|
An ionophore is a chemical species that reversibly binds ions. Many ionophores are lipid-soluble entities that transport ions across a cell membrane. Ionophore means "ion carriers" as these compounds catalyze ion transport across hydrophobic membranes such as liquid polymeric membranes (carrier-based ion selective electrodes) or lipid bilayer - in the living cell or synthetic vesicles (liposomes) Some ionophores are synthesized by microorganisms to import ions into their cells. Synthetic ion carriers have also been prepared. Ionophores selective for cations and anions have found many applications in analysis. The two broad classifications of ionophores synthesized by microorganisms are: # Carrier ionophores that bind to a particular ion and shield its charge from the surrounding environment. This makes it easier for the ion to pass through the hydrophobic interior of the lipid membrane. An example of a carrier ionophore is valinomycin, a molecule that transports a single potassium cation. Carrier ionophores may be proteins or other molecules. # Channel formers that introduce a hydrophilic pore into the membrane, allowing ions to pass through without coming into contact with the membrane's hydrophobic interior. An example of a channel former is gramicidin A. Channel forming ionophores are usually large proteins. ==Mechanism of action of biologically relevant ionophores== Transmembrane ion concentration gradients (membrane potential) are required for living organisms. Ionophores can disrupt the membrane potential by conducting ions through a lipid membrane in the absence of a protein pore, and thus could exhibit cytotoxic properties. They are produced naturally by a variety of microbes and act as a defense against competing microbes. Many antibiotics, particularly the macrolide antibiotics, are ionophores. Some exhibit high affinities for Na+, others high affinities for K+. The structure of the sodium and potassium complexes of antibiotics have been verified by X-ray crystallography. Ionophores have been used to modify the permeability of biological membranes toward certain ions. Additionally, some ionophores are used as antibiotics and/or as growth-enhancing feed additives for certain animals, such as cattle (see monensin) and chickens.〔The U.S. Department of Agriculture sent a letter to Tyson Foods to remove labels for chickens that said "raised without antibiotics" because of the use of ionophores in their feed. 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ionophore」の詳細全文を読む スポンサード リンク
|