.