Leu-Enkephalin
Leu-enkephalin (YGGFL) is an endogenous opioid pentapeptide and preferential delta-opioid receptor agonist involved in pain modulation, mood regulation, immune function, and stress response.
Leu-enkephalin (leucine-enkephalin, Tyr-Gly-Gly-Phe-Leu) is one of two endogenous enkephalin pentapeptides discovered by John Hughes and Hans Kosterlitz in 1975. As a preferential delta-opioid receptor (DOR) agonist, leu-enkephalin plays fundamental roles in endogenous pain modulation, mood regulation, reward processing, and immune system function.
Overview
Leu-enkephalin and met-enkephalin were the first endogenous opioid peptides identified, fundamentally changing our understanding of pain perception and reward neuroscience. Their discovery in pig brain extracts by Hughes, Kosterlitz, and colleagues demonstrated that the brain produces its own morphine-like compounds, explaining why opiate drugs are effective -- they mimic endogenous signaling molecules. Leu-enkephalin differs from met-enkephalin only at the C-terminal position (leucine versus methionine), yet this single substitution confers distinct receptor selectivity, metabolic stability, and biological roles. Leu-enkephalin is derived from the proenkephalin gene product but also from prodynorphin processing, giving it a broader distribution pattern than met-enkephalin.
Mechanism of Action
Leu-enkephalin exerts its primary effects through activation of delta-opioid receptors (DOR), with lower affinity for mu-opioid receptors (MOR) and minimal kappa-opioid receptor (KOR) activity. The peptide's N-terminal tyrosine residue is critical for opioid receptor binding -- the free amine and phenolic hydroxyl groups form essential hydrogen bonds with conserved receptor residues in the binding pocket.
Delta-opioid receptor signaling: DOR activation by leu-enkephalin triggers Gi/Go protein coupling, leading to (1) inhibition of adenylyl cyclase and reduced cAMP production, (2) opening of G protein-coupled inwardly rectifying potassium channels (GIRKs) producing membrane hyperpolarization, and (3) inhibition of voltage-gated calcium channels, reducing neurotransmitter release. In pain circuits, this presynaptic inhibition reduces substance P and glutamate release from nociceptive primary afferents in the spinal dorsal horn (Stein et al., 2003).
Immune modulation: Leu-enkephalin enhances natural killer (NK) cell cytotoxic activity and modulates T cell proliferation through opioid receptors expressed on immune cells. DOR activation on lymphocytes influences cytokine production and cell migration, establishing enkephalins as neuroimmune mediators (Plotnikoff et al., 1986).
Enzymatic degradation: Leu-enkephalin is rapidly inactivated by two primary peptidases: neutral endopeptidase (NEP/enkephalinase, EC 3.4.24.11), which cleaves the Gly3-Phe4 bond, and aminopeptidase N (APN, EC 3.4.11.2), which removes the N-terminal tyrosine. This dual degradation pathway produces an in vivo half-life of approximately 2-3 minutes, severely limiting therapeutic utility of the native peptide.
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Research
Pain Modulation
Leu-enkephalin contributes to descending pain inhibition through DOR activation in the periaqueductal gray (PAG), rostral ventromedial medulla (RVM), and spinal dorsal horn. DOR-selective agonism produces analgesia with reduced respiratory depression and constipation compared to mu-opioid agonists, making delta-selective compounds an active area of analgesic drug development. Ossipov et al. (2004) demonstrated that DOR activation modulates chronic pain states particularly effectively, suggesting enkephalin-based approaches may be better suited for persistent rather than acute pain.
Mood and Reward
Enkephalinergic neurons are densely distributed in the nucleus accumbens, ventral tegmental area, and amygdala -- key nodes of the reward and emotional regulation circuits. Leu-enkephalin release in these regions contributes to natural reward processing, hedonic experience, and stress buffering. DOR knockout mice show increased anxiety-like behavior and depressive phenotypes, confirming the role of delta-opioid signaling in mood regulation (Filliol et al., 2000).
Immune Function
Leu-enkephalin enhances NK cell activity at low concentrations (10^-12 to 10^-10 mol/L) through DOR expressed on NK cells. This immunostimulatory effect is blocked by the opioid antagonist naloxone, confirming receptor-mediated action. Research has also demonstrated that leu-enkephalin modulates T cell responses, macrophage chemotaxis, and antibody production, establishing it as a bidirectional immunomodulator whose effects depend on concentration and immune activation state (Plotnikoff et al., 1986).
Cardioprotection
DOR activation by leu-enkephalin and related agonists provides cardioprotection during ischemia-reperfusion injury. DOR activation preconditions cardiomyocytes by activating mitochondrial KATP channels and reducing intracellular calcium overload during reperfusion. Schultz et al. (1997) demonstrated that endogenous enkephalin release contributes to ischemic preconditioning in cardiac tissue.
Safety Profile
As an endogenous peptide, leu-enkephalin operates within established physiological signaling systems. The primary limitation is its extremely short half-life (~2-3 minutes), which restricts exogenous therapeutic use. At supraphysiological concentrations, DOR activation can produce seizure-like activity in rodent models, though this effect is less pronounced than with delta-selective non-peptide agonists. Unlike mu-opioid agonists, DOR-selective compounds produce minimal respiratory depression, constipation, and physical dependence at analgesic doses. Tolerance to delta-opioid analgesia develops more slowly than to mu-opioid analgesia. The rapid degradation of native leu-enkephalin by NEP and aminopeptidases serves as a built-in safety mechanism preventing accumulation.
Pharmacokinetic Profile
Leu-Enkephalin — Pharmacokinetic Curve
Molecular Structure
- Formula
- C28H37N5O7
- Weight
- 555.6 Da
- CAS
- 58822-25-6
- PubChem CID
- 443363
- Exact Mass
- 573.2257 Da
- LogP
- -2.1
- TPSA
- 225 Ų
- H-Bond Donors
- 7
- H-Bond Acceptors
- 9
- Rotatable Bonds
- 16
- Complexity
- 847
Identifiers (SMILES, InChI)
InChI=1S/C27H35N5O7S/c1-40-12-11-21(27(38)39)32-26(37)22(14-17-5-3-2-4-6-17)31-24(35)16-29-23(34)15-30-25(36)20(28)13-18-7-9-19(33)10-8-18/h2-10,20-22,33H,11-16,28H2,1H3,(H,29,34)(H,30,36)(H,31,35)(H,32,37)(H,38,39)/t20-,21-,22-/m0/s1
YFGBQHOOROIVKG-FKBYEOEOSA-NResearch Protocols
intranasal Injection
Intranasal delivery has been explored as a route to CNS access, bypassing the blood-brain barrier through olfactory nerve transport.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| Rodents producing analgesia equivalent to | 40-160 mg, 3-5 mg | Per protocol | — |
intravenous Injection
Following intravenous administration, the peptide is immediately available systemically but is degraded within minutes.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| Rodents producing analgesia equivalent to | 40-160 mg, 3-5 mg | Per protocol | — |
oral
Absorption Leu-enkephalin is not orally bioavailable due to rapid degradation by gastrointestinal peptidases and first-pass hepatic metabolism.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| Rodents producing analgesia equivalent to | 40-160 mg, 3-5 mg | Per protocol | — |
intrathecal Injection
Intrathecal leu-enkephalin at 10-100 microg produces spinal analgesia lasting 15-30 minutes in rodent models. Continuous intrathecal infusion at 1-5 microg/hour maintains analgesia but requires implanted pumps due to the short half-life.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| RB101 at 40-160 mg/kg IV | 40-160 mg, 3-5 mg | Per protocol | —(Route: Intrathecal) |
Interactions
Peptide Interactions
The most pharmacologically rational approach to enhancing leu-enkephalin signaling is co-administration with NEP and/or APN inhibitors. Thiorphan (NEP inhibitor) and bestatin (APN inhibitor) individually extend enkephalin half-life 2-3 fold; combined, they extend it 5-10 fold and produce robust a...
Leu-Enkephalin + Met-Enkephalin (Endogenous Synergy) Both enkephalins are co-released from proenkephalin-expressing neurons.
Selank, a synthetic tuftsin analogue, modulates enkephalin metabolism by influencing carboxypeptidase activity and may indirectly enhance endogenous enkephalin levels. The anxiolytic effects of selank may partially operate through potentiation of enkephalinergic signaling.
What to Expect
What to Expect
Rapid onset expected; half-life of ~2-3 minutes (plasma) indicates fast-acting pharmacokinetics
Intrathecal leu-enkephalin at 10-100 microg produces spinal analgesia lasting 15-30 minutes in rodent models.
Due to short half-life (~2-3 minutes (plasma)), effects are expected per-dose; consistent daily administration maintains therapeutic levels
Regular administration schedule required; effects are dose-dependent and do not persist between doses
Quality Indicators
What to look for
- Multiple peer-reviewed studies available
- Oral administration available
Caution
- Short half-life may require frequent dosing
Frequently Asked Questions
References (11)
- [1]Hughes et al *Nature* Nature (1975)
- [9]Dripps et al -- Delta opioid receptor agonists as novel analgesics with improved safety profiles Trends Pharmacol Sci (2023)
- [10]Gendron et al -- Delta opioid receptor-mediated analgesia: from molecular mechanisms to clinical translation Pain (2022)
- [11]Cahill et al -- Enkephalin regulation of pain and affect: implications for therapeutic development Neuropsychopharmacology (2022)
- [2]
- [3]Stein et al *Pharmacol Rev* Pharmacol Rev (2003)
- [4]
- [5]
- [6]Roques et al *Pharmacol Ther* Pharmacol Ther (2012)
- [7]Ossipov et al *Life Sci* Life Sci (2004)
- [8]Schultz et al *Am J Physiol* Am J Physiol (1997)
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