Use of 125I-Tyr27 beta-endorphin for the study of beta-endorphin binding sites in rat cortex.

An iodine-labelled derivative of beta-endorphin, 125I-Tyr27 beta h-endorphin, was used in carrier-free form to study the binding of beta-endorphin to brain opioid receptors. The experiments were carried out with rat cortex membranes in vitro under conditions that gave a high degree of naloxone reversible binding. Beta h-Endorphin and nonradioactive iodo-Tyr27 beta h-endorphin were found to be identical in their ability to inhibit the binding of 125I-Tyr27 beta h-endorphin. Competition experiments demonstrated the existence of binding sites with higher affinity for beta-endorphin than for a variety of other opioids, including naturally occurring fragments of beta-endorphin. The experiments show that 125I-Tyr27 beta-endorphin possesses similar binding properties to the unmodified peptide and can be used with the advantages of iodine-125 as an isotope for the investigation of beta-endorphin receptors in brain. In experiments employing 125I-Tyr27 beta-endorphin 1-27 as the radioiodinated ligand, binding curves were obtained which showed that beta-endorphin 1-31 was more potent than beta-endorphin 1-27 in inhibiting the binding of the labelled 27 residue peptide. With both the 27 and 31 residue radioligands, magnesium ion enhanced the specific binding whereas sodium ion and guanylyl-imidodiphosphate had a strong inhibitory effect. The data indicate that beta-endorphin 1-27 bindings with reduced affinity to the same receptors as beta-endorphin 1-31 and like the 31 residue peptide exhibits properties characteristic of an agonist.