Circulating opioids in man

Currently, all known mammalian endogenous opioid peptides belong to one of three peptide families, each derived from a distinct precursor molecule: pro-enkephalin (PENK), pro-opiomelanocortin (POMC) or pro-dynorphin (PDYN). Enzymatic processing of these precursors at specific cleavage sites gives rise to opioid pep tides which share the opiate active sequence Tyr-GlyGly- Phe- at the NH2-terminal. This sequence is followed by either Met- or Leu- and COOH-terminal extensions of varying lengths which appear to confer increased resistance to degradation, altered receptor selectivity and often increased potency.