Enkephalin
A class of short endogenous opioid peptides (5 amino acids) found throughout the nervous system that modulate pain, mood, and stress responses. Met-enkephalin and leu-enkephalin act on delta and mu opioid receptors. They are part of the endogenous opioid system relevant to peptide-based pain research.
Technical Context
Met-enkephalin (Tyr-Gly-Gly-Phe-Met) and leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) were the first endogenous opioid peptides discovered (1975). They are produced from the proenkephalin precursor and are widely distributed in the CNS (dorsal horn, periaqueductal grey, limbic system) and periphery (adrenal medulla, GI tract). Enkephalins preferentially bind delta opioid receptors but also activate mu receptors. They are rapidly degraded by enkephalinases (aminopeptidase N, neutral endopeptidase/neprilysin) with half-lives of seconds to minutes. Enkephalin degradation inhibitors (racecadotril) have been developed as antidiarrhoeals, demonstrating the therapeutic potential of modulating endogenous opioid peptide levels rather than using exogenous receptor agonists.