[Met5]-Enkephalin, amide, is an agonist for δ opioid receptors and putative ζ (zeta) opioid receptors. Met-enkephalin circulates in several forms, some potentially derived from proteins other than proenkephalin A, with plasma levels including both free and bound forms.
Phe-Met-Arg-Phe, amide dose-dependently (ED50=23 nM) activates a K+ current in peptidergic caudodorsal neurons and appears to localize with neuropeptide Y in some brain regions.