Ionophores

The lipid bilayer of natural membranes presents a complete barrier to the free diffusion of inorganic ions which, being strongly hydrated, lack lipophilicity. However, ionophores are usually on hand to facilitate this transport. There are two kinds of ionophore, the mobile and the stationary. The latter ( ion pumps ) consist of a water-filled channel spanning the bilayer. Much of what we know of ionophores has been learnt from the mobile type, derived from microbes (e.g. valinomycin, gramicidin). Such foreign ionophores, if effective, are toxic to mammalian cells except in low doses. 

Several fungal metabolites have been found which enhance the passive uptake of potassium. Some, like nonactin and monactin 14.6), are macro-tetrolides others, like valinomycin, are depsipeptides. Valinomycin 14.7) (from Streptomycesfulvissimus) is a macrocycle composed of three residues of each of L-valine, D-valine, L-lactic acid, and D-a-hydroxy-isovaleric acid, linked alternatively by ester and amide bonds to form a 36-membered ring (Shemyakin 

Valinomycin assists potassium ions to penetrate erythrocytes, mitochondria (Moore and Pressman, 1964), and bacterial plasma membranes (Harold and Baarda, 1967) but has little effect on the permeability of sodium, lithium, or hydrogen ions. Nonactin acts similarly, but monactin also slightly assists the permeability of sodium (Henderson, McGivan and Chappell, 1969). Valinomycin is active against cancer in mice (Carter, Sakurai and Umezawa, 1981). 

Of greater interest, because it may indicate how potassium pores are formed and controlled in mammalian physiology, is alamethicin, a linear peptide from the fungus Trichoderma viride. It has 20 peptide-linked components which, except for the terminal L-phenylalaninol, are all amino acids eight of them are a-aminoisobutyric acid residues, and the others are normal protein constituents. X-ray crystal structure analysis shows that the residues facing in one direction are hydrophilic and those of opposite situation are lipophilic. 

The Crowns have been used as models for the transport of anions across membranes, against a concentration gradient (cf Type 2 transport, p. 68). The anions transported were A -benzoylated amino acids and small peptides the membrane was a stirred layer of chloroform bounded on each face by water. Because the Crowns had little solubility in water, they remained mainly in the chloroform. At the first water/chloroform interface, potassium ions were introduced. This led to extraction of the ions into the chloroform layer to form a ternary complex (anion, K , ionophore) which released both ions into the post-membrane aqueous phase. The depleted ionophore continued the cycle by extracting more anion and cation from the first aqueous phase, thus starting another round of the cycle (Tsukube, 1982). For anion-complexing Cryptands, see Dietrich et al. (1978). 

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Faraday s law (p. 496) galvanostat (p. 464) glass electrode (p. 477) hanging mercury drop electrode (p. 509) hydrodynamic voltammetry (p. 513) indicator electrode (p. 462) ionophore (p. 482) ion-selective electrode (p. 475) liquid-based ion-selective electrode (p. 482) liquid junction potential (p. 470) mass transport (p. 511) mediator (p. 500) membrane potential (p. 475) migration (p. 512) nonfaradaic current (p. 512)... 

This experiment describes the preparation and evaluation of two liquid-membrane Na+ ion-selective electrodes, using either the sodium salt of monensin or a hemisodium ionophore as ion exchangers incorporated into a PVG matrix. Electrodes prepared using monensin performed poorly, but those prepared using hemisodium showed a linear response over a range of 0.1 M to 3 X 10 M Na+ with slopes close to the theoretical value. 

The FDA first approved use of a polyether ionophore as a feed additive for catde ia 1975. Ionophores were first isolated from bacteria generally of the Streptomjces genus, but are produced commercially by bacterial fermentation (qv). Monensia [17090-79-8] and other ionophores are being fed to over 90% of feedlot cattle grown for beef (53) to enhance efficiency of gain improvements of 5—10% are common. Ionophores also are used as anticoccidial dmgs ia poultry production and have similar, but lesser, effects ia mminants (54). 

Doses range from 6 to 33 ppm ia the diet, but very htde if any ionophore can be measured ia the circulation after feeding. Monensia is absorbed from the gut, metabolized by the Hver, and excreted iato the bile and back iato the gut. Thus tissue and blood concentrations are very low. Over 20 metabohtes of monensia, which have Htde or ao biological activity, have beea ideatified (47,55). 

Dietary adrninistration of ionophores is coupled with the use of anaboHc steroid implants to maximize rate and efficiency of gain in growing catde. Effects of ionophores and anaboHc steroid implants are generally additive. 

Other auxin-like herbicides (2,48) include the chlorobenzoic acids, eg, dicamba and chloramben, and miscellaneous compounds such as picloram, a substituted picolinic acid, and naptalam (see Table 1). Naptalam is not halogenated and is reported to function as an antiauxin, competitively blocking lAA action (199). TIBA is an antiauxin used in receptor site and other plant growth studies at the molecular level (201). Diclofop-methyl and diclofop are also potent, rapid inhibitors of auxin-stimulated response in monocots (93,94). Diclofop is reported to act as a proton ionophore, dissipating cell membrane potential and perturbing membrane functions. 

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

Polyethers. Antibiotics within this family contain a number of cycHc ether and ketal units and have a carboxyHc acid group. They form complexes with mono- and divalent cations that ate soluble ia aoapolar organic solvents. They iateract with bacterial cell membranes and allow cations to pass through the membranes causiag cell death. Because of this property they have been classified as ionophores. Monensia, lasalocid, and maduramicia are examples of polyethers that are used commercially as anticoccidial agents ia poultry and as growth promotants ia mmiaants. 

Coccidiosis is a proto2oal disease of the intestinal tract of animals that leads to severe loss of productivity and death. The development and widespread use of anticoccidials has revolutionized the poultry industry. The estimated world market for anticoccidial agents in 1989 was 425 million and this was dominated by the polyether ionophore antibiotics monensin, salinomycin [53003-10-4], n imsm [55134-13-9], la.s9locid, and maduramicin [84878-61-5] (26). 

Valinomycin and the Enniatins. Neutral ionophores such as the cycHc dodecadepsipeptide valinomycin [2001-95-8] C H QN O g, (Fig. 8) from StreptomjcesJulvissimus (179), and the cycHc hexadepsipeptides enniatin [11113-62-5] and beauvericin [26048-05-5] from fungi (180—182),... 

Table 1 Hsts the polyether antibiotics arranged by the number of carbons in the skeleton. Many of these compounds were isolated independendy in separate laboratories and thus have more than one designation. The groups are subdivided depending on the number of spiroketals. Two classes fall outside this scheme the pyrrole ether type containing a heterocycHc ring, and the acyltetronic acid type, that has an acyHdene tetronic acid instead of a carboxyHc acid. These compounds are ionophores and because of their common features are included as polyethers.

Anticoccidial Activity. The 1968 report that claimed monensin has activity against Eimeria sp., particularly E. tenella E. macdma., and E. acervulina greatly altered the prevention and control of coccidiosis in poultry (172). It is estimated that the polyether ionophores presently constitute more than 80% of the total worldwide usage of anticoccidials (173). Lasalocid and monensin have been approved for use in control of coccidiosis in cattle. 

The total world market for the use of ionophores for feed efficiency improvement in mminants is approximately 80— 90 million. The United States is the largest market. Lasalocid and monensin are the only members of this class cleared for use. Outside the United States, salinomycin is used in limited quantities. Worldwide usage is about 1.5 million kg. 

The majoiity of the various analyte measurements made in automated clinical chemistry analyzers involve optical techniques such as absorbance, reflectance, luminescence, and turbidimetric and nephelometric detection means. Some of these ate illustrated in Figure 3. The measurement of electrolytes such as sodium and potassium have generally been accomphshed by flame photometry or ion-selective electrode sensors (qv). However, the development of chromogenic ionophores permits these measurements to be done by absorbance photometry also. 

Ion-selective electrodes (ISEs) with ionophore-based membranes allow for quantification of a large number of analytes in various matrixes. Tailoring of the composition of the membranes to comply with the analytical task, requires advanced theory of membrane response. Most of theoretical descriptions include nonrealistic extra-thermodynamic assumptions, in the first place it is assumed that some kind of species strongly predominate in membranes. Ideally, a rigorous theory of ISE response should be based on strict thermodynamics. However, real ISE membranes are too complex. Therefore, known attempts aimed at rigorous thermodynamic description of ISEs proved to be fraritless. 

Some of these compounds could be considered as dietary additives, but various other terms, including pesticides, can also be used. They can have beneficial effects on the environment and this aspect will be discussed later. The ionophore monensin, which is an alicyclic polyether (Figure 1), is a secondary metabolite of Streptomyces and aids the prevention of coccidiosis in poultry. Monensin is used as a growth promoter in cattle and also to decrease methane production, but it is toxic to equine animals. " Its ability to act as an ionophore is dependent on its cyclic chelating effect on metal ions. ° The hormones bovine somatotropin (BST) and porcine somatotropin (PST), both of which are polypeptides, occur naturally in lactating cattle and pigs, respectively, but can also be produced synthetically using recombinant DNA methods and administered to such animals in order to increase milk yields and lean meat production. "... 

M 644.9, m 156-158". Purified by chromatography on a Kieselgel column and eluted with CH2Cl2-EtOAc (5 1), and recryst from EtOH-Me2CO as colourless crystals. It is an electrically neutral ionophore with high selectivity for Ba " ions and with high lipophilicity. [Chem Ber 118 1071 1985.]... 

Cadmium ionophore I [, , , -tetramethyl-3,6-dioxooctanedi-(thioamide)] [73487-00-0] M 432.7, m 35-36°. Wash well with pet ether, then several times with 2N HCl (if it has a slight odour of pyridine) then H2O and dry in a vacuum over H2SO4. It is a polar selectrophore for Cd. [Helv Chim Acta 63 217 1980.]... 

Calcium ionophore I (ETH 1001) [58801-34-6] M 685.0. This is a neutral Ca selectophore. It can be purified by thick layer (2mm) chromatography (Kieselgel F245) and eluted with Me2CO-CHCl3 (2 1). [Helv Chim Acta 56 1780 1 973.]... 

Carbonate ionophore I [ETH 6010] (heptyl 4-trifluoroacetylbenzoate) [129476-47-7] M 316.3, b 170°/0.02 Torr, d 0.909. Purified by flash chromatography (2g of reagent with 30g of Silica Gel 60) and eluted with EtOAc/hexane (1 19). The fractions that absorbed at 260nm were pooled, evapd and dried at room temp (10.3 Torr). The oily residue was distd in a bubbled-tube apparatus (170°/0.02 Torr). Its IR (CHCI3) had peaks at 1720, 1280, 940cm and its sol in tetrahydrofuran is 50mg/0.5mL. It is a lipophilic neutral ionophore selective for carbonate as well as being an optical humidity sensor. [Anal Chim Acta 233 41 1990.]... 

Crown-4 (lithium ionophore V, 1,4,7,10-tetraoxacyclododecane) [294-93-9] M 176.2, m 17°. The distilled crude product had to be crystd from pentane at -20° to remove acyclic material. It is then dried over P2O5. [Acta Chem Scand 27 3395 1973.]... 

Dibenzyl-14-crown-4 (lithium ionophore VI 6,6-dibenzyl-l,4,8,ll-tetra-oxa-cyclo-tetradecane) [106868-21-7] M 384.5, m 102-103°. Dissolve in CHCI3, wash with saturated aqueous NaCl, dry with MgSOa, evaporate and purify by chromatography on silica gel and gradient elution with C6Hg-MeOH followed by preparative reverse phase HPLC on an octadecyl silanised silica (ODS) column and eluting with MeOH. It can be crystd from MeOH (v Br 120 cm , C-O-C). [7 Chem Soc Perkin Trans 1 1945 1986.]... 

A -Diheptyl-A, A -5,5-tetramethyl-3,7-dioxanonanediamide [lithium ionophore I (ETH 149)] [58821-96-8J M 442.7. Purified by chromatography on Kieselgel using CHCI3 as eluent (IR v 1640cm ). [Helv Chim Acta 60 2326 1977.]... 

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