Leu-enkephalin
[Leu5]-enkephalin, YGGFL, proenkephalin-derived pentapeptide
Leu-enkephalin is the second of the two opioid pentapeptides identified in 1975, differing from met-enkephalin by a single C-terminal residue. It is a preferred endogenous delta-opioid ligand and also forms the N-terminus of every dynorphin, which makes sequence identity a poor guide to receptor selectivity. It has no human therapeutic development of its own.
Mechanism
Leu-enkephalin derives principally from proenkephalin, which yields one copy alongside four met-enkephalins, and also from prodynorphin, where the YGGFL motif begins dynorphin A, dynorphin B and the neoendorphins. It binds delta-opioid receptors with clear preference over mu and essentially no kappa activity. Delta receptors couple to Gi/Go: inhibition of adenylate cyclase, GIRK channel opening and N-type calcium channel closure, giving presynaptic suppression of neurotransmitter release. Notably, extending the same pentapeptide with basic C-terminal residues, as in dynorphin, converts it into a kappa-selective ligand, a striking demonstration that the address portion of a peptide, not the message portion, determines subtype selectivity.
Anatomically, enkephalinergic interneurons are dense in the striatum, dorsal horn, amygdala, periaqueductal grey and enteric nervous system. In the gut, delta and mu signalling by enkephalins reduces electrolyte and water secretion into the lumen, the basis of the antisecretory action exploited by enkephalinase inhibitors. In the striatum, enkephalin marks the indirect pathway medium spiny neurons and is a standard molecular marker of that population.
What the research shows
The peptide's own clinical record is empty. Because it is degraded within seconds and does not cross the blood-brain barrier, direct administration has never been a viable strategy, and drug development moved instead to peptidase inhibition and to stable delta agonists. Racecadotril and the newer dual enkephalinase inhibitors act by preventing enkephalin breakdown, and Cochrane-level evidence in paediatric acute diarrhoea shows modest, low-certainty benefit. That is evidence about an enzyme inhibitor, not about leu-enkephalin.
The delta-agonist route has repeatedly disappointed. Small-molecule delta agonists produced convulsions in preclinical species, and clinical candidates advanced for depression and pain have not delivered convincing efficacy. This is the honest summary: enkephalin biology is beautifully characterised and therapeutically unfulfilled. Anything sold as leu-enkephalin for human use rests on no clinical evidence whatsoever.
Evidence assessment
Preclinical only
No human study has ever administered leu-enkephalin. The one human-level citation retained, a Cochrane review of racecadotril, concerns inhibition of the enzyme that degrades enkephalins, not the peptide, and cannot be used to rate the peptide itself. All direct evidence for leu-enkephalin is from cell and animal systems, so the tier is preclinical.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Identification of two related pentapeptides from the brain with potent opiate agonist activity Preclinical only
Simultaneous identification of leu-enkephalin and met-enkephalin as endogenous opiate agonists in brain, establishing endogenous opioid signalling.
Racecadotril for acute diarrhoea in children Preclinical only
Preventing enkephalin degradation reduced illness duration and stool output with low-certainty evidence. This tests an enzyme inhibitor, not leu-enkephalin itself, and is cited only as indirect pathway support.
Dynorphin is a specific endogenous ligand of the kappa opioid receptor Preclinical only
Showed that extending the leu-enkephalin sequence with basic residues produces kappa selectivity, establishing the message-address principle for opioid peptides.
Safety
There is no human safety database for leu-enkephalin administration. Delta-opioid agonism has been associated with seizure activity in animal studies, which remains the principal theoretical hazard. Because the peptide is destroyed almost immediately in blood and by digestion, oral products cannot deliver a meaningful systemic dose regardless of claims.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | No UK marketing authorisation. Unlicensed substance with no lawful route of supply for human consumption. |
| United States | No FDA approval and no clinical development programme. Research reagent only, explicitly not for human use. |
| WADA (sport) | Not individually listed on the WADA Prohibited List. Prohibited at all times under section S0 as a non-approved substance. |
Questions
One residue. Leu-enkephalin ends in leucine, met-enkephalin in methionine. Both prefer delta-opioid receptors, but they come from different precursor processing patterns and met-enkephalin has a second identity as opioid growth factor.
It survives seconds in blood, does not cross the blood-brain barrier, and delta agonism has been dogged by seizure risk in animals. Development moved to inhibiting the enzymes that degrade enkephalins instead.