What Is Kallidin? Definition and What Research Reports
Kallidin is a short peptide — Lys-bradykinin, ten amino acids long — released from low-molecular-weight kininogen by tissue kallikrein. It belongs to the kinin family alongside bradykinin and their des-Arg metabolites, and it signals through the B1 and B2 bradykinin receptors. Published work has mapped how kinins bind and activate these receptors, quantified kinin peptides in human samples by mass spectrometry, and described kinin-driven inflammatory signalling in cell and tissue models. This entry is definitional and reports literature only.
Definition
Kallidin (also written Lys-bradykinin or Lys-BK) is a ten-amino-acid peptide of the kinin family. It is generated in tissues when the enzyme tissue kallikrein cleaves low-molecular-weight kininogen, a circulating precursor protein. Structurally it is bradykinin with an additional lysine residue at the N-terminus, which is why the two peptides share most of their receptor pharmacology. Kallidin and bradykinin are both trimmed by carboxypeptidases to their "des-Arg" forms — des-Arg10-kallidin (also called Lys-des-Arg9-bradykinin) and des-Arg9-bradykinin — which shifts their receptor preference. Together these four peptides make up the effector arm of what the literature calls the kinin–kallikrein system.
What Class of Molecule It Is
Kallidin is an endogenous signalling peptide, not a synthetic research analogue and not a hormone in the classical endocrine sense. It is produced locally, acts locally, and is degraded within minutes by peptidases including kininase I and kininase II (angiotensin-converting enzyme). Its receptors are G protein-coupled receptors: the B1 bradykinin receptor (B1R) and the B2 bradykinin receptor (B2R). Cryo-EM work has resolved human bradykinin receptor–Gq protein complexes, describing how the receptors couple to Gq signalling (PMID 35132089), and a separate structural study set out the molecular basis for kinin selectivity and activation at the human bradykinin receptors — the feature that distinguishes which kinin prefers B1R versus B2R (PMID 34518695).
Where It Comes From
- Precursor: low-molecular-weight kininogen (in contrast, bradykinin is released from high-molecular-weight kininogen by plasma kallikrein).
- Releasing enzyme: tissue kallikrein (KLK1).
- Sequence relationship: kallidin = lysine + bradykinin sequence, hence "Lys-bradykinin".
- Metabolites: des-Arg10-kallidin, formed by carboxypeptidase removal of the C-terminal arginine.
- Receptors: B2R (constitutively expressed) and B1R (strongly induced during inflammation).
How the Term Is Used in Peptide Research
In the published literature "kallidin" appears mostly as one member of a panel rather than as a standalone subject. Three usage patterns are common.
1. As an analyte in kinin–kallikrein system panels
Because kinins are short-lived and present at low concentrations, analytical chemistry papers treat kallidin as one peak among several. A sensitive mass spectrometric method was developed for the determination of kinin–kallikrein system peptides, with the authors framing the work in light of COVID-19 research interest in that system (PMID 33542252). Papers of this type report measurement performance rather than physiological actions.
2. As a reference agonist in receptor pharmacology
Kallidin and its des-Arg metabolite are used to probe which receptor subtype a tissue expresses. A pharmacological and functional characterisation study examined the bradykinin B2 receptor in human prostate tissue (PMID 15541417), and structural studies used kinin peptides to define the determinants of selectivity and activation at the human receptors (PMID 34518695).
3. As a marker of kinin-system activity in disease models
Reviews and mechanistic papers discuss kallidin as part of the upstream supply of ligand for B1R and B2R. A review of the kallikrein system considered its roles in retinal damage and protection (PMID 25448306), and a separate expert review discussed kinin B1 receptors as a therapeutic target in inflammation (PMID 29168929).
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Try it freeKallidin Compared With Related Kinins
| Peptide | Other names | Precursor / enzyme | Preferred receptor (as described in the literature) |
|---|---|---|---|
| Kallidin | Lys-bradykinin | Low-molecular-weight kininogen / tissue kallikrein | B2R, with activity also reported at B1R |
| Bradykinin | BK | High-molecular-weight kininogen / plasma kallikrein | B2R |
| des-Arg10-kallidin | Lys-des-Arg9-BK | Carboxypeptidase trimming of kallidin | B1R |
| des-Arg9-bradykinin | — | Carboxypeptidase trimming of bradykinin | B1R |
The practical consequence of this table, as structural work described, is that the length and C-terminus of the kinin determine receptor selectivity; the molecular basis for that selectivity at the human bradykinin receptors was mapped in a 2021 structural study (PMID 34518695).
What the Published Literature Reports
Reports in this area are largely mechanistic — receptor structure, cell signalling, enzymology and analytical detection — rather than human dosing studies of kallidin itself.
Receptor structure and signalling
Cryo-EM structures of human bradykinin receptor–Gq protein complexes were reported, and the study described how agonist binding is coupled to Gq activation at these receptors (PMID 35132089). Researchers in a companion line of work reported the molecular determinants that allow the receptors to discriminate between kinin peptides and become activated (PMID 34518695).
Inflammation and the B1 receptor
A 2018 review discussed kinin B1 receptors as a therapeutic target for inflammation, summarising the induced expression of this receptor in inflamed tissue (PMID 29168929). In cardiac fibroblasts, a study reported TLR4-induced expression and function of the B1R bradykinin receptor (PMID 29775649). In a cellular model of Parkinson's disease, researchers reported that kinin peptides enhanced inflammatory and oxidative responses and promoted apoptosis (PMID 27721576).
Enzymes that make and break kinins
Enzymology papers touch kallidin indirectly. Nardilysin was reported to cleave peptides at monobasic sites, a specificity relevant to processing of basic-residue-containing peptides (PMID 12590613). A structural and functional analysis of phosphoglucose isomerase from Vibrio vulnificus characterised an associated lysyl aminopeptidase activity — the class of activity that removes N-terminal lysine from peptides such as kallidin (PMID 15450853). On the inhibitor side, kallistatin concentrations were measured in relation to hypertension in heart transplant recipients (PMID 30177118).
Other ligands at kinin receptors
The receptors kallidin acts on are not exclusive to kinins. One report described blockade of non-opioid excitatory effects of spinal dynorphin A at bradykinin receptors (PMID 26221618), illustrating that pharmacology at B1R and B2R is studied with several ligand classes.
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Get the appAdverse Events and Safety Signals: What Studies Report
The verified literature summarised here does not contain human administration trials of kallidin, and therefore reports no dose-related adverse-event data for it. What it does report are context-dependent effects of kinin receptor signalling: kinin peptides enhanced inflammatory and oxidative responses and promoted apoptosis in a Parkinson's disease cellular model (PMID 27721576), while a review of the kallikrein system described both damaging and protective roles in the retina (PMID 25448306). Researchers also reported induced B1R expression and function in cardiac fibroblasts downstream of TLR4 (PMID 29775649). No safety conclusion for any use in people can be drawn from these models.
Limitations of the Evidence
- Most cited work is structural, analytical or cell-based; it describes receptors and enzymes rather than outcomes in people given kallidin.
- Kinins are rapidly degraded, which is precisely why sensitive mass spectrometric methods were developed for kinin–kallikrein system peptides (PMID 33542252); measured concentrations depend heavily on sample handling.
- Effects reported for "kinins" as a group cannot automatically be attributed to kallidin specifically.
- B1R and B2R signalling has been reported as both injurious and protective depending on tissue and model (PMID 25448306).
This page is for educational purposes only and is not medical advice; consult a licensed physician before making any health decision. Kallidin is discussed here as a subject of published research, and nothing above describes a protocol, product or intended use.
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- Cryo-EM structures of human bradykinin receptor-G(q) proteins complexes (Nature Communications, 2022)
- Molecular basis for kinin selectivity and activation of the human bradykinin receptors (Nature Structural & Molecular Biology, 2021)
- Sensitive mass spectrometric determination of kinin-kallikrein system peptides in light of COVID-19 (Scientific Reports, 2021)
- Kinin B1 receptors as a therapeutic target for inflammation (Expert Opinion on Therapeutic Targets, 2018)
- Expression and function of TLR4-induced B1R bradykinin receptor on cardiac fibroblasts (Toxicology and Applied Pharmacology, 2018)
- Kallistatin Concentration and Hypertension in Heart Transplant Recipients (Transplantation Proceedings, 2018)
- Kinin Peptides Enhance Inflammatory and Oxidative Responses Promoting Apoptosis in a Parkinson's Disease Cellular Model (Mediators of Inflammation, 2016)
- Blockade of non-opioid excitatory effects of spinal dynorphin A at bradykinin receptors (Receptors & Clinical Investigation, 2015)
- The kallikrein system in retinal damage/protection (European Journal of Pharmacology, 2015)
- Pharmacological and functional characterization of bradykinin B2 receptor in human prostate (European Journal of Pharmacology, 2004)
- Structural and functional analyses of phosphoglucose isomerase from Vibrio vulnificus and its lysyl aminopeptidase activity (Biochimica et Biophysica Acta, 2004)
- Nardilysin cleaves peptides at monobasic sites (Biochemistry, 2003)
Frequently asked questions
What is kallidin in one sentence?▾
Kallidin is a ten-amino-acid kinin peptide — bradykinin plus an N-terminal lysine, hence "Lys-bradykinin" — released from low-molecular-weight kininogen by tissue kallikrein, and it signals through the B1 and B2 bradykinin receptors. Structural studies described the molecular basis for how these receptors distinguish and are activated by kinin peptides (PMID 34518695).
How is kallidin different from bradykinin?▾
Both are kinins with nearly identical sequences, but kallidin carries an extra N-terminal lysine and is released from low-molecular-weight kininogen by tissue kallikrein, whereas bradykinin comes from high-molecular-weight kininogen via plasma kallikrein. Structural work reported that kinin length and terminus shape selectivity at the human bradykinin receptors (PMID 34518695, PMID 35132089).
Which receptors does kallidin act on?▾
The kinin B2 receptor, which is constitutively expressed, and the kinin B1 receptor, which is strongly induced in inflamed tissue. Cryo-EM structures of human bradykinin receptor–Gq protein complexes described the coupling of these receptors to Gq signalling (PMID 35132089), and a review discussed B1 receptors as an inflammation target (PMID 29168929).
Is kallidin used as a therapeutic peptide?▾
The verified literature summarised on this page does not report human administration trials of kallidin. It appears instead as an endogenous analyte and a reference agonist — for example in a sensitive mass spectrometric method for kinin–kallikrein system peptides (PMID 33542252) and in pharmacological characterisation of the B2 receptor in human prostate tissue (PMID 15541417).
What have studies reported about kinin signalling and inflammation?▾
Researchers reported that kinin peptides enhanced inflammatory and oxidative responses and promoted apoptosis in a Parkinson's disease cellular model (PMID 27721576), and that TLR4 stimulation induced expression and function of the B1R bradykinin receptor on cardiac fibroblasts (PMID 29775649). A review also described both damaging and protective kallikrein-system roles in the retina (PMID 25448306).
Why is kallidin hard to measure?▾
Kinins are released locally and degraded within minutes by peptidases, so circulating concentrations are low and sensitive to sample handling. That is why a dedicated mass spectrometric method was developed for determination of kinin–kallikrein system peptides, with the authors framing the work in the context of COVID-19 research interest in that system (PMID 33542252).
What regulates kallidin levels?▾
Generation depends on tissue kallikrein activity and kininogen supply, while breakdown involves carboxypeptidases and aminopeptidases; a lysyl aminopeptidase activity of this general class was characterised structurally in one report (PMID 15450853), and nardilysin was reported to cleave peptides at monobasic sites (PMID 12590613). Kallistatin, an endogenous kallikrein inhibitor, was measured in heart transplant recipients with hypertension (PMID 30177118).
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References
This page summarises published research for education — it is not medical advice, and nothing here is a recommendation to use, purchase, or dose any substance. Study parameters described are what researchers reported, not instructions. Consult a qualified clinician before any health decision.