Opioid peptides oropiate peptides arepeptides that bind toopioid receptors in the brain;opiates andopioids mimic the effect of these peptides. Such peptides may be produced by the body itself, for exampleendorphins. The effects of these peptides vary, but they all resemble those of opiates. Brain opioid peptide systems are known to play an important role inmotivation,emotion,attachment behaviour, the response tostress andpain,control of food intake, and the rewarding effects ofalcohol andnicotine.

Opioid-like peptides may also be absorbed from partiallydigestedfood (casomorphins,exorphins, andrubiscolins).Opioid peptides from food typically have lengths between 4–8amino acids. Endogenous opioids are generally much longer.

Opioid peptides are released bypost-translationalproteolytic cleavage ofprecursor proteins. The precursors consist of the following components: asignal sequence that precedes aconserved region of about 50 residues; a variable-length region; and the sequence of theneuropeptides themselves.Sequence analysis reveals that theconservedN-terminal region of the precursors contains 6cysteines, which are probably involved indisulfide bond formation. It is speculated that this region might be important for neuropeptide processing.^[1]

The humangenome contains severalhomologousgenes that are known tocode for endogenous opioid peptides.

• The nucleotide sequence of the human gene forproopiomelanocortin (POMC) was characterized in 1980.^[2] The POMC gene codes for endogenous opioids such asβ-endorphin andγ-endorphin.^[3]
• The human gene for theenkephalins was isolated and its sequence described in 1982.^[4]
• The human gene fordynorphins (originally called the "Enkephalin B" gene because of sequence similarity to the enkephalin gene) was isolated and its sequence described in 1983.^[5]
• The PNOC gene encodingprepronociceptin, which is cleaved intonociceptin and potentially two additional neuropeptides.^[1]
• Adrenorphin,amidorphin, andleumorphin were discovered in the 1980s.
• Theendomorphins were discovered in the 1990s.
• Opiorphin andspinorphin,enkephalinase inhibitors (i.e., prevent the metabolism of enkephalins).
• Hemorphins,hemoglobin-derived opioid peptides, includinghemorphin-4,valorphin, andspinorphin, among others.

While not peptides,codeine andmorphine are also produced in the human body.^[6][7]

Exogenous opioid substances are calledexorphins, as opposed toendorphins. Exorphins includeopioid food peptides, such asgluten exorphin andopioid food peptides, and are often contained in cereals and animal milk. Exorphins mimic the actions of endorphins by binding to and activatingopioid receptors in the brain.

Common exorphins include:

• Casomorphin (fromcasein found in milk of mammals, including cows and humans)
• Gluten exorphin (fromgluten found incereals wheat, rye, barley), including:
  • Gliadorphin/gluteomorphin
• Soymorphin-5 (fromsoybean)
• Rubiscolin (fromspinach)

• Deltorphin
• Deltorphin I
• Deltorphin II
• Dermorphin

• Zyklophin – semisynthetic KOR antagonist derived fromdynorphin A

• Opioid+Peptides at the U.S. National Library of MedicineMedical Subject Headings (MeSH)

• Protein families
• Opioid peptides

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