Leucine pyroglutamate

Leucine Pyroglutamate (Pyroglutamyl-Leucine)

Identity and Chemical Characterization

Names and Systematic Chemistry

Leucine pyroglutamate — also known as pyroglutamyl-leucine, pyroGlu-Leu, pEL, or L-pyroglutamyl-L-leucine — is a naturally occurring dipeptide composed of two amino acid residues: pyroglutamic acid (pyroglutamate; 5-oxoproline) at the N-terminus, covalently bonded to L-leucine at the C-terminus via a standard peptide bond.

Its molecular formula is C₁₁H₁₈N₂O₄, and it is registered in the PubChem database under Compound ID (CID) 152417. The compound is formally named L-pyroglutamyl-L-leucine in the IUPAC convention, reflecting the stereospecificity of both constituent residues. In synthetic chemistry, the free acid form has been characterized as white crystals with a melting point of 152°–154°C, and an optical rotation [α]_D^23°C of −19.35° (c 1.0, MeOH).

Pyroglutamyl (pGlu) peptides are formed by the intramolecular cyclization of a glutamine or glutamic acid residue at the N-terminal position of peptides. This process can occur endogenously or during processing of foods containing the peptides. Factors such as heat, high pressure, and enzymatic modifications contribute to pGlu formation. The reaction can be catalyzed by heat, glutamyl-peptide cyclotransferase, or glutaminyl cyclase.

Pyroglutamic acid is formed through the intramolecular cyclization of N-terminal glutamine (Q) or glutamic acid (E) residues via both non-enzymatic and enzymatic processes. This transformation can occur under physiological conditions and is influenced by factors such as temperature, pH, enzymatic activity, and pressure.

The role of the pGlu residue in peptide bioactivity is not completely established, although the hydrophobic γ-lactam ring is thought to enhance the peptide's stability against degradation by gastrointestinal proteases.

Structural Relationship to Its Constituent Amino Acids

The N-terminal moiety, pyroglutamic acid (5-oxoproline; L-pyroglutamic acid; molecular formula C₅H₇NO₃), is a cyclic derivative of glutamine formed by intramolecular dehydration of the amino group onto the side-chain amide. Glutamine is stable in a dry state but unstable in aqueous solution, where it undergoes hydrolysis of the side-chain amide group to yield glutamate and ammonia, and at elevated temperatures it cyclizes to pyroglutamate. The C-terminal residue, L-leucine (CID 6106), is a standard branched-chain essential amino acid. Their peptide bond union produces the unique dipeptide structure of leucine pyroglutamate.

Natural Sources and Occurrence in Foods

PyroGlu-Leu is widely distributed in food protein hydrolysates, such as wheat gluten and corn gluten hydrolysates, as well as in Japanese fermented foods produced by Aspergillus oryzae, such as the Japanese rice wine known as sake, the salted fermented soy paste known as miso, and a type of soy sauce, shoyu.

PyroGlu-Leu is also present in the enzymatic hydrolysates of corn gluten and fish, as well as in certain traditional Japanese fermented foods.

Although it was initially assumed that all the peptides found in food are degraded into amino acids during the digestion and absorption processes, it has been demonstrated that some peptides can resist protease digestion and can be absorbed directly into the blood. Peptides in foods have been found to have biological functions in addition to being sources of amino acids. Food protein hydrolysates and fermented foods contain pyroglutamyl peptides, which are spontaneously generated from peptides with a glutaminyl residue at the amino terminal during storage and processing.

Regarding quantified concentrations in specific foods, sake contains approximately 1.0–1.5 mg/100 mL of pyroGlu-Leu. Both miso and shoyu contain higher amounts of pyroGlu-Leu than sake. Commercial sake products (n = 5) contained pyroGlu-Leu at concentrations ranging from 40 to 60 μM (10–15 mg/L).

Japanese rice wine, sake, is made from steamed rice, water, and lactic acid by "multiple parallel fermentation" with mold (Aspergillus oryzae) and yeast (Saccharomyces cerevisiae). Nineteen pyroglutamyl peptides were identified in commercially available sake; among them, pyroGlu-Leu and pyroGlu-Gln were the major constituents. The pyroGlu-Leu content in sake mash increased during the fermentation processes. However, no pyroGlu-Leu was produced by yeast inoculated into preheated mash. Furthermore, addition of ¹³C-Leu to the mash did not increase the ratio of pyroGlu-¹³C-Leu to pyroGlu-¹²C-Leu. On the other hand, digestion of steamed rice with A. oryzae proteases increased the pyroGlu-Leu content. These data collectively indicate that the production of pyroGlu-Leu in sake fermentation is mediated by A. oryzae proteases acting on the rice protein substrate, rather than by yeast or non-enzymatic cyclization alone.

Industrial Protein Hydrolysates

Food-derived peptides have been reported to yield a variety of health-promoting activities. Pyroglutamyl peptides are contained in wheat gluten hydrolysate. Corn and wheat gluten hydrolysates were digested using pancreatin and leucine aminopeptidase; the resultant peptides were identified via size-exclusion chromatography and reverse-phase HPLC-tandem mass spectrometry (MS/MS). Structures of indigestible peptides were confirmed via LC-MS/MS in multi-reaction monitoring mode. All indigestible peptides in the exopeptidase digest were diprolyl and di- and tripyroglutamyl peptides. The resistance to exopeptidase digestion is a key feature of pyroGlu-Leu that distinguishes it from regular dipeptides and underlies its ability to reach the circulation intact after oral ingestion.

Traditional and Historical Use

Leucine pyroglutamate as an isolated, identified compound is a product of modern food chemistry research; no discrete traditional use of the purified dipeptide has been documented in historical medical literature. However, the compound exists at measurable concentrations in fermented foods with long histories of dietary use in East Asian cultures.

Sake is not only consumed as an alcoholic beverage, but is also used as a seasoning in traditional Japanese dishes. Miso and shoyu (soy sauce) similarly have centuries of use in Japanese cuisine and functional food traditions. The empirical observation of health associations attributed to fermented grain and legume products in East Asian dietary traditions — including their use for digestive health and liver support — may partly reflect, in retrospect, the presence of bioactive pyroglutamyl peptides including pyroGlu-Leu. However, no historical source specifically identified pyroGlu-Leu as the active constituent, and any such causal connection remains speculative in the absence of historical analytical chemistry. The compound was first formally isolated and characterized from wheat gluten hydrolysate in modern peer-reviewed research in the early 2000s.

A hepatoprotective peptide, pyroglutamyl leucine (pyroGlu-Leu), was identified in wheat gluten hydrolysate through an in vivo activity-guided fractionation approach based on D-galactosamine-induced acute hepatitis in rats and fractionation of peptides with large-scale preparative ampholine-free isoelectric focusing. This discovery represents the foundational event in scientific recognition of the compound.

Key Constituents and Active Compounds

Leucine pyroglutamate is itself the single bioactive entity of interest; it is a dipeptide and not a botanical extract containing multiple constituents. Its activity is attributed to the intact dipeptide structure, with both the N-terminal pyroglutamyl moiety and the C-terminal leucine residue contributing to biological recognition and effects.

Some health-promoting properties have been reported for pGlu peptides, including hepatoprotective, antidepressant, and anti-inflammatory activities. However, the role of the pGlu residue in the peptide bioactivity is not completely established, although the hydrophobic γ-lactam ring is thought to enhance the peptide stability against degradation by gastrointestinal proteases.

The key structural features relevant to bioactivity are:

• Resistance to proteolytic degradation: Short-chain pyroglutamyl peptides are resistant to digestion by endoproteinases and exopeptidases. This resistance, conferred by the blocked (cyclized) N-terminus, allows the intact dipeptide to survive passage through the gastrointestinal tract and enter systemic circulation.
• Absorption and bioavailability: The levels of free and peptide forms of pyroglutamic acid increased significantly and reached a maximum (approximately 40 nmol/mL) at 15 and 30 min after administration in rat portal blood experiments following ingestion of wheat gluten hydrolysate containing pyroglutamyl peptides. Identification of food-derived peptides in human blood after ingestion of corn and wheat gluten hydrolysates has also been undertaken, providing further evidence for systemic absorption, though detailed published quantitative data in humans remains limited.
• Intact peptide activity: Small peptides such as di- or tripeptides can be mainly absorbed via paracellular diffusion, transcellular passive diffusion, transcytosis, and carrier-mediated transport.

Mechanisms of Action

Peer-reviewed studies have identified several distinct mechanisms through which pyroGlu-Leu exerts its observed biological effects, primarily in preclinical (in vitro and animal) models:

Anti-inflammatory Signaling: NF-κB and MAPK Pathway Inhibition

PyroGlu-Leu inhibits LPS-induced inflammatory response via the blocking of NF-κB and MAPK pathways in RAW 264.7 macrophages. More specifically, pyroGlu-Leu dose-dependently suppressed IκBα degradation and MAPK (JNK, ERK, and p38) phosphorylation in LPS-stimulated RAW 264.7 cells. This dual pathway inhibition — targeting both the canonical NF-κB arm and the stress-activated MAPK kinases — suggests a broad suppressive effect on inflammatory transcription factor activation.

Inhibition of Inducible Nitric Oxide Synthase (iNOS) in Hepatocytes

Pyroglutamyl leucine (pyroGlu-Leu), which is a peptide isolated from wheat gluten hydrolysate, has been reported to be a hepatoprotective compound in acute liver failure. In inflamed liver, proinflammatory cytokines including interleukin (IL)-1β and tumor necrosis factor (TNF)-α stimulate the induction of inducible nitric oxide synthase (iNOS). Excess production of nitric oxide (NO) by iNOS is an inflammatory biomarker in liver injury.

IL-1β stimulated the enhancement of NO production in hepatocytes and this effect was inhibited by pyroGlu-Leu. pyroGlu-Leu decreased the expression of iNOS protein and its mRNA. Transfection experiments with iNOS-luciferase constructs revealed that pyroGlu-Leu inhibited both iNOS promoter transactivation and its mRNA stabilization. pyroGlu-Leu also decreased the expression of an iNOS gene antisense transcript, which is involved in iNOS mRNA stability. However, pyroGlu-Leu had no effects on IκB degradation and NF-κB activation. This last finding indicates that the hepatocyte iNOS-suppressing mechanism operates via an IκB/NF-κB-independent pathway — distinct from the mechanism observed in macrophages — and points to transcriptional and post-transcriptional regulation as the primary hepatoprotective route.

Upregulation of Intestinal Antimicrobial Peptides (α-Defensins)

By focusing on the production of intestinal antimicrobial peptides, researchers found that pyroGlu-Leu significantly increased the level of 4962 Da peptides, which were identified as the propeptide of rattusin or defensin alpha 9, in ileum. Increased tryptic fragment peptides from rattusin were also observed in the lumen. PyroGlu-Leu modulated gut microbiota by increasing secretion of host antimicrobial peptides classified as α-defensin in rats. This mechanism — stimulating Paneth cell-derived α-defensins — represents an indirect mechanism of microbiota modulation that does not require direct antimicrobial activity from the dipeptide itself.

Scientific Evidence by Area of Use

1. Anti-inflammatory Activity

Evidence type: In vitro (cell culture), animal model

RAW 264.7 macrophages were treated with LPS and various concentrations of pyroglutamyl-leucine (pyroGlu-Leu), -valine (pyroGlu-Val), -methionine (pyroGlu-Met), and -phenylalanine (pyroGlu-Phe). Cell viability/proliferation and various inflammatory parameters were measured by established methods including ELISA and western blotting. All the tested dipeptides significantly inhibited the secretion of nitric oxide, tumor necrosis factor (TNF)-α, and interleukin (IL)-6 from LPS-stimulated RAW 264.7 macrophages. Above all, pyroGlu-Leu inhibited the secretion of all these inflammatory mediators even at the lowest dose (200 μg/mL).

Evidence strength: Preliminary. This work was conducted entirely in a murine macrophage cell line (RAW 264.7), which does not replicate the complexity of systemic human inflammation. No human clinical trials testing anti-inflammatory outcomes of orally administered pyroGlu-Leu have been published as of the available literature. The in vitro doses used (200 μg/mL and above) are pharmacological concentrations that may not be physiologically achievable via oral consumption. Results are hypothesis-generating only.

2. Hepatoprotective Activity

Evidence type: Animal model (rat), in vitro (primary hepatocytes)

Studies have shown that the oral administration of pyroglutamyl leucine (pyroGlu-Leu or pEL), which was initially identified in wheat gluten hydrolysate, attenuates hepatitis in animal models. The initial discovery involved an in vivo activity-guided fractionation approach using D-galactosamine-induced acute hepatitis in rats. Results indicated that pyroGlu-Leu in sake is produced from rice proteins by digestion with A. oryzae proteases, and has been demonstrated to attenuate hepatitis and colitis in animal models.

The mechanism in liver cells was subsequently studied in vitro: researchers examined proinflammatory cytokine-stimulated hepatocytes as a simple "in vitro inflammation model" to determine liver protective effects of pyroGlu-Leu and its mechanisms of action, hypothesizing that pyroGlu-Leu inhibits the induction of iNOS gene expression, resulting in the attenuation of hepatic inflammation. These hypotheses were confirmed by the transcriptional and post-transcriptional findings described in the mechanisms section above.

Evidence strength: Preclinical only. Evidence is limited to rodent acute liver injury models and rat hepatocyte cell culture. No human clinical data on hepatoprotective effects exist in the peer-reviewed literature. The relevance of the D-galactosamine acute hepatitis model to chronic human liver disease or dietary hepatoprotection is uncertain.

3. Gut Microbiota Modulation and Anti-colitic Activity

Evidence type: Animal models (mice with DSS-induced colitis, rats on high-fat diet)

Researchers previously found the anti-inflammatory peptide pyroGlu-Leu in the enzymatic hydrolysate of wheat gluten. The objective was to elucidate improvement of colitis by oral administration of pyroGlu-Leu in an animal model. Acute colitis was induced by dextran sulfate sodium (DSS), and various concentrations of pyroGlu-Leu were administered by oral gavage for 7 days. A dose of 0.1 mg/kg body weight/day showed the most significant improvement. The pyroGlu-Leu concentration was significantly increased 24 h after oral administration both in the small intestine and the colon compared with the baseline.

The oral administration of very low doses (0.1–1.0 mg/kg body weight) of pyroGlu-Leu can normalize the disturbances in the colonic microbiota of mice with dextran sulfate sodium (DSS)-induced colitis.

Regarding high-fat diet-induced dysbiosis: high-fat diet extensively increased the ratio of Firmicutes/Bacteroidetes in feces of rats compared to control diet. Oral administration of pyroGlu-Leu (1 mg/kg body weight) significantly attenuated high fat diet-induced dysbiosis.

The proposed mechanism involved the upregulation of innate immune antimicrobial peptides: orally administered pyroGlu-Leu attenuates dysbiosis by increasing in the host antimicrobial peptide, rattusin. Short-chain pyroglutamyl peptides such as pyroglutamyl-leucine in traditional Japanese seasonings and food protein hydrolysates have been demonstrated to attenuate colitis and dysbiosis in animal models after oral administration at 0.1–1.0 mg/kg body weight, which were less than the doses of prebiotics and probiotics exerting a similar effect. PyroGlu-Leu modulated gut microbiota by increasing secretion of host antimicrobial peptides classified as α-defensin in rats.

Anti-colitic pyroglutamyl peptides have also been identified in sake: three Japanese rice wine pyroglutamyl peptides consisting of pyroGlu-Tyr, pyroGlu-Asn-Ile, and pyroGlu-Leu were reported to have anti-colitis effects in mice.

Evidence strength: Preclinical only. All gut microbiota and colitis data derive from murine and rat models. No human randomized controlled trials examining pyroGlu-Leu's effects on gut dysbiosis, inflammatory bowel disease, or microbiome composition have been published. The doses used in rodent studies (0.1–1.0 mg/kg body weight) must be considered with caution when extrapolating to human therapeutic contexts, as inter-species differences in gut physiology, defensin biology, and microbiome composition are substantial.

4. Antidepressant-Like Activity

Evidence type: Animal model (mice), behavioral pharmacology

An antidepressant-like effect of food-derived pyroglutamyl peptides in mice was reported in a 2015 study published in Neuropeptides. The effects of L-pGlu and pyroglutamyl peptides on depression, anxiety, analgesia, and locomotion were investigated using the forced swim test (FST), elevated plus maze (EPM), hot plate test, and open field test. BALB/c mice were treated intraperitoneally and acutely with pyroglutamic acid-leucine (pGlu-Leu; 10 mg/kg; n=10) or vehicle.

Evidence strength: Preliminary and animal-only. Behavioral tests such as the FST and tail suspension test are standard rodent screening tools for antidepressant-like effects, but their predictive validity for clinical antidepressant efficacy in humans is imperfect and controversial. The route of administration in these experiments (intraperitoneal injection) differs from the oral route typically relevant to dietary supplementation, limiting direct translational relevance. No human clinical trials on mood, depression, or anxiety effects of pyroGlu-Leu have been identified in the peer-reviewed literature.

5. Sensory Properties — Umami/Taste Modulation

pGlu peptides are thought to have different characteristics, especially bitter and umami tastes, and thus can affect the sensory properties of foods that contain them. PyroGlu-Leu has been detected in traditional Japanese soy sauce preparations in the context of research on umami-enhancing compounds in these condiments. This is a flavor-science observation rather than a therapeutic claim, but it contextualizes the compound's broader relevance to fermented food research.

6. Anti-obesity and Metabolic Activity (Preliminary)

Results from at least one study suggest that the short-chain hydrophobic pyroglutamyl peptides present in miso are effective in suppressing high fat diet-induced obesity in animal models. This research, focused on the pyroglutamyl peptide fraction of miso, aligns with the gut dysbiosis data showing attenuation of high-fat diet-induced changes in the Firmicutes/Bacteroidetes ratio, and with the known role of gut microbiota composition in metabolic regulation. However, this work has not been confirmed in human studies specific to pyroGlu-Leu.

Body Systems and Health Areas of Association

Based on the available peer-reviewed evidence, leucine pyroglutamate has been studied in connection with the following body systems:

• Gastrointestinal system: Colitis attenuation, gut microbiota modulation (reduction of dysbiosis), stimulation of intestinal antimicrobial peptide production in the ileum.
• Hepatic system / liver: Hepatoprotective activity in acute liver injury models; inhibition of iNOS-mediated inflammatory nitric oxide in hepatocytes.
• Immune system: Inhibition of macrophage activation and pro-inflammatory cytokine secretion (TNF-α, IL-6, NO); modulation of innate intestinal immunity via α-defensin upregulation.
• Central nervous system: Antidepressant-like behavioral effects in mouse models (preliminary, animal-only).
• Metabolic system: Preliminary evidence for attenuation of high-fat diet-induced metabolic dysbiosis and possibly obesity-related changes in gut microbiota.

Some health-promoting properties have been reported for pGlu peptides overall, including hepatoprotective, antidepressant, and anti-inflammatory activities.

Dosage: Forms and Amounts Reported in Research

No regulatory body (FDA, EFSA, EMA, Health Canada) has established a recommended dietary allowance, adequate intake, or tolerable upper intake level for leucine pyroglutamate as an isolated supplement. All dosage information below derives directly from peer-reviewed preclinical studies and must not be interpreted as validated human therapeutic doses.

• Anti-colitis / dysbiosis model (mice and rats, oral administration): The oral administration of very low doses of 0.1–1.0 mg/kg body weight of pyroGlu-Leu can normalize the disturbances in the colonic microbiota of mice with dextran sulfate sodium (DSS)-induced colitis. A dose of 0.1 mg/kg body weight/day showed the most significant improvement in the DSS colitis model.
• High-fat diet dysbiosis model (rats): Oral administration of pyroGlu-Leu at 1 mg/kg body weight significantly attenuated high fat diet-induced dysbiosis.
• Anti-inflammatory cell culture: pyroGlu-Leu inhibited the secretion of all inflammatory mediators even at the lowest dose of 200 μg/mL in the RAW 264.7 macrophage assay system.
• Antidepressant-like effect (mice, intraperitoneal): pGlu-Leu was administered at 10 mg/kg (n=10) by intraperitoneal injection in mouse behavioral studies.
• Natural food source context: Sake contains approximately 1.0–1.5 mg/100 mL of pyroGlu-Leu. Both miso and shoyu contain higher amounts of pyroGlu-Leu than sake. Fermented foods are consumed in the diet, while protein hydrolysates are generally consumed as supplements.

Researchers noted that effective anti-dysbiosis doses in animal models were less than the doses of prebiotics and probiotics exerting a similar effect, emphasizing the apparent potency of pyroGlu-Leu at low concentrations in preclinical systems.

Safety Considerations

Leucine pyroglutamate has not been subjected to formal toxicological assessment programs (such as multi-dose rat toxicity studies, genotoxicity assays, or reproductive toxicity evaluations) that have been published in accessible peer-reviewed literature. No safety monograph by the NIH Office of Dietary Supplements, EFSA, EMA, or comparable body for pyroGlu-Leu as an isolated supplement has been identified.

The following factual observations from research are relevant to a safety assessment:

• Dietary exposure context: PyroGlu-Leu is widely distributed in food protein hydrolysates, such as wheat gluten and corn gluten hydrolysates, as well as in Japanese fermented foods produced by Aspergillus oryzae. Populations consuming traditional Japanese diets have chronically ingested pyroGlu-Leu as a food constituent without documented adverse effects attributed to this compound specifically, although this is observational and not a controlled safety evaluation.
• Doses in preclinical models: Animal model doses of 0.1–1.0 mg/kg body weight/day (oral) resulted in measurable biological effects without reported overt toxicity in published studies. No lethal dose (LD50) or no-observed-adverse-effect level (NOAEL) data for isolated pyroGlu-Leu have been identified in the accessible literature.
• Cell viability in macrophage studies: Cell viability/proliferation was measured by established methods in the macrophage anti-inflammatory study, implying that the peptide's inhibitory effects on inflammatory mediators were not attributable to cytotoxicity at the tested doses (200 μg/mL and above), though formal cytotoxicity endpoints were a methodological control rather than a comprehensive safety study.
• Wheat gluten source considerations: Wheat gluten hydrolysate is a common source of pyroGlu-Leu in both research and commercial applications. Individuals with celiac disease or non-celiac gluten sensitivity may wish to evaluate whether any residual immunogenic gluten epitopes persist in a given preparation, as this would be a product-specific consideration unrelated to pyroGlu-Leu itself.
• Absence of human clinical safety data: No human clinical trial reporting adverse events, tolerability, or safety outcomes for isolated pyroGlu-Leu supplementation has been identified in the peer-reviewed literature. The gap between animal model doses and human-equivalent supplemental doses, and the absence of human pharmacokinetic data for pyroGlu-Leu specifically, means that the safety profile in humans cannot be characterized from the current evidence base.
• Drug interaction data: No peer-reviewed studies examining pharmacokinetic or pharmacodynamic drug interactions with leucine pyroglutamate have been identified.

Current Research Status and Evidence Limitations

The scientific investigation of leucine pyroglutamate is an active but early-stage field, centered primarily in Japanese food science research groups (notably associated with Kyoto University and Kyoto Prefectural University) and in the broader context of bioactive peptide research. Some short chain pyroglutamyl peptides have been demonstrated to have in vivo and in vitro activities, but the body of published human clinical evidence specifically for pyroGlu-Leu remains absent as of the available literature. All claimed health benefits rest on in vitro cell culture experiments or animal models. The compound has not been evaluated by any major national or international regulatory body as a supplement ingredient with established efficacy or safety profile. A 2019 review in Food Science and Human Wellness addresses the occurrence, properties, and biological significance of pyroglutamyl peptides derived from different food sources, providing a broader context for understanding where pyroGlu-Leu fits within this peptide class.

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