Three distinct families of classical opioid peptides have been identified: the enkephalins, endorphins, and dynorphins. Each family derives from a distinct precursor protein, prepro-opiome-lanocortin (POMC), preproenkephalin, and preprodynorphin, respectively, which are encoded by distinct genes. Each precursor is subject to complex cleavages and posttranslational modifications that result in the synthesis of multiple active peptides. The opioid peptides share a common amino-terminal sequence of Tyr-Gly-Gly-Phe-(Met or Leu), the opioid motif. This motif is followed by C-terminal extensions yielding peptides ranging from 5 to 31 residues (Table 21-1).
The major opioid peptide derived from POMC is ^-endorphin. In addition to ^-endorphin, the POMC precursor also is processed into the nonopioid peptides adrenocorticotropic hormone (ACTH), melanocyte-stimulating hormone (a-MSH), and ^-lipotropin ^-LPH). Proenkephalin contains multiple copies of met-enkephalin, as well as a single copy of leu-enkephalin. Prodynor-phin contains three peptides of differing lengths that all begin with the leu-enkephalin sequence: dynorphin A, dynorphin B, and neoendorphin (Figure 21-1).
A novel endogenous opioid peptide with significant sequence homology to dynorphin A was alternatively termed nociceptin or orphanin FQ (now termed N/OFQ; Table 21-1). The substitution of Phe for Tyr in the opioid motif is sufficient to abolish interactions with the three classical opioid peptide receptors. N/OFQ has behavioral and pain modulatory properties distinct from those of the three classical opioid peptides.
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