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

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.