There are three well-characterized families of opioid peptides produced by the body: endorphins, enkephalins and dynorphins. Endorphins are processed from the precursor proopiomelanocortin (POMC) (Takahashi H et al, 1981), which can also be processed to yield adrenocorticotropic hormone (ACTH) and alpha- and gamma-melanocyte stimulating hormone (MSH). Beta-endorphin (Dragon N et al, 1977) is a 31 amino acid peptide found in neurons of the hypothalamus and pituitary gland. It is released into the blood (from the pituitary gland) and into the spinal cord and brain from hypothalamic neurons during vigourous exercise, excitement and orgasm. Beta-endorphin binds with the highest affinity to the mu-opioid receptor but it also possesses some affinity towards the delta- and kappa-opioid receptors. Once bound, it acts as an analgesic in the body by dulling pain. It does this by breaking down bradykinins which are peptides which accumulate in response to injury. The mu-opioid receptor (MOR) (Wang JB et al, 1994) possesses high affinity for enkephalins and beta-endorphin but low affinity for dynorphins.
The enkephalins are endogenous ligands, or specifically endorphins, as they are internally derived and bind to the body's opioid receptors. There are two forms of enkephalin, one containing leucine ("leu"), while the other contains methionine ("met"). The met-enkephalin peptide sequence is coded by the POMC gene whereas leu-enkephalin is coded by both POMC and dynorphin genes. Enkephalins are pentapeptides involved in regulating pain and nociception in the body. Their action is mediated through the delta-opioid receptor (DOR) (Knapp RJ et al, 1994).
Dynorphins constitute a class of opioid peptides that arise from the precursor protein prodynorphin. When prodynorphin is cleaved during processing by proprotein convertase 2 (PC2), multiple active peptides are released, amongst which are dynorphin A, dynorphin B, big-dyn and alpha/beta-neoendorphin (Day R et al, 1998). Dynorphins primarily exert their effects through the kappa-opioid receptor (KOR), a G-protein coupled receptor (Simonin F et al, 1995). Two subtypes of KORs have been identified: K1 and K2. Although KOR is the primary receptor for all dynorphins (James IF et al, 1982), the peptides do have some affinity for MOR and DOR.