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Endogenous opioid peptides, their precursors, and receptor affinity.
| Precursor | Endogenous Opioid Peptide | Relative Opioid Receptor Affinity |
|---|---|---|
| Proenkephalin (PENK) | [Met]-enkephalin [Leu]-enkephalin |
µ, δ (δ >> µ) |
| Proopiomelanocortin (POMC) | β-endorphin | µ and δ (δ = µ) |
| Prodynorphin (PDYN) | Dynorphin A Dynorphin A (1–8) Dynorphin B α-neoendorphin β-neoendorphin |
κ, µ, δ (κ >> µ, δ) |
| Prepronociceptin (PNOC) | Nociceptin/Orphanin FQ | ORL-1 |
| Unknown | Endomorphin-1 Endomorphin-2 |
µ |
