Opioid peptide

Opioid peptides are short sequences of amino acids that bind to opioid receptors in the brain; opiates and opioids mimic the effect of these peptides. Opioid peptides may be produced by the body itself, for example endorphins. The effects of these peptides vary, but they all resemble opiates. Brain opioid peptide systems are known to play an important role in motivation, emotion, attachment behaviour, the response to stress and pain, and the control of food intake.

Opioid-like peptides may also be absorbed from partially digested food (casomorphins, exorphins, and rubiscolins), but have limited physiological activity. The opioid food peptides have lengths of typically 4-8 amino acids. The body's own opioids are generally much longer.

Opioid peptides are released by post-translational proteolytic cleavage of precursor proteins. The precursors consist of the following components: a signal sequence that precedes a conserved region of about 50 residues; a variable-length region; and the sequence of the neuropeptides themselves. Sequence analysis reveals that the conserved N-terminal region of the precursors contains 6 cysteines, which are probably involved in disulphide bond formation. It is speculated that this region might be important for neuropeptide processing.^[1]

Opioid peptides produced by the body

The human genome contains several homologous genes that are known to code for endogenous opioid peptides.

• The nucleotide sequence of the human gene for proopiomelanocortin (POMC) was characterized in 1980.^[2] The POMC gene codes for endogenous opiates such as β-endorphin and gamma-endorphin.^[3] The peptides with opiate activity that are derived from proopiomelanocortin comprise the class of endogenous opioid peptides called "endorphins".
• The human gene for enkephalins was isolated and its sequence described in 1982.^[4]
• The human gene for dynorphins (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 encoding prepronociceptin, which is cleaved into nociceptin and potentially two additional neuropeptides.^[1]
• Adrenorphin, amidorphin, and leumorphin were discovered in the 1980s.
• Opiorphin and spinorphin, enkephalinase inhibitors (i.e., prevent the metabolism of enkephalins).
• Hemorphins, hemoglobin-derived opioid peptides, including hemorphin-4, valorphin, and spinorphin, among others.

Opioid food peptides

• Casomorphin (from milk)
• Gluten exorphin (from gluten)
• Gliadorphin/gluteomorphin (from gluten)
• Rubiscolin (from spinach)

Microbial opioid peptides

• Deltorphin I and II (fungal)
• Dermorphin (from an unknown microbe)

References

1. ^ ^{a b} Mollereau C, Simons MJ, Soularue P, Liners F, Vassart G, Meunier JC, Parmentier M (August 1996). "Structure, tissue distribution, and chromosomal localization of the prepronociceptin gene". Proc. Natl. Acad. Sci. U.S.A. 93 (16): 8666–70. PMC 38730. PMID 8710928. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=38730. 
2. ^ Chang AC, Cochet M, Cohen SN (August 1980). "Structural organization of human genomic DNA encoding the pro-opiomelanocortin peptide". Proc. Natl. Acad. Sci. U.S.A. 77 (8): 4890–4. PMC 349954. PMID 6254047. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=6254047. 
3. ^ Ling N, Burgus R, Guillemin R (November 1976). "Isolation, primary structure, and synthesis of alpha-endorphin and gamma-endorphin, two peptides of hypothalamic-hypophysial origin with morphinomimetic activity". Proc. Natl. Acad. Sci. U.S.A. 73 (11): 3942–6. PMC 431275. PMID 1069261. http://www.pubmedcentral.nih.gov/pagerender.fcgi?tool=pmcentrez&artid=431275&pageindex=1. 
4. ^ Noda M, Teranishi Y, Takahashi H, Toyosato M, Notake M, Nakanishi S, Numa S (June 1982). "Isolation and structural organization of the human preproenkephalin gene." Nature". 1982 Jun 3;' 297 (5865): 431–4. PMID 6281660. 
5. ^ Horikawa S, Takai T, Toyosato M, Takahashi H, Noda M, Kakidani H et al. (Dec 1983). "Isolation and structural organization of the human preproenkephalin B gene". Nature 306 (5943): 611–4. PMID 6316163. 

External links

• MeSH Opioid+Peptides

This article incorporates text from the public domain Pfam and InterPro IPR006024