Met 5-Enkephalin is a 5 amino acid endogenous opioid neuropeptide that is one of the two forms of enkephalin. It is a opioid growth factor receptor agonist and endogenous potent and selective ligand of the delta opioid receptor. Active growth factor, neurotransmitter and neuromodulator that enhances immune function through met-enkephalin receptors on leukocytes.