Correlation of hydrophobic character of opioid peptides with their biological activity measured in various bioassay systems

Relative hydrophobicity of a series of enkephalin-like peptides has been studied by partitioning in the aqueous polymeric Ficoll-dextran biphasic system. The activities of the peptides in the guinea pig ileum, mouse vas deferens and rat brain receptor binding assay systems have been examined. The biological activities of the peptides in the two latter assays were correlated with their hydrophobic character. No correlation between the relative hydrophobicity of the compounds and their activity in the guinea pig ileum could be established. The correlations obtained appear to indicate that the drug-receptor interactions in the two bioassays occur under various conditions in regard to the local ionic composition at the membrane and/or the ionizable state of the receptor site. The activities of morphine and a number of similar drugs are described by a relationship different from that found for the peptides, which seems to support the multiple opiate receptor hypothesis.