Opioid activities of fragments of beta-endorphin and of its leucine65-analogue. Comparison of the binding properties of methionine- and leucine-enkephalin.

For characterisation in vitro, four parallel assays were used: the guinea-pig ileum and mouse vas deferens as pharmacological models at 36 degrees C and the inhibition of binding of [3H]-naltrexone, [3H]-leucine-enkephalin and [3H]-methione-enkephalin at 0 degrees C. The Leu65-analogue of beta-andorphin and its fragments (61-65, 61-76 and 61-77) have a lower affinity to the [3H]-naltrexone binding site of mu-receptors than the corresponding Met65-peptides wereheas no such difference was found for the [3H]leucine-enkephalin binding sites or delta-receptors. When the binding of [3H]-methionine-enkephalin or [3H]-leucine-enkephalin was inhibited by cold ligands interacting with delta-, mu-, or kappa-receptors, no evidence was obtained for more than one type of delta-binding site.