[125I] Tyr27 beta-endorphin: a radio-iodinated derivative of beta-endorphin for the study of opiate receptors.

Evidence is presented that a new derivative of beta-endorphin, [125I] Tyr27 beta-endorphin, is a suitable ligand for the study of beta-endorphin binding sites in rat brain. The results obtained with this homogeneous mono-iodinated peptide demonstrated high affinity agonist binding sensitive to the presence of sodium and magnesium ion and to guanine nucleotide. Competition experiments using a series of opiates and opioid peptides including shorter forms of beta-endorphin revealed a range of potencies; beta-endorphin 1-31 exhibited the highest affinity. The findings suggest that binding sites that complement the structure of beta-endorphin are present in rat cortex.