The third member of the endogenous opioid family is b-endorphin, a 31-amino acid peptide that is localized primarily in the pituitary and the cells of the arcuate nucleus within the brain. The initial amino terminus of b-endorphin is identical to that of [Met5]enkephalin, raising the question as to whether the enkephalins might simply be degradation products of longer peptides. However, this is not the case, as clearly demonstrated with the cloning of the peptide precursors.

The enkephalins and dynorphins are rapidly broken down by peptidases. Their extreme lability leads to very short durations of action, hampering early work on their pharmacology. However, substituting the glycine at the second position with a D-amino acid stabilizes the enkephalins and many stable derivatives have now been synthesized. The selectivity of these synthetic peptides for the opioid receptor classes can also be dramatically affected by their amino acid sequences. b-Endorphin, on the other hand, is more stable with a more prolonged duration of action. When steps are taken to minimize degradation, all three families of peptides share many actions, including the ability to produce analgesia. These actions are reversed by opioid-selective antagonists, confirming an opioid mechanism of action for the peptides.

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