What Is Proenkephalin? Definition and What Research Reports
Proenkephalin, often abbreviated PENK, is a precursor protein (a proprotein) encoded by the PENK gene and processed by cells into the small opioid peptides known as enkephalins. In research settings the term usually refers either to the gene and the neurons that express it, or to a stable circulating fragment measured in blood as a laboratory marker. Published studies have examined proenkephalin in kidney function, heart failure, sepsis, critical care, kidney transplantation, stroke and neuroscience contexts.
Definition
Proenkephalin (commonly abbreviated PENK, and sometimes written proenkephalin A) is a precursor protein — a proprotein — that cells synthesise and then enzymatically cut into several much smaller peptides, the best known of which are the enkephalins (Met-enkephalin and Leu-enkephalin). The enkephalins are endogenous opioid peptides, meaning they are produced by the body itself and interact with opioid receptors. Because the intact precursor and its active peptide products are short-lived and difficult to measure directly, laboratories usually quantify a stable mid-region fragment of the precursor, written as proenkephalin A 119–159 and often shortened to penKid. In most published clinical papers, “proenkephalin” refers to that measurable fragment rather than to a substance being given to anyone.
What Class of Molecule Is It?
Proenkephalin belongs to the family of opioid peptide precursors, alongside proopiomelanocortin (POMC, the precursor of beta-endorphin) and prodynorphin (the precursor of the dynorphins). All three are longer polypeptide chains that are trafficked through the secretory pathway and cleaved at specific basic amino-acid sites, releasing shorter bioactive peptides. Proenkephalin and prodynorphin have been examined side by side in human biofluids; researchers measured both precursors in cerebrospinal fluid in a study of sporadic Creutzfeldt-Jakob disease (PMID 35216166).
Where it comes from
- Gene: the PENK gene, expressed in neurons, endocrine and immune cells, and in a range of peripheral tissues.
- Processing: the precursor is cleaved intracellularly into enkephalins and several intermediate fragments, one of which — proenkephalin A 119–159 — is stable enough to be assayed in plasma or serum.
- Peripheral sources: renal proximal tubules have been identified as a source of secreted proenkephalin A, and researchers reported in a mouse study that proximal tubule–derived proenkephalin A acted as a brake on kidney regeneration (PMID 37935684).
How the Term Is Used in Peptide Research
The word appears in three fairly distinct literatures, and confusing them is a common source of misreading.
- As a laboratory biomarker. The circulating fragment proenkephalin A 119–159 is freely filtered by the glomerulus, so its concentration in blood rises as filtration falls. Papers in this group describe it as a marker of glomerular filtration rate and of acute kidney injury (PMID 32739920).
- As a marker of the endogenous opioid system. Because the precursor gives rise to enkephalins, some cardiology papers describe it as an opioid system surrogate; one study characterised proenkephalin as a comprehensive renal marker in heart failure and framed it in exactly those terms (PMID 31091993), and a review chapter covered proenkephalin in heart failure more broadly (PMID 29153195).
- As a cell-type label in neuroscience. Neuroscientists use “proenkephalin neurons” or “PENK neurons” to identify populations of cells that express the gene; researchers described sexually dimorphic cellular architecture and neural circuitry of proenkephalin neurons in the oval bed nucleus of the stria terminalis (ovBNST) in a 2025 report (PMID 40445490).
This page is for educational purposes only and is not medical advice; consult a licensed physician for questions about testing, diagnosis or treatment.
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Kidney function and acute kidney injury
The largest share of the clinical literature treats proenkephalin as a filtration marker. One review-style paper presented proenkephalin as a new biomarker for glomerular filtration rate and acute kidney injury (PMID 32739920). In end-stage kidney disease, researchers measured proenkephalin levels and examined their determinants in patients treated with haemodialysis and peritoneal dialysis (PMID 37834463). In transplantation, a 2025 study evaluated proenkephalin A 119–159 as a biomarker for tracking graft function trajectories after kidney transplantation and reported superior tracking of those trajectories compared with the comparators examined (PMID 40475974).
Critical illness and sepsis
In sepsis, researchers reported that proenkephalin predicted organ failure, need for renal replacement therapy and mortality in the patients studied (PMID 32539302). A separate 2025 paper described the implementation of proenkephalin A testing in a critical care setting and its evaluation over one year of routine use (PMID 40141244).
Cardiovascular contexts
Cardiology work has looked at whether the marker adds information beyond established measures. One study characterised proenkephalin as a comprehensive renal marker in heart failure and as a surrogate for the opioid system (PMID 31091993), while a 2024 analysis examined the association between proenkephalin A and cardiovascular outcomes in ambulatory Veterans (PMID 39633843). Not every result has been positive: in acute ischaemic stroke, the study reported that proenkephalin A added no incremental prognostic value over the models tested (PMID 31973568).
Neurological and neuroscience contexts
Beyond blood-based measurement, researchers quantified prodynorphin and proenkephalin in the cerebrospinal fluid of people with sporadic Creutzfeldt-Jakob disease (PMID 35216166). In basic neuroscience, the 2025 ovBNST work mapped proenkephalin-expressing neurons and reported sex differences in their cellular architecture and circuitry (PMID 40445490).
Research Contexts at a Glance
| Context | How proenkephalin was used | Reference |
|---|---|---|
| Kidney filtration / AKI | Described as a marker of glomerular filtration rate and acute kidney injury | PMID 32739920 |
| Sepsis | Reported to predict organ failure, renal replacement therapy and mortality | PMID 32539302 |
| Critical care operations | Implementation and one-year evaluation of proenkephalin A testing | PMID 40141244 |
| Kidney transplantation | Tracking of graft function trajectories using proenkephalin A 119–159 | PMID 40475974 |
| Dialysis populations | Levels and determinants in haemodialysis and peritoneal dialysis patients | PMID 37834463 |
| Heart failure | Framed as an opioid-system surrogate and comprehensive renal marker | PMID 31091993 |
| Acute ischaemic stroke | Reported to add no incremental prognostic value | PMID 31973568 |
| Basic neuroscience | Used as a label for ovBNST neuron populations | PMID 40445490 |
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The verified papers summarised on this page were observational or mechanistic in design — measuring proenkephalin in blood, cerebrospinal fluid or tissue, or characterising proenkephalin-expressing cells — rather than administering proenkephalin or its fragments to human participants. Consequently, none of the cited reports described an adverse-event profile for proenkephalin as an administered substance, and no safety, tolerability or exposure data can be drawn from them. Papers such as the one-year critical care evaluation described the use of a laboratory assay (PMID 40141244), not an intervention.
Terms Readers Encounter
- PENK — the gene symbol for proenkephalin; also used loosely for the protein and for neurons expressing it.
- Proenkephalin A 119–159 / penKid — the stable mid-region fragment measured in plasma or serum in clinical studies, including in kidney transplantation (PMID 40475974).
- Enkephalins — the short opioid peptides released when the precursor is processed.
- Prodynorphin — a related opioid peptide precursor measured alongside proenkephalin in cerebrospinal fluid work (PMID 35216166).
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Start learning freeLimitations of the Current Literature
Most of the human data are associational: they describe how concentrations of a circulating fragment track with kidney function or outcomes in defined patient groups, not what happens when the molecule is manipulated. Findings have also differed by setting, since the stroke study reported no incremental prognostic value (PMID 31973568) while the sepsis study reported prediction of organ failure and mortality (PMID 32539302). Mechanistic work in animals, such as the report that renal proximal tubule–secreted proenkephalin A acted as a brake in kidney regeneration (PMID 37935684), has not been shown to translate directly to humans. Assay availability, reference ranges and dialysis status all influence measured values, as the dialysis determinants analysis illustrated (PMID 37834463).
References
- Prodynorphin and Proenkephalin in Cerebrospinal Fluid of Sporadic Creutzfeldt-Jakob Disease (International Journal of Molecular Sciences, 2022)
- Implementation and One-Year Evaluation of Proenkephalin A in Critical Care (International Journal of Molecular Sciences, 2025)
- Proenkephalin A 119-159 in Kidney Transplantation: A Novel Biomarker for Superior Tracking of Graft Function Trajectories (Transplant International, 2025)
- Proenkephalin Predicts Organ Failure, Renal Replacement Therapy, and Mortality in Patients With Sepsis (Annals of Laboratory Medicine, 2020)
- Proenkephalin A Adds No Incremental Prognostic Value After Acute Ischemic Stroke (Clinical and Applied Thrombosis/Hemostasis, 2020)
- Sexually Dimorphic Cellular Architecture and Neural Circuity of ovBNST Proenkephalin Neurons (Neuroscience Bulletin, 2025)
- Proenkephalin, an Opioid System Surrogate, as a Novel Comprehensive Renal Marker in Heart Failure (Circulation: Heart Failure, 2019)
- Proenkephalin: A New Biomarker for Glomerular Filtration Rate and Acute Kidney Injury (Nephron, 2020)
- Proenkephalin in Heart Failure (Heart Failure Clinics, 2018)
- Proenkephalin Levels and Its Determinants in Patients with End-Stage Kidney Disease Treated with Hemodialysis and Peritoneal Dialysis (International Journal of Molecular Sciences, 2023)
- Proenkephalin-A secreted by renal proximal tubules functions as a brake in kidney regeneration (Nature Communications, 2023)
- Association between Proenkephalin A and cardiovascular outcomes in ambulatory Veterans (IJC Heart & Vasculature, 2024)
Frequently asked questions
Is proenkephalin a peptide or a protein?▾
It is a precursor polypeptide — a proprotein — that cells cleave into much shorter opioid peptides called enkephalins. In clinical papers the term usually refers to a stable measurable fragment, proenkephalin A 119-159, which researchers assayed in blood as a marker of glomerular filtration rate and acute kidney injury (PMID 32739920). It sits in the same family as prodynorphin (PMID 35216166).
Why is proenkephalin measured instead of enkephalins?▾
Enkephalins themselves are short-lived and difficult to quantify, so laboratories measure a stable mid-region fragment of the precursor. Studies used that fragment in kidney transplantation to track graft function trajectories (PMID 40475974) and in critical care, where a 2025 report described implementing proenkephalin A testing and evaluating it over one year of routine use (PMID 40141244).
What did studies report about proenkephalin in sepsis?▾
In a sepsis cohort, researchers reported that proenkephalin predicted organ failure, the need for renal replacement therapy, and mortality among the patients studied (PMID 32539302). A separate critical care paper described the practical implementation and one-year evaluation of proenkephalin A measurement in that setting (PMID 40141244). These were observational analyses of a laboratory marker, not studies of an administered compound.
Has proenkephalin been useful in every clinical setting studied?▾
No. Results have varied by condition. In acute ischaemic stroke, the study reported that proenkephalin A added no incremental prognostic value beyond the models tested (PMID 31973568). By contrast, heart failure work framed it as an opioid-system surrogate and comprehensive renal marker (PMID 31091993), and a 2024 analysis examined its association with cardiovascular outcomes in ambulatory Veterans (PMID 39633843).
What does proenkephalin do in the kidney?▾
Beyond being filtered by the glomerulus, proenkephalin appears to have a local role. Researchers reported that proenkephalin-A secreted by renal proximal tubules functioned as a brake on kidney regeneration in a 2023 mechanistic study (PMID 37935684). Separately, an analysis of end-stage kidney disease measured proenkephalin levels and their determinants in haemodialysis and peritoneal dialysis patients (PMID 37834463).
What are proenkephalin neurons?▾
In neuroscience, "proenkephalin" or "PENK" neurons denotes cell populations that express the gene, used as an anatomical and functional label rather than as a biomarker. A 2025 report described sexually dimorphic cellular architecture and neural circuitry of proenkephalin neurons in the oval bed nucleus of the stria terminalis (PMID 40445490). This page is educational only and is not medical advice.
Is proenkephalin something that is given to people in studies?▾
The cited literature did not describe administering proenkephalin to human participants. The papers measured it in blood, cerebrospinal fluid or tissue, or characterised proenkephalin-expressing cells — for example in Creutzfeldt-Jakob disease cerebrospinal fluid (PMID 35216166) and in heart failure reviews (PMID 29153195). Because of that design, no dosing or adverse-event profile for an administered form was reported.
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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.