Proinsulin: Physiology and What Research Reports

What Proinsulin Is

Proinsulin is the single-chain polypeptide precursor from which mature insulin is derived. Inside pancreatic beta cells it is assembled from preproinsulin, folds into a defined three-dimensional shape stabilised by disulfide bonds, and is then cleaved so that the connecting peptide (C-peptide) is removed and the A- and B-chains of insulin remain joined by those bonds. A review of proinsulin physiology and measurement described this precursor role, the enzymatic processing steps in secretory granules, and the fact that a minority of proinsulin escapes complete conversion and is co-secreted into the circulation alongside insulin and C-peptide (PMID 38018823).

Because of that incomplete conversion, proinsulin and its partially processed intermediates are measurable in blood, and the ratio of proinsulin to insulin has been studied as an index of how efficiently beta cells are handling their secretory load.

How proinsulin relates to neighbouring molecules

Where It Is Produced and What It Does

Proinsulin is produced in the beta cells of the pancreatic islets. Its immediate biological job is structural rather than hormonal: the connecting peptide holds the A- and B-chains in an orientation that allows correct disulfide pairing, after which processing enzymes liberate the active hormone. The same review noted that proinsulin itself has far weaker metabolic activity than insulin, which is part of why elevated circulating proinsulin has been treated as a signal about beta-cell behaviour rather than as a hormone effect in its own right (PMID 38018823).

Beta-cell proinsulin content is not static. A study in The Journal of Biological Chemistry examined nutrient-dependent regulation of beta-cell proinsulin content and reported that proinsulin levels within cells responded to nutrient conditions rather than remaining fixed (PMID 37209827). Researchers have also described a degradation route: a 2022 report in Scientific Reports identified UDP-glucose as a cereblon-dependent proinsulin degrader, implicating a ubiquitin-ligase pathway in controlling how much proinsulin persists (PMID 36028536).

Folding and Quality Control: What Studies Report

A recurring theme in the literature is that proinsulin folding is error-prone and must be policed. A 2019 study in Molecular Cell reported that cells deploy a two-pronged strategy to rectify misfolded proinsulin aggregates, describing coordinated handling of aggregated precursor inside the endoplasmic reticulum (PMID 31176671). Work in Biomolecules reported that FK506-binding protein 2 participates in proinsulin folding, adding a specific chaperone to the list of proteins involved (PMID 36671537).

Misfolding has also been framed as a risk factor. A 2021 study in Diabetes examined predisposition to proinsulin misfolding as a genetic risk for diet-induced diabetes, linking an inherited tendency toward faulty folding with metabolic outcomes under dietary challenge (PMID 34462258). More recently, a Nature Communications report combined a CRISPR screen with in vivo mouse quantitative trait locus mapping to identify regulators of proinsulin, an approach the authors used to nominate genes that set proinsulin levels (PMID 41974708).

How Proinsulin Is Measured or Studied

In clinical biology, proinsulin is quantified in plasma or serum by immunoassay, and the review of proinsulin physiology and measurement discussed assay specificity — in particular the need to distinguish intact proinsulin from partially processed intermediates and from insulin — as well as the interest of proinsulin measurement in laboratory practice (PMID 38018823). Fasting samples and ratio measures such as proinsulin-to-insulin or proinsulin-to-C-peptide are common ways the literature expresses the result.

In basic research, proinsulin is studied in cultured beta-cell lines, isolated islets and rodent models, using pulse-chase experiments, aggregation assays, genetic screens and mapping studies of the kind reported in the CRISPR and QTL work (PMID 41974708) and the nutrient-regulation experiments in beta cells (PMID 37209827).

Proinsulin, Beta-Cell Workload and Metabolic Risk

Human physiology studies have used proinsulin ratios as a readout of secretory demand. A 2018 study in Diabetes reported that increased beta-cell workload modulated the proinsulin-to-insulin ratio in humans, consistent with the idea that when demand on beta cells rises, a larger share of released material is incompletely processed precursor (PMID 30131390).

Prospective epidemiology has examined the same measure as a predictor. A 2022 analysis in the Journal of Personalized Medicine reported that fasting proinsulin independently predicted incident type 2 diabetes in a general-population sample (PMID 35887628). Age is another variable: a 2023 study in Physiological Research examined age-related changes in proinsulin processing among normoglycemic individuals, reporting that processing indices differed across age groups even in people without hyperglycaemia (PMID 38116775).

Proinsulin has also been detected long after beta-cell function was assumed lost. A 2025 report in Diabetes, Obesity & Metabolism described persisting plasma proinsulin levels in a cohort of 482 individuals with long-standing type 1 diabetes mellitus (PMID 40654035).

Why the Term Appears in Peptide Reading

Readers encounter "proinsulin" in three main contexts. First, as a laboratory value reported alongside insulin and C-peptide, where the review literature explains what the assay represents (PMID 38018823). Second, as a model system: because proinsulin folds in the endoplasmic reticulum and is prone to aggregation, it has become a standard test case for studying secretory-protein quality control, as in the aggregate-clearance work (PMID 31176671) and the chaperone study (PMID 36671537). Third, as a protein-engineering scaffold: a 2025 study in Molecular Pharmaceutics reported that engineered nonhormonal, cationic proinsulin variants prolonged lymphatic drainage, using the proinsulin framework as a carrier-like construct rather than as a hormone (PMID 41222114).

Limitations of the Current Literature

• Several mechanistic findings, including the degradation and misfolding-risk reports, came from cell and rodent models, and the cited papers did not establish that the same relationships hold identically in humans (PMID 36028536, PMID 34462258).
• Association findings such as fasting proinsulin predicting incident type 2 diabetes described statistical prediction in a population, not an individual diagnosis or treatment decision (PMID 35887628).
• Assay differences complicate comparison across studies, a point raised in the measurement review (PMID 38018823).

This page is for educational purposes only and is not medical advice; consult a licensed physician about any question concerning insulin, glucose metabolism, laboratory testing or diabetes care. Nothing here describes a protocol, and none of the cited work is presented as guidance for use in people.

References

• [Proinsulin: physiology, measurement, and interest in clinical biology] (Annales de biologie clinique, 2023)
• Nutrient-dependent regulation of β-cell proinsulin content (The Journal of Biological Chemistry, 2023)
• UDP-glucose, cereblon-dependent proinsulin degrader (Scientific Reports, 2022)
• Cells Deploy a Two-Pronged Strategy to Rectify Misfolded Proinsulin Aggregates (Molecular Cell, 2019)
• FK506-Binding Protein 2 Participates in Proinsulin Folding (Biomolecules, 2023)
• Predisposition to Proinsulin Misfolding as a Genetic Risk to Diet-Induced Diabetes (Diabetes, 2021)
• Proinsulin regulators identified with CRISPR screen and in vivo mouse QTL mapping (Nature Communications, 2026)
• Increased β-Cell Workload Modulates Proinsulin-to-Insulin Ratio in Humans (Diabetes, 2018)
• Fasting Proinsulin Independently Predicts Incident Type 2 Diabetes in the General Population (Journal of Personalized Medicine, 2022)
• Age-Related Changes in Proinsulin Processing in Normoglycemic Individuals (Physiological Research, 2023)
• Persisting plasma proinsulin levels in a cohort of 482 individuals with long-standing type 1 diabetes mellitus (Diabetes, Obesity & Metabolism, 2025)
• Engineered Nonhormonal, Cationic Proinsulin Variants Prolong Lymphatic Drainage (Molecular Pharmaceutics, 2025)