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 produced by the body
The human genome contains three homologous genes that are known to code for endogenous opioid peptides. Each gene codes for a large protein that can be processed to yield smaller peptides that have opiate-like activity.
 * The nucleotide sequence of the human gene for proopiomelanocortin (POMC) was characterized in 1980 . The POMC gene codes for endogenous opiates such as β-endorphin and gamma-endorphin . 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.
 * 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.
 * Adrenorphin and amidorphin were discovered in the 1980s.
 * Opiorphin, found in human saliva, is an enkephalinase inhibitor, i.e. it prevents the metabolism of enkephalins.

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)