Mn2+ does not uncouple adenosine "Ra" receptors from the liver adenylate cyclase

Four analogs of oxymorphone, oxymorphaminoethylthiol, oxymorphamino-ethyldisulfide, oxymorphaminoethyl-nitrobenzoic acid disulfide and oxymorphone thiazolidine, as well as the enkephalin analogs, enkephalin-thiol, Tyr-D-Ala-Gly-Phe-Leu-Lys(?-NH)COCH₂CH₂SH and the enkephalin-dimer, [Tyr-D-Ala-Gly-Phe-Leu-Lys(?-NH)COCH₂CH₂S-]₂, were examined for binding to enkephalin and morphine receptors. The analogs gained substantial affinity for enkephalin and lost affinity for morphine receptors. The affinity of the dimers of both opiates and enkephalins was slightly greater than that achieved by the corresponding thiol monomers. However, in the guinea pig ileum the dimeric analogs were much more active than the monomers. Receptor dimerization or cross-linking may be involved in the biological activity of opiates and opioid peptides.