# Plasminogen activator inhibitor-1

**Plasminogen activator inhibitor-1** (PAI-1), also known as serpin E1, is a serine protease inhibitor (serpin) that is the primary inhibitor of the fibrinolytic system, rapidly inactivating both tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), the enzymes that convert plasminogen into the clot-dissolving protease plasmin.<sup>[1](https://www.sciencedirect.com/science/article/pii/S0268949995800158)</sup> By blocking this conversion, PAI-1 helps ensure that blood clots are dissolved only when they are no longer needed to stop bleeding.<sup>[2](https://medlineplus.gov/genetics/gene/serpine1/)</sup> Beyond hemostasis, elevated PAI-1 is associated with cardiovascular disease, metabolic disorders, cancer, inflammation, and cellular senescence, and it has emerged as a biomarker of aging.<sup>[3](https://www.sciencedirect.com/science/article/pii/S2950327226000069)</sup>

| Key fact | Detail |
|---|---|
| Protein class | Serpin superfamily; primary physiological inhibitor of t-PA and u-PA in plasma and pericellular matrices<sup>[1](https://www.sciencedirect.com/science/article/pii/S0268949995800158)</sup><sup> • </sup><sup>[4](https://www.ncbi.nlm.nih.gov/books/NBK6146/)</sup> |
| Size and structure | 45-kDa glycoprotein of 379 to 381 amino acids, with three β-sheets (A, B, C), nine α-helices, and a 26-residue reactive center loop<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC12499650/)</sup> |
| Active-state instability | Spontaneously converts to an inactive latent form with a half-life of about 2 hours at 37 °C and neutral pH<sup>[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC4214104/)</sup> |
| Stabilization by vitronectin | The plasma protein vitronectin prolongs the active half-life by roughly 50 to 100 percent through binding at the somatomedin B domain<sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC9731859/)</sup> |
| Gene | SERPINE1 on chromosome 7 (7q21.3-q22), 12.3 kb, nine exons; a 4G/5G promoter polymorphism at −675 bp affects transcription<sup>[8](https://link.springer.com/article/10.1186/s12964-024-01910-5)</sup><sup> • </sup><sup>[9](https://www.sciencedirect.com/science/article/abs/pii/S0049384801002924)</sup> |
| Deficiency phenotype | Complete PAI-1 deficiency causes abnormal, sometimes excessive bleeding, including menorrhagia and bleeding in pregnancy and childbirth<sup>[2](https://medlineplus.gov/genetics/gene/serpine1/)</sup> |
| Therapeutic status | No FDA- or EMA-approved therapeutics specifically target PAI-1, despite extensive research<sup>[10](https://pubmed.ncbi.nlm.nih.gov/42546744/)</sup> |

## Structure and mechanism of inhibition

PAI-1 exhibits the canonical serpin fold, with a central β-sheet from which protrudes the reactive center loop (RCL), a surface-exposed peptide sequence that houses the target recognized by t-PA and u-PA.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC12499650/)</sup> The RCL comprises 26 residues (331-SGTVASSSTAVIVSARMAPEEIIMDR-356), and the Arg346-Met347 bond (P1-P1′) acts as a substrate mimic toward the plasminogen activators.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC12499650/)</sup>

Inhibition follows the standard serpin "suicide substrate" mechanism. The protease attacks the bait peptide bond, forming a 1:1 stoichiometric acyl-enzyme complex; the N-terminal portion of the RCL then inserts into the central β-sheet A, translocating the protease to the opposite end of the molecule and producing an irreversible inhibitory complex.<sup>[8](https://link.springer.com/article/10.1186/s12964-024-01910-5)</sup><sup> • </sup><sup>[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC7782431/)</sup> The reaction follows a branched pathway that can instead yield cleaved, inactive PAI-1 through hydrolysis of the acyl-enzyme intermediate.<sup>[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC7782431/)</sup>

Substrate specificity is not fixed. In the presence of either vitronectin or the glycosaminoglycan heparin, PAI-1 also efficiently inhibits thrombin, a protease outside its usual targets.<sup>[1](https://www.sciencedirect.com/science/article/pii/S0268949995800158)</sup>

## The latency transition and vitronectin

PAI-1 is unusual among serpins in existing in structurally and functionally distinct active, latent, and cleaved conformations.<sup>[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC4476021/)</sup> It is synthesized in an active but unstable state and converts spontaneously to the latent form, in which the reactive center loop inserts into the central β-sheet A; this transition is irreversible under physiological conditions.<sup>[8](https://link.springer.com/article/10.1186/s12964-024-01910-5)</sup><sup> • </sup><sup>[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC4476021/)</sup>

**Vitronectin binding is the physiological stabilizer.** Vitronectin, a multifunctional glycoprotein found in blood and the extracellular matrix, binds PAI-1 at its N-terminal somatomedin B (SMB) domain and prolongs the active half-life by roughly 50 to 100 percent.<sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC9731859/)</sup><sup> • </sup><sup>[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC4214104/)</sup> Structural work showed that the bound SMB domain sterically blocks the movement of strands 1 and 2A into the gap between helices E and F, a movement required for the latency transition; simulations also indicate that vitronectin suppresses mobility in the gate region and other parts of the molecule.<sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC9731859/)</sup> VN-bound PAI-1 is considered the physiologically relevant form for maintaining inhibitory function in human tissue.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC12499650/)</sup> Expansion of β-sheet A, whether during latency transition or during interaction with a plasminogen activator, causes vitronectin to dissociate from the now-inactive PAI-1.<sup>[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC7782431/)</sup>

Environmental factors also modulate stability. In a study of metal ions, magnesium, calcium, and manganese slightly stabilized the active conformation, whereas cobalt, nickel, and copper destabilized it; nickel reduced the half-life to about 5 minutes, while copper together with vitronectin extended it to 3 hours, compared with about 30 minutes with copper alone.<sup>[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC3048420/)</sup>

## Gene, regulation, and sources

The SERPINE1 gene spans 12.3 kb on chromosome 7 (7q21.3-q22) and contains eight introns and nine exons, encoding a 23-amino acid signal peptide and a mature protein of about 379 amino acids.<sup>[8](https://link.springer.com/article/10.1186/s12964-024-01910-5)</sup> Two mRNA transcripts of 2.3 and 3.2 kb are produced through alternative polyadenylation.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC12499650/)</sup>

Expression is regulated by numerous biochemical factors, and plasma levels vary with plasma lipids, body mass index, and insulin resistance; concentrations also differ by sex and ethnicity and show circadian variability, peaking in the early morning.<sup>[8](https://link.springer.com/article/10.1186/s12964-024-01910-5)</sup> PAI-1 is produced by endothelial cells, fibroblasts, smooth muscle cells, megakaryocytes, liver cells, adipose tissue, and macrophages, and is primarily stored in platelets.<sup>[8](https://link.springer.com/article/10.1186/s12964-024-01910-5)</sup>

A single nucleotide insertion/deletion polymorphism in the promoter at position −675 bp creates a sequence of four or five guanines (4G or 5G). Both alleles bind a transcription factor, but with the 5G allele a repressor protein can bind at an overlapping site and reduce transcription; the 4G allele and 4G/4G genotype are associated with increased plasma PAI-1 activity.<sup>[9](https://www.sciencedirect.com/science/article/abs/pii/S0049384801002924)</sup>

## Role in disease

**Cardiovascular and metabolic disease.** PAI-1 has well-described roles in clinical thrombosis, atherosclerosis, arteriosclerosis, and myocardial infarction.<sup>[3](https://www.sciencedirect.com/science/article/pii/S2950327226000069)</sup> Increased PAI-1 expression is considered an independent risk factor for cardiovascular disease.<sup>[9](https://www.sciencedirect.com/science/article/abs/pii/S0049384801002924)</sup> Obesity-related thrombosis is linked to decreased fibrinolytic activity, largely attributed to rising plasma PAI-1 antigen and activity, and PAI-1 has been interpreted as a component of the metabolic syndrome; PAI-1 activity is reduced by the antidiabetic drug metformin, with corresponding improvement in glycemic control.<sup>[14](https://www.frontiersin.org/articles/10.3389/fcvm.2021.653655/pdf)</sup> Elevated PAI-1 levels predict failure of fibrinolytic therapy and early re-occlusion after tPA-mediated lysis in myocardial infarction.<sup>[15](https://academic.oup.com/ehjacc/article/10/3/343/5959874)</sup>

**Bleeding from deficiency.** The clinical importance of PAI-1 is confirmed from both directions: at least three SERPINE1 mutations cause complete PAI-1 deficiency, in which absent functional protein allows plasminogen activators to dissolve clots prematurely, producing abnormal bleeding, menorrhagia, and bleeding complications in pregnancy and childbirth.<sup>[2](https://medlineplus.gov/genetics/gene/serpine1/)</sup>

**Cancer and senescence.** PAI-1 is expressed in many cancer cell types and modulates cancer growth, invasion, and angiogenesis; a meta-analysis of 25 case-control studies found the 4G/4G genotype associated with a modestly increased cancer risk (allele contrast 4G vs 5G: OR = 1.10, 95% CI 1.03–1.18).<sup>[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC3577655/)</sup> Recent work also identifies PAI-1 as a mediator and marker of cellular senescence: overexpression of PAI-1 alone can trigger proliferative senescence in fibroblasts even without p53, and in prematurely aging kl/kl mice, partial PAI-1 deficiency extended median survival nearly threefold and preserved telomere length.<sup>[8](https://link.springer.com/article/10.1186/s12964-024-01910-5)</sup>

## Therapeutic targeting

Despite its clinical significance, no FDA- or EMA-approved therapeutic specifically targets PAI-1.<sup>[10](https://pubmed.ncbi.nlm.nih.gov/42546744/)</sup> The complexity of PAI-1's molecular interactions, its instability, and its pleiotropic effects have complicated inhibitor development.<sup>[17](https://pmc.ncbi.nlm.nih.gov/articles/PMC11433585/)</sup>

Several small-molecule inhibitors have been characterized. Tiplaxtinin (PAI-039) is a selective active-site inhibitor that binds the active conformation and induces substrate behavior; it showed an IC<sub>50</sub> of 2.7 µM in vitro and prolonged occlusion time in rat carotid injury models, but evidence remains limited to small preclinical cohorts.<sup>[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC13078979/)</sup> The orally active compounds TM5275 and TM5441 block PAI-1 activity and downstream profibrotic signaling; TM5275 reduced liver fibrosis by about 50 percent of fibrotic area in rat models, though antitumor activity in xenografts was modest.<sup>[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC13078979/)</sup> The inhibitor TM5614 has entered phase II open-label combination trials with nivolumab in non-small cell lung cancer and with paclitaxel in angiosarcoma, and a placebo-controlled phase II trial of TM5614 is being conducted for interstitial lung disease associated with systemic sclerosis.<sup>[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC12859776/)</sup>

Inhibiting PAI-1 enhances endogenous fibrinolytic activity without directly affecting blood coagulation or platelet function, and serious or fatal bleeding has not been a reported issue in preclinical or clinical studies of PAI-1 inhibitors, in contrast to plasminogen activators themselves.<sup>[20](https://www.mdpi.com/1422-0067/22/6/3170/pdf?version=1616226592)</sup><sup> • </sup><sup>[17](https://pmc.ncbi.nlm.nih.gov/articles/PMC11433585/)</sup>

## Measurement

PAI-1 can be measured as antigen (including free active, inactive, and complexed forms) or as activity, but commercial methods show poor harmonization; a multicenter study of five plasma pools measured in 12 laboratories with seven different methods found poor agreement.<sup>[21](https://pmc.ncbi.nlm.nih.gov/articles/PMC5947570/)</sup> Global assays such as the euglobulin clot lysis time reflect the balance between t-PA and PAI-1 and can detect fibrinolysis shutdown caused by a sudden rise in plasma PAI-1, though when PAI-1 concentration is high the lysis time exceeds 6 hours and becomes impractical.<sup>[22](https://pmc.ncbi.nlm.nih.gov/articles/PMC10109242/)</sup><sup> • </sup><sup>[23](https://onlinelibrary.wiley.com/doi/full/10.1002/rth2.12293)</sup>

## References

1. [Structure of plasminogen activator inhibitor 1 (PAI-1) and its function in fibrinolysis: an update, Fibrinolysis (1995)](https://www.sciencedirect.com/science/article/pii/S0268949995800158)
2. [SERPINE1 gene, MedlinePlus Genetics, National Library of Medicine](https://medlineplus.gov/genetics/gene/serpine1/)
3. [A contemporary review of plasminogen activator inhibitor 1 in cardiovascular disease and broader disease state implications, Blood Vessels, Thrombosis & Hemostasis (2026)](https://www.sciencedirect.com/science/article/pii/S2950327226000069)
4. [The Plasminogen Activation System in Cell Invasion, Madame Curie Bioscience Database, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK6146/)
5. [A Contemporary Review of Plasminogen Activator Inhibitor Type 1: Structure, Function, Genetic Architecture, and Intracellular/Extracellular Roles, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12499650/)
6. [Functional Stability of Plasminogen Activator Inhibitor-1, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4214104/)
7. [Long-range allostery mediates the regulation of plasminogen activator inhibitor-1 by cell adhesion factor vitronectin, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC9731859/)
8. [A signaling pathway map of plasminogen activator inhibitor-1 (PAI-1/SERPINE-1), Cell Communication and Signaling (2024)](https://link.springer.com/article/10.1186/s12964-024-01910-5)
9. [Regulation of PAI-1 Expression by Genetic Polymorphisms: Impact on Atherogenesis, Thrombosis Research (2001)](https://www.sciencedirect.com/science/article/abs/pii/S0049384801002924)
10. [PAI-1 as a Therapeutic Target: A State-of-the-Art Review, Seminars in Thrombosis and Hemostasis (2026)](https://pubmed.ncbi.nlm.nih.gov/42546744/)
11. [Targeting PAI-1 in Cardiovascular Disease: Structural Insights Into PAI-1 Functionality and Inhibition, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7782431/)
12. [Chemical Antagonists of Plasminogen Activator Inhibitor-1: Mechanisms of Action and Therapeutic Potential in Vascular Disease, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4476021/)
13. [Metals affect the structure and activity of human plasminogen activator inhibitor-1, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3048420/)
14. [PAI-1's Central Function in Thromboinflammation and Cardiovascular Disease, Frontiers in Cardiovascular Medicine (2021)](https://www.frontiersin.org/articles/10.3389/fcvm.2021.653655/pdf)
15. [Biomarkers of coagulation and fibrinolysis in acute myocardial infarction, European Heart Journal: Acute Cardiovascular Care (2021)](https://academic.oup.com/ehjacc/article/10/3/343/5959874)
16. [PAI-1 4G/5G Polymorphism Contributes to Cancer Susceptibility: Evidence from Meta-Analysis, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3577655/)
17. [Targeting Fibrinolytic Inhibition for Venous Thromboembolism Treatment, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11433585/)
18. [Plasminogen activator inhibitor-1 as an oncologic target: biology, therapeutic inhibitors, and clinical translation, Frontiers in Oncology (2026)](https://pmc.ncbi.nlm.nih.gov/articles/PMC13078979/)
19. [Overview: PAI-1 inhibitors and clinical applications, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12859776/)
20. [PAI-1 in Diabetes: Pathophysiology and Role as a Therapeutic Target, International Journal of Molecular Sciences (2021)](https://www.mdpi.com/1422-0067/22/6/3170/pdf?version=1616226592)
21. [Measuring fibrinolysis: from research to routine diagnostic assays, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5947570/)
22. [Assays to quantify fibrinolysis: strengths and limitations, ISTH SSC Subcommittee on Fibrinolysis, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10109242/)
23. [Assessing plasminogen activation potential with global fibrinolytic assays, Research and Practice in Thrombosis and Haemostasis (2020)](https://onlinelibrary.wiley.com/doi/full/10.1002/rth2.12293)

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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Enzyme classes and activities › Proteolytic and peptidase enzymes › Protease regulation and inhibitors › Serpins and serpinopathies-as-molecules*

*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*

*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*

License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license