# Ornithine decarboxylase
**Ornithine decarboxylase** (ODC; EC 4.1.1.17), also known as ornithine carboxy-lyase, is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the decarboxylation of L-ornithine to putrescine, the first and, in animal cells, rate-limiting step in the biosynthesis of polyamines[1](https://www.ncbi.nlm.nih.gov/mesh/68009955) • [2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full). Polyamines (putrescine, spermidine, and spermine) are low-molecular-mass aliphatic polycations essential for fundamental cellular processes including cell growth and proliferation[3](https://www.sciencedirect.com/science/article/pii/S1470872816001478). Because rapidly dividing cells require high polyamine flux, ODC is a long-standing drug target in cancer chemoprevention and in the treatment of parasitic infections[4](https://pubmed.ncbi.nlm.nih.gov/1438532/).
| Key fact | Detail |
|---|---|
| Reaction | Decarboxylation of L-ornithine to putrescine via Schiff base formation with the PLP cofactor[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full) |
| Classification | EC 4.1.1.17, a carbon-carbon lyase (carboxy-lyase); MeSH Unique ID D009955[1](https://www.ncbi.nlm.nih.gov/mesh/68009955) |
| Quaternary structure | Active as a homodimer in dynamic equilibrium with inactive monomers; the two active sites sit at the dimer interface[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full) |
| Key residues | Lys69 binds PLP covalently; Cys360 forms the covalent adduct with the inhibitor DFMO[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full) |
| Degradation | Ubiquitin-independent proteasomal degradation directed by antizyme, whose own translation requires polyamine-stimulated ribosomal frameshifting[5](https://www.jstage.jst.go.jp/article/jsnfs1983/49/1/49_1_1/_article/-char/en) |
| Principal inhibitor | α-difluoromethylornithine (DFMO, eflornithine), an enzyme-activated irreversible inhibitor on the WHO Model List of Essential Medicines for sleeping sickness[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full) |
| Disease link | Elevated ODC expression and polyamine levels correlate with cancer; ODC is considered an oncogenic enzyme[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full) |
## Position in polyamine metabolism
ODC initiates the polyamine biosynthetic pathway. The putrescine it produces is the precursor of the higher polyamines: spermidine synthase links putrescine to an aminopropyl group donated by decarboxylated S-adenosylmethionine (produced by S-adenosylmethionine decarboxylase) to form spermidine, and spermine synthase extends spermidine to spermine[1](https://www.ncbi.nlm.nih.gov/mesh/68009955) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC4027058/). In bacteria and plants an alternative route to putrescine exists through [arginine decarboxylase](https://www.edgepedia.org/arginine-decarboxylase)[7](https://www.sciencedirect.com/science/article/abs/pii/S0981942810000331). Because ODC catalyzes the rate-limiting step in animal cells, cellular polyamine pools depend heavily on how much active ODC is present, which the cell controls at several levels[7](https://www.sciencedirect.com/science/article/abs/pii/S0981942810000331).
## Structure and catalytic mechanism
ODC functions only as a homodimer, with the two monomers interacting weakly; the dimer exists in equilibrium with inactive monomers[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full) • [8](https://onlinelibrary.wiley.com/doi/10.1002/cbdv.202302067). Each of the two identical active sites is assembled from residues contributed by different subunits: Lys69 from the N-terminal domain of one subunit is the covalent binding site for PLP, while Cys360 from the C-terminal domain of the other subunit participates in catalysis and in covalent adduct formation with DFMO[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full). Other active-site residues support catalysis: Glu274 stabilizes the positive charge on the pyridine nitrogen of PLP, and Arg277 binds the cofactor's 5'-phosphate[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full).
Catalysis follows the standard mechanism of PLP-dependent decarboxylases. The substrate ornithine displaces the internal aldimine that PLP forms with the enzyme's lysine, generating an external [Schiff base](https://www.edgepedia.org/schiff-base); electron withdrawal by the PLP ring then facilitates loss of the substrate's carboxyl group as CO₂, releasing putrescine[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full) • [9](https://feradical.utsa.edu/labdoc/BO12_Decarboxylation.pdf).
## Regulation by antizyme
**Rapid turnover.** ODC turns over unusually fast and sits at the center of a negative feedback loop: polyamines accelerate ODC degradation and induce an ODC-inhibitory protein, antizyme[5](https://www.jstage.jst.go.jp/article/jsnfs1983/49/1/49_1_1/_article/-char/en). Antizyme binds strongly to ODC monomers, preventing their reassociation into the active homodimer and directing the enzyme to the 26S proteasome[3](https://www.sciencedirect.com/science/article/pii/S1470872816001478) • [8](https://onlinelibrary.wiley.com/doi/10.1002/cbdv.202302067).
ODC is one of a small set of proteasome substrates that are degraded without ubiquitination; antizyme substitutes for the ubiquitin tag in recruiting the proteasome[10](https://www.nature.com/articles/35056508). [In vitro](https://www.edgepedia.org/in-vitro) systems showed the degradation is ATP- and antizyme-dependent and proceeds without ubiquitin[5](https://www.jstage.jst.go.jp/article/jsnfs1983/49/1/49_1_1/_article/-char/en). Antizyme itself escapes proteasomal destruction, so even low levels catalyze repeated rounds of ODC degradation, and polyamines additionally stabilize antizyme by protecting it from ubiquitin-dependent proteolysis[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC5349138/).
**Frameshifting sensor.** Antizyme expression is itself polyamine-gated: its mRNA is translated only after a programmed ribosomal frameshifting event evoked by polyamines, making antizyme a sensor of free intracellular polyamine pools[5](https://www.jstage.jst.go.jp/article/jsnfs1983/49/1/49_1_1/_article/-char/en) • [3](https://www.sciencedirect.com/science/article/pii/S1470872816001478). Spermidine and spermine stimulate antizyme mRNA translation more effectively than putrescine[8](https://onlinelibrary.wiley.com/doi/10.1002/cbdv.202302067). Antizyme also inhibits polyamine uptake, closing the loop from both the synthetic and the import side[3](https://www.sciencedirect.com/science/article/pii/S1470872816001478).
**Antizyme inhibitor.** A counterweight exists in antizyme inhibitor (AzI), a protein highly homologous to ODC but lacking ornithine-decarboxylating activity. AzI binds antizyme more tightly than ODC does, pulling antizyme off ODC and thereby stabilizing the enzyme[3](https://www.sciencedirect.com/science/article/pii/S1470872816001478) • [12](https://pubmed.ncbi.nlm.nih.gov/17371267/). A second homolog, antizyme inhibitor 2, similarly rescues ODC from degradation[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC11115672/).
Regulation also operates upstream of the protein. ODC levels are controlled at transcription, where the ODC gene is a target of the Myc/Max transcription factor family, and at translation of ODC mRNA[14](https://www.jbc.org/article/S0021925820723926/pdf) • [15](https://www.jbc.org/article/S0021925820311832/pdf).
## ODC and cancer
Polyamines are required for proliferation, and elevated ODC expression and polyamine levels correlate with cancer and other diseases[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full). Mouse genetics supports a causal role: transgenic mice expressing high ODC levels are more susceptible to tumor development, while mice with reduced ODC, from loss of one ODC allele or elevated antizyme expression, are less susceptible[14](https://www.jbc.org/article/S0021925820723926/pdf). In neuroblastoma, tumors overexpressing ODC1 show inferior survival compared with low-ODC1 tumors even after stratification by MYCN status[16](https://www.accessdata.fda.gov/drugsatfda_docs/nda/2024/215500Orig1s000MultidisciplineR.pdf).
Depleting polyamines produces a cytostatic growth response[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC4027058/). In a mouse model, combining ODC and polyamine oxidase inhibitors, gut decontamination antibiotics, and a polyamine-deficient diet reduced Lewis lung carcinoma growth rate by more than 80% and prevented 70 to 100% of lung metastases, with the effect accompanied by lower tissue polyamine concentrations and reduced tumor cell proliferation and protein synthesis[17](https://aacrjournals.org/cancerres/article/50/16/5077/495587/Endogenous-and-Exogenous-Polyamines-in-Support-of). Natural-product work has also yielded allosteric inhibitors: herbacetin suppressed HCT116 xenograft tumor growth and reduced polyp number and size in an ApcMin/+ mouse model of colon cancer[18](https://aacrjournals.org/cancerres/article/76/5/1146/616096/Herbacetin-Is-a-Novel-Allosteric-Inhibitor-of).
## Inhibitors and therapeutic use
Interest in ODC inhibition as therapy has been widespread since the late 1970s and 1980s, covering cancer and chemoprevention, autoimmune disease, ischemia, hearing loss, [African trypanosomiasis](https://www.edgepedia.org/african-trypanosomiasis), and opportunistic infections in AIDS[4](https://pubmed.ncbi.nlm.nih.gov/1438532/).
**DFMO (eflornithine).** α-difluoromethylornithine, an ornithine analog, is the most important and best-known ODC inhibitor[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full). It is accepted by the enzyme as a substrate and forms an irreversible decarboxylated adduct: Cys360 displaces one of DFMO's fluorines, deactivating the enzyme[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC11912471/) • [14](https://www.jbc.org/article/S0021925820723926/pdf). The compound is water-soluble with a significant safety record but has low ODC affinity and rapid clearance, so it requires high dosing to reach effective exposures[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC11912471/). The World Health Organization recommends DFMO with nifurtimox for human African trypanosomiasis (sleeping sickness), and it is on the [WHO Model List of Essential Medicines](https://www.edgepedia.org/who-model-list-of-essential-medicines)[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full). DFMO has also reached FDA approval as an intravenous formulation for West African sleeping sickness and as a topical formulation for excessive facial hair growth (hirsutism)[15](https://www.jbc.org/article/S0021925820311832/pdf). In 2024, an FDA review examined eflornithine tablets (IWILFIN) proposed to reduce the risk of relapse in pediatric patients with high-risk neuroblastoma after multimodality therapy[16](https://www.accessdata.fda.gov/drugsatfda_docs/nda/2024/215500Orig1s000MultidisciplineR.pdf).
**APA.** 1-amino-oxy-3-aminopropane, first described in 1985, is the most potent ODC inhibitor, with a Ki of 3.2 nM against homogeneous mouse kidney ODC[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC11912471/). A crystal structure of APA-bound ODC solved at 2.49 Å resolution showed a covalent oxime between APA and PLP in the catalytic site, confirmed in solution by mass spectrometry; the stable oxime cannot be catabolized, explaining the inhibitor's high potency[20](https://pmc.ncbi.nlm.nih.gov/articles/PMC9125680/).
## Measuring ODC activity
Assays for ODC activity fall into several families, each with trade-offs for screening[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full):
- **Radiolabeling** with [1-¹⁴C]-L-ornithine captures released ¹⁴CO₂ on hyamine/sodium hydroxide-impregnated discs for liquid scintillation counting; specific activity is expressed as nmol CO₂ released/min/mg protein. It monitors reaction course but is expensive for high-throughput screening[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full).
- **Coupled-enzyme CO₂ detection (PEPC-MDH)** converts released CO₂ into an NADH absorbance change at 340 nm; it runs in real time in 384-well plates and was used to screen a library of 316,114 compounds against T. brucei ODC, identifying inhibitor classes that bind outside the active site[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full).
- **Putrescine oxidation** detects H₂O₂ released by amine oxidase action on the product, colorimetrically at 505 nm or by chemiluminescence down to picomolar putrescine detection[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full).
- **Chromatographic and spectrometric methods** (HPLC with putrescine derivatization, mass spectrometry with fmol-range detection) offer specificity and sensitivity but assess only the reaction end point and require laborious sample preparation[2](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full).
## References
1. [Ornithine Decarboxylase, Medical Subject Headings (MeSH), NCBI](https://www.ncbi.nlm.nih.gov/mesh/68009955)
2. [Tinoco LW et al. (2022). Analytical assays to evaluate enzymatic activity and screening of inhibitors for ornithine decarboxylase. Frontiers in Analytical Science.](https://www.frontiersin.org/journals/analytical-science/articles/10.3389/frans.2022.1018080/full)
3. [Pegg AE (2016). Protein degradation, the main hub in the regulation of cellular polyamines. Biochemical Journal 473:4551-4558.](https://www.sciencedirect.com/science/article/pii/S1470872816001478)
4. [McCann PP, Pegg AE (1992). Ornithine decarboxylase as an enzyme target for therapy. Pharmacology & Therapeutics 54:195-215.](https://pubmed.ncbi.nlm.nih.gov/1438532/)
5. [Hayashi S (1996). A Track Record of Studies on the Regulatory Mechanism of Ornithine Decarboxylase. Nippon Eiyo Shokuryo Gakkaishi 49:1-11.](https://www.jstage.jst.go.jp/article/jsnfs1983/49/1/49_1_1/_article/-char/en)
6. [Gerner EW, Meyskens FL. Polyamines and cancer: implications for chemoprevention and chemotherapy. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC4027058/)
7. [Wallace HM. The early history of polyamine research. ScienceDirect.](https://www.sciencedirect.com/science/article/abs/pii/S0981942810000331)
8. [Sahu et al. (2024). Preventing Cancer by Inhibiting Ornithine Decarboxylase: A Comparative Perspective on Synthetic vs. Natural Drugs. Chemistry & Biodiversity.](https://onlinelibrary.wiley.com/doi/10.1002/cbdv.202302067)
9. [Decarboxylation mechanisms in biological system. UTSA.](https://feradical.utsa.edu/labdoc/BO12_Decarboxylation.pdf)
10. [Coffino P (2001). Regulation of cellular polyamines by antizyme. Nature Reviews Molecular Cell Biology 2:188-194.](https://www.nature.com/articles/35056508)
11. [A safety catch for ornithine decarboxylase degradation. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC5349138/)
12. [Keren-Paz A, Bercovich Z, Kahana C (2007). Antizyme inhibitor. Biochemical Society Transactions 35:311-313.](https://pubmed.ncbi.nlm.nih.gov/17371267/)
13. [Antizyme and antizyme inhibitor, a regulatory tango. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC11115672/)
14. [Pegg AE (2006). Regulation of Ornithine Decarboxylase. Journal of Biological Chemistry.](https://www.jbc.org/article/S0021925820723926/pdf)
15. [Polyamine synthesis as a target of MYC oncogenes. Journal of Biological Chemistry.](https://www.jbc.org/article/S0021925820311832/pdf)
16. [FDA Multi-discipline Review, NDA 215500, eflornithine tablets (IWILFIN), 2024.](https://www.accessdata.fda.gov/drugsatfda_docs/nda/2024/215500Orig1s000MultidisciplineR.pdf)
17. [Endogenous and Exogenous Polyamines in Support of Tumor Growth. Cancer Research 50:5077-5083 (1990).](https://aacrjournals.org/cancerres/article/50/16/5077/495587/Endogenous-and-Exogenous-Polyamines-in-Support-of)
18. [Herbacetin Is a Novel Allosteric Inhibitor of Ornithine Decarboxylase with Antitumor Activity. Cancer Research 76:1146 (2016).](https://aacrjournals.org/cancerres/article/76/5/1146/616096/Herbacetin-Is-a-Novel-Allosteric-Inhibitor-of)
19. [Design, Synthesis, and Biological Activity of Novel Ornithine Decarboxylase (ODC) Inhibitors. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC11912471/)
20. [Structural basis of binding and inhibition of ornithine decarboxylase by 1-amino-oxy-3-aminopropane. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC9125680/)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolism and metabolic pathways › Amino acid and nitrogen metabolism › Polyamine and decarboxylated-amino-acid metabolism › Polyamine biosynthesis*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license