3H-Leu5-enkephalin specific binding to synaptic membrane. Comparison with 3H-dihydromorphine and 3H-naloxone

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Specific binding of 3H-Leu5 -enkephalin to synaptic membrane in the presence of a protease inhibitor, bacitracin, was observed to be affected differently than that of 3H-dihydromorphine and of 3H-naloxone at varying physical conditions. The association and disassociation rates and the dissociation constants of the enkephalin were affected to a larger degree than those of the opiates when the incubation temperature was altered from 0° to 37°C. Although 100 mM Na+ inhibited 3H-Leu5 -enkephalin binding, the magnitude of the sodium inhibition was increased when the incubation temperature was raised from 25° to 37°C. When the membrane was treated with N-ethylmaleimide or 3,5-diiodo-4-diazosulfanilic acid, selective inhibition of enkephalin, dihydromorphine and naloxone binding was observed. Furthermore, the affinities of various opioid peptides for 3H- Leu5 -enkephalin were observed to be different than those for 3H-dihydromorphine. In general, all alkaloids have higher affinity for 3H-dihydromorphine binding sites than 3H- Leu5 -enkephalin binding sites. The converse was the case with opioid peptides with the exception of β-endorphin. Thus it is concluded that 3H- Leu5 -enkephalin's interaction with the opiate receptor is distinctly different from those of the narcotic analgesics and so offers another possible explanation for the low in vivo analgesic potency displayed by the enkephalins.

Original languageEnglish (US)
Pages (from-to)409-434
Number of pages26
JournalResearch Communications in Chemical Pathology and Pharmacology
Issue number3
StatePublished - Jan 1 1978


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