# Ionophore
An ionophore (from Greek, "ion bearer") is a compound that can carry specific ions through the membranes of cells or organelles, a definition formalized by IUPAC in its glossary of bioinorganic chemistry[1](https://goldbook.iupac.org/terms/view/IT06772). The term was first used in 1967 to describe a molecule's ability to bind a metal ion and facilitate its transport across cellular membranes[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4896753/). Many ionophores are small, lipid-soluble molecules of microbial origin, and several are antibiotics; by disrupting the ionic gradients a cell depends on, they can kill bacteria, fungi, and protozoa[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/). Others are synthetic and serve as research tools or as the active components of [ion-selective electrodes](https://www.edgepedia.org/ion-selective-electrode)[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/).
| Key fact | Detail |
|---|---|
| Definition | A compound that can carry specific ions through membranes of cells or organelles (IUPAC)[1](https://goldbook.iupac.org/terms/view/IT06772) |
| Term coined | 1967, for molecules that bind a metal ion and facilitate its transport across cellular membranes[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4896753/) |
| Two basic classes | Mobile carriers, which shuttle one ion at a time, and channel formers, which pass many ions simultaneously[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/) |
| Transport speed | A single ion channel can pass upwards of 1 × 109 ions/sec, about three orders of magnitude faster than a mobile carrier[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC2849168/) |
| Natural diversity | Over 120 naturally occurring ionophores known, mostly polyether antibiotics from Streptomyces spp.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/) |
| Major commercial use | Control of coccidiosis and growth promotion in ruminants; six ionophores had combined yearly sales above $150 million in 2003[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/) |
| Classic example | Valinomycin, a K+ carrier whose binding constant for K+ is 106 versus only 10 for Na+ • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/) |
## Classes and transport mechanisms
Ionophores fall into two basic classes that differ in how they move ions across a lipid bilayer[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/).
**Mobile carriers** bind an ion on one side of the membrane, dissolve in and diffuse across the bilayer, and release the ion on the other side. They carry one ion at a time, so their flux is limited by the rate of transmembrane diffusion, but they are often highly selective[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC2849168/). **Channel formers** are long-lasting, stationary structures that span the membrane and allow many ions at a time to flow rapidly through a hydrophilic pore; a single channel can pass upwards of 1 × 109 ions/sec, roughly a thousand times faster than a carrier[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC2849168/). Gramicidin and nystatin are channel-forming examples, while [valinomycin](https://www.edgepedia.org/valinomycin) is the textbook carrier[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/) • [6](https://openmd.com/define/ionophore).
A third grouping, used in reviews of natural products, distinguishes mobile carriers, quasi-ionophores that form transmembrane hydrophilic channels, and neutral ionophores that facilitate cation diffusion according to the membrane potential[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4896753/).
## Valinomycin and ion selectivity
**Valinomycin** is the most studied carrier ionophore. It is a cyclic dodecadepsipeptide produced by [Streptomyces](https://www.edgepedia.org/streptomyces) fulvissimus, built from three repeats of alternating L- and D-amino and hydroxy acids (L-Val, D-α-hydroxyvaleric acid, D-Val, and L-lactic acid), and it prefers larger cations such as K+ and Rb+ • [7](https://journals.physiology.org/doi/10.1152/physrev.00008.2006). When it binds K+, six carbonyl oxygens from the hydroxy-acid residues point inward to form the ion-binding cavity while the amino- and hydroxy-acid side chains point outward, creating a hydrophobic exterior that lets the complex dissolve in the membrane[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC2390915/). The binding constant for K+-valinomycin is 106 while that for Na+ is only 10, a selectivity difference of five orders of magnitude[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/).
The physical basis of this selectivity has been examined by quantum chemical calculations. Valinomycin achieves K+/Na+ discrimination with only six carbonyl ligands because its cavity-size constraints, enforced by a combination of intramolecular hydrogen bonds, its specific ring size, and the spacing between connected ligands, physically prevent the ligands from collapsing onto the smaller Na+ ion[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC2390915/). When the six intramolecular hydrogen bonds are chemically disabled, the free energy difference between the Na+ and K+ complexes rises from 12.3 to 17.8 kcal/mol, showing that the hydrogen bonds make the molecule less flexible and prevent an energetically more stable configuration around Na+ • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC2390915/). This mechanism differs from that of potassium channels, which rely on over-coordination by eight carbonyl ligands in a hydrogen-bond-donor-free local environment rather than on a fixed cavity size[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC2390915/).
Valinomycin was first recognized as a potassium ionophore by Bernard Pressman, an American biochemist, in the early 1960s, when he reported that the known antibiotic stimulated K+ uptake and H+ efflux from mitochondria[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/). By dissipating essential transmembrane electrochemical gradients it causes metabolic upheaval in microorganisms, which is why it was recognized as an antibiotic long before its transport function was understood[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/).
## Polyether (carboxylic) ionophores
**Polyether ionophores** are the largest natural group, with over 120 naturally occurring members, produced mainly by Streptomyces spp. and related [Gram-positive bacteria](https://www.edgepedia.org/gram-positive-bacteria)[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/). They are relatively high-molecular-weight lipid-soluble compounds, 500 to 1,000 amu, with a lipophilic alkyl exterior and an oxygen-rich internal cavity that binds cations such as K+, Na+, and Ca2+ • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4896753/). Complexation typically wraps the molecule into a pseudocyclic structure stabilized by intramolecular hydrogen bonds, rendering the whole complex lipophilic[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/).
Their transport mechanism is an electroneutral exchange of protons and cations: the deprotonated carboxylate anion (I–COO−) binds either a metal cation, giving a neutral salt (I–COO−M+), or a proton, regenerating the neutral acid (I–COOH), and only these uncharged species cross the membrane[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/). Monensin, the best-known member, facilitates a transmembrane exchange of principally Na+ for H+; the resulting neutralization of acidic intracellular compartments such as the trans-Golgi cisternae, lysosomes, and some endosomes disrupts intracellular traffic and contributes to its toxicity[9](https://pubmed.ncbi.nlm.nih.gov/2160275/).
The consequence for bacteria is a collapse of the Na+/K+ gradient and an increase in intracellular osmotic pressure, causing swelling, vacuolization, and cell death[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/). Selectivity follows cage size: only cations of the right radius fit the cavity perfectly, larger ones deform it, and smaller ones find a nonoptimal coordination geometry[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/).
Ionophores are most effective against Gram-positive bacteria, whose porous peptidoglycan layer lets the lipophilic molecule reach the cytoplasmic membrane. [Gram-negative bacteria](https://www.edgepedia.org/gram-negative-bacteria) are shielded by a lipopolysaccharide layer, an outer membrane, and the periplasmic space; [Escherichia coli](https://www.edgepedia.org/escherichia-coli) is insensitive to ionophores unless the outer membrane is removed[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/).
## Calcium ionophores
A23187 (calcimycin) and ionomycin are lipid-soluble carboxylic acid antibiotics that transport divalent but not monovalent cations, and they are the standard tools for raising cytosolic Ca2+ in experiments[10](https://www.ncbi.nlm.nih.gov/books/NBK531424/?report=printable) • [11](https://pmc.ncbi.nlm.nih.gov/articles/PMC5983822/). They differ in stoichiometry: ionomycin can lose two acidic hydrogens and, as a dianion, forms an uncharged 1:1 complex with a divalent ion such as Ca2+ or Mg2+, whereas Br-A23187 loses one acidic hydrogen and forms an uncharged 2:1 complex, so two molecules of Br-A23187 are needed to carry a single Ca2+ • [12](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/ionomycin).
Concentration determines where these ionophores act. Below about 1 μM, ionomycin appears to selectively partition into intracellular membranes and release intracellular Ca2+ stores (ED50 = 50 nM) without greatly increasing plasma membrane permeability; at 1 to 10 μM it increases the permeability of the plasma membrane, endoplasmic reticulum, and mitochondria to Ca2+ • [10](https://www.ncbi.nlm.nih.gov/books/NBK531424/?report=printable) • [13](https://pmc.ncbi.nlm.nih.gov/articles/PMC2643845/). The electroneutral calcium/proton antiport ionophores ionomycin and A23187 have been used in more than 20,000 publications to raise cytoplasmic free calcium, but they show little membrane selectivity, and only a narrow concentration range enhances cytoplasmic calcium without damaging mitochondrial function[14](https://pmc.ncbi.nlm.nih.gov/articles/PMC3076726/).
## Applications
**Animal health and production** is the largest commercial use. In animal care, the ionophores employed are monensin, lasalocid, salinomycin, narasin, maduramicin, semduramicin, and laidlomycin propionate; of these, monensin sees especially broad use in cattle raised for beef and milk, being added to feed to guard against coccidiosis and make feed use more efficient.[15](https://www.merckvetmanual.com/pharmacology/antibacterial-agents/ionophores-use-in-animals) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/) In 2003, ionophores were the antimicrobials most commonly used in beef cattle production, with lasalocid, monensin, salinomycin, narasin, maduramycin, and laidlomycin propionate totaling more than $150 million in yearly sales[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/). In October 2004, the FDA cleared monensin as a means of improving how efficiently dairy cows produce milk when fed via total mixed ration, and it extended this approval to component feeding setups in December 2005.[16](https://www.fda.gov/animal-veterinary/frequently-asked-questions-about-animal-drugs/questions-and-answers-proper-use-monensin-dairy-cows)
**Analytical chemistry** uses ionophores as the selective agents in ion-selective electrodes, where the target ion is extracted into a polymeric membrane through coordination with an incorporated ionophore; valinomycin is the standard potassium ionophore in such membranes[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/) • [17](https://pmc.ncbi.nlm.nih.gov/articles/PMC9699433/).
**Cell biology** relies on ionophores as experimental tools. Ionomycin depletes endoplasmic reticulum Ca2+ pools as an alternative to SERCA pump inhibition, and protonophores such as 2,4-dinitrophenol (DNP), a synthetic lipid-soluble H+ carrier, are used to dissipate proton gradients across bioenergetic membranes[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC2643845/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/). DNP's uncoupling of electron transport from ATP synthesis was an essential component in support of Peter Mitchell's chemiosmotic hypothesis[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/).
## Toxicity
Ionophore toxicity follows directly from their mechanism of action, the disruption of normal ionic gradients[15](https://www.merckvetmanual.com/pharmacology/antibacterial-agents/ionophores-use-in-animals). Species sensitivity varies sharply: monensin medicated cattle feed is safe for cattle only, and ingestion of monensin by horses has been fatal, so feed labels prohibit access by horses and other equines[18](https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadBBL/8339). Mixing errors producing high monensin concentrations have also been fatal to cattle[18](https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadBBL/8339). In humans, DNP illustrates the same principle at the whole-organism level: it was used to treat obesity from 1933 to 1938, and doses of 20 to 50 mg/kg can be lethal, with death caused by a dangerous rise in body temperature as uncoupled electron transport releases energy as heat[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/).
## References
1. [IUPAC Gold Book: ionophore](https://goldbook.iupac.org/terms/view/IT06772)
2. [Polyether ionophores: broad-spectrum and promising biologically active molecules (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4896753/)
3. [Membrane Transport, Chapter 19, Section 5: Ionophores (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/)
4. [Rutkowski J, Brzezinski B (2013). Structures and Properties of Naturally Occurring Polyether Antibiotics (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613094/)
5. [Recent Advances in Synthetic Membrane Transporters (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2849168/)
6. [ionophore, OpenMD definition aggregator](https://openmd.com/define/ionophore)
7. [Page MJ, Di Cera E (2006). Role of Na+ and K+ in Enzyme Function, Physiological Reviews](https://journals.physiology.org/doi/10.1152/physrev.00008.2006)
8. [Varma S, Rempe SB (2008). K+/Na+ Selectivity in K-channels and Valinomycin (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2390915/)
9. [Mollenhauer HH et al. (1990). Alteration of intracellular traffic by monensin, PubMed](https://pubmed.ncbi.nlm.nih.gov/2160275/)
10. [Pharmacology of Store-Operated Calcium Entry Channels, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK531424/?report=printable)
11. [Pharmacological Strategies for Manipulating Plant Ca2+ Signalling (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC5983822/)
12. [Ionomycin, ScienceDirect Topics](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/ionomycin)
13. [Methods for Studying Store-Operated Calcium Entry (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2643845/)
14. [Assessing mitochondrial dysfunction in cells (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3076726/)
15. [Ionophores Use in Animals, Merck Veterinary Manual](https://www.merckvetmanual.com/pharmacology/antibacterial-agents/ionophores-use-in-animals)
16. [Questions and Answers on the Proper Use of Monensin in Dairy Cows, FDA](https://www.fda.gov/animal-veterinary/frequently-asked-questions-about-animal-drugs/questions-and-answers-proper-use-monensin-dairy-cows)
17. [Voltammetric Ion Sensing with Ionophore-Based Ion-Selective Electrodes (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9699433/)
18. [Monensin Medicated Feedlot Cattle Feed, Type C Medicated Feed, FDA](https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadBBL/8339)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Organic reactions, structure and reference › Organic polymer classes › Polyethers and polyolefins › Polyether natural products and toxins*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license