Dopamine D1 receptors of the calf parathyroid gland: identification of a ligand binding subunit with lower apparent molecular weight but similar primary structure to neuronal D1-receptors.

The ligand binding subunit of the calf parathyroid D1 dopamine receptor was visualized by autoradiography following photoaffinity labeling with (+/-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4'-azidophenyl)-2,3,4,5- tetrahydro-1H-benzazepine ([125I]IMAB) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein comprising the D1 binding subunit migrated with an apparent Mr approximately equal to 62,000. Photoincorporation of [125I]IMAB into the Mr approximately equal to 62,000 polypeptide required the presence of protease inhibitors and was stereoselectively antagonized by dopaminergic agonists and antagonists with an appropriate pharmacological specificity for D1 receptors. The electrophoretic mobility of the [125I]IMAB-labeled receptor was not altered by the absence or presence of urea or thiol-reducing/oxidizing reagents. The Mr approximately equal to 62,000 protein representing the ligand binding subunit of bovine parathyroid D1 receptors corresponds to one of three D1 receptor binding subunits (Mr = 74,000, 62,000, and 51,000) identified in bovine brain. Peptide map comparisons of radiolabeled D1 receptors from calf parathyroid and brain following limited proteolytic digestion with Staphylococcus aureus V8 and papain revealed marked structural similarities. These data suggest that, despite tissue-specific differences in overall molecular weight, both parathyroid and neuronal D1 dopamine binding subunits appear to be pharmacologically and structurally homologous and that the molecular mechanism(s) responsible for the apparent lack of a one to one correspondence in the subunit composition of the D1 receptor in these tissues probably reflect(s) tissue-specific posttranslational modifications.