ionophore

n.
Any of a group of organic compounds that facilitate the transport of ions across the cell membrane.

A substance that can transfer ions from a hydrophilic medium, such as water, into a hydrophobic medium, such as hexane or a biological membrane, where the ions typically would not be soluble; also known as an ion carrier. The ions transferred are usually metal ions, for example, lithium (Li⁺), sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺), and calcium (Ca²⁺); but there are ionophores that promote the transfer of other ions, such as ammonium ion (NH₄⁺) or amines of biological interest. See also Ion.

There are two mechanisms by which ionophores promote the transfer of ions across hydrophobic barriers: ion-ionophore complex formation and ion channel formation. In complex formation, the ion forms a coordination complex with the ionophore in which there is a well-defined ratio (typically 1:1) of ion to ionophore. The ionophore wraps around the ion so that the ion exists in the polar interior of the complex, while the exterior is predominantly hydrophobic in character and as such is soluble in nonpolar media. The ion is coordinated by oxygen atoms present in the ionophore molecule through iondipole interactions. The ionophore molecule essentially acts as the solvent for the ion, replacing the aqueous solvation shell that normally surrounds the ion. See also Coordination complexes.

Ionophores that act via ion channel formation are found in biological environments. The molecule forms a polar channel in an otherwise nonpolar cell membrane, allowing passage of small ions either into or out of the cell.

Naturally occurring ionophores fall into four classes, each of which has antibiotic activity: peptide, cyclic depsipeptide, macrotetrolide, and polyether ionophores. The biological activity of ionophore antibiotics is due to their ability to disrupt the flow of ions either into or out of cells. Under normal conditions, cells have a high internal concentration of potassium ions but a low concentration of sodium ions. The concentration of ions in the extracellular medium is just the reverse, high in sodium ions but low in potassium ions. This imbalance, which is necessary for normal cell function, is maintained by a specific transport protein (sodium-potassium adenosine triphosphatase) in the cell membrane that pumps sodium ions out of the cell in exchange for potassium ions. Ionophore antibiotics can disrupt this ionic imbalance by allowing ions to penetrate the cell membrane as ion-ionophore complexes or via the formation of ion channels. Gram-positive bacteria appear to be particularly sensitive to the effect of ionophores perturbing normal ion transport.

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Any molecule, as of a drug, that increases the permeability of cell membranes to a specific ion.

• i. antibiotic — monocarboxylic polyether antibiotics. They may be monovalent (e.g. monensin, salinomycin) or divalent (e.g. lasolocid). They facilitate transport of ions across biological membranes by forming lipid-soluble complexes with mono or divalent cations; latter day additions to the list include narasin, maduramycin.
• i. poisoning — see monensin.

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