Vertebrate endogenous opioids neuropeptides are released by post-translational proteolytic processing of precursor proteins. These precursor proteins consist of a signal sequence followed by a conserved region of about 50 residues, a variable length region and the sequence of the various neuropeptides. Three types of precursors are currently known:

• Preproenkephalin A (gene PENK) which is processed to produce the peptides Met-enkephalin (6 copies) and Leu-enkephalin,
• Preproenkephalin B (gene PDYN) which is processed to produce the peptides neoendorphin, dynorphin, leumorphin, rimorphin and leu-enkephalin.
• Prepronocipeptin (gene PNOC) which is processed to produce the peptides nociceptin (orphanin FQ) as well as two other potential neuropeptides.

The conserved region in the N-terminal of these precursors contains six cysteines that are probably involved in disulfide bonds. This region could be important for the processing of the neuropeptides [1].