Molecular comparison of alpha 1- and alpha 2-adrenergic receptors suggests that these proteins are structurally related "isoreceptors".

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Abstract
The structures of human platelet .alpha.2-adrenergic receptors and rat liver .alpha.1-adrenergic receptors were compared by using isoelectric focusing, NaDodSO4/PAGE [sodium dodecyl sulfate/polyacrylamide gel electrophoresis] and monoclonal antibody crossreactivity. Digitonin-solubilized .alpha.1- and .alpha.2-adrenergic receptors have an identical isoelectric point of 4.6. Under reducing conditions in NaDodSO4/polyacrylamdie gels, the .alpha.1-adrenergic receptor has an apparent MW of 85 [kilodalton]. The .alpha.2-adrenergic receptor, which had been affinity-labeled with [3H]phenoxybenzamine and partially purified by isoelectric focusing or photoaffinity-labled with .pi.-[3,5-3H]azidoclonidine, was also found to have an apparent MW of 85 kDa. One hybridoma, developed from a fusion between SP2/O myeloma cells and splenic lymphocytes from BALB/c mice immunized with human platelet .alpha.2-adrenergic receptors, secreted a monoclonal antibody (.alpha.2-116p) against the ligand binding site of .alpha.2-adrenergic but not .alpha.1-adrenergic receptors. Three monoclonal antibodies raised against the .alpha.1-receptor polypeptide backbone but not the ligand binding site were found to specifically immunoprecipitate human platelet .alpha.2-adrenergic receptors are isoreceptors sharing immunogenic and, by implication, structural determinants that most likely evolved as a result of gene duplication.