Identification of the D2-dopamine receptor binding subunit in mammalian tissues
Photoaffinity labeling of the D2-dopamine receptor in plasma membrane preparations of various tissues from several mammalian species has been performed using the recently developed D2-dopaminergic antagonist probe [125I]N-(p-azidophenethyl)spiperone ([125I]N3-NAPS). In tissues known to contain D2-receptors such as the corpus striatum from rat, dog, calf, hamster, guinea pig, bovine, and rabbit as well as the anterior pituitary of bovine and rat, the probe covalently labels a peptide of M(r) = 94,000. Specificity of the labeling is typically D2-dopaminergic. The covalent labeling is blocked by (-)butaclamol but not by the inactive (+)isomer. Agonists block incorporation with the potency N-propylnorapomorphine > apomorphine > dopamine. The D2-selective antagonist spiperone blocks labeling of the M(r) = 94,000 peptide whereas the D1-selective antagonist SCH-23390 is inactive. Under conditions where proteolysis is not stringently controlled, peptides of lower M(r) (30,-40,000) are labeled at the expense of the M(r) = 94,000 peptide. In the rat neurointermediate lobe, a tissue containing D2-receptors, [125I]N3-NAPS specifically labels in addition to the M(r) = 94,000 peptide, a peptide of M(r) ≃ 120,000. This peptide may represent an unprocessed precursor form of the receptor in this tissue. Alternatively, the M(r) = 94,000 peptide labeled in all the other tissues, even under the conditions used, may already represent a fragment of a high M(r) peptide.
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