Induced fit as a determinative of ionophore selectivity

Selectivity of cation transport across lipid membranes by the antibiotic salinomycin

The ionophoric antibiotic salinomycin is in the phase of preclinical tests against several types of malignant tumors including breast cancer. Notwithstanding, the data on its ion selectivity, although being critical for its therapeutic activity, are rather scarce. In the present work, we studied the ability of salinomycin to exert cation/H+-exchange across artificial bilayer lipid membranes (BLM) by measuring electrical potential on planar BLM in the presence of a protonophore and fluorescence responses of the pH-sensitive dye pyranine entrapped in liposomes. The following order of ion selectivity was obtained by these two methods: K+ > Na+ > Rb+ > Cs+ > Li+. Measurements of the monovalent cation-induced quenching of fluorescence of thallium ions in methanol showed that salinomycin effectively binds potassium and calcium but poorly binds sodium and lithium ions. At high concentrations, salinomycin transports Ca2+ through membranes of liposomes and mitochondria, as measured by using the calcium-sensitive dye Fluo-5 N. The data obtained can be used in the mechanistic studies of the anti-tumor activity of salinomycin and its selective cytotoxicity towards cancer stem cells.

Circular Dichroism is Sensitive to Monovalent Cation Binding in Monensin Complexes

Monensin is a natural antibiotic that exhibits high affinity to certain metal ions. In order to explore its potential in coordination chemistry, circular dichroism (CD) spectra of monensic acid A (MonH) and its derivatives containing monovalent cations (Li(+) , Na(+) , K(+) , Rb(+) , Ag(+) , and Et4 N(+) ) in methanolic solutions were measured and compared to computational models. Whereas the conventional CD spectroscopy allowed recording of the transitions down to 192 nm, synchrotron radiation circular dichroism (SRCD) revealed other bands in the 178-192 nm wavelength range. CD signs and intensities significantly varied in the studied compounds, in spite of their similar crystal structure. Computational modeling based on the Density Functional Theory (DFT) and continuum solvent model suggests that the solid state monensin structure is largely conserved in the solutions as well. Time-dependent Density Functional Theory (TDDFT) simulations did not allow band-to-band comparison with experimental spectra due to their limited precision, but indicated that the spectral changes were caused by a combination of minor conformational changes upon the monovalent cation binding and a direct involvement of the metal electrons in monensin electronic transitions. Both the experiment and simulations thus show that the CD spectra of monensin complexes are very sensitive to the captured ions and can be used for their discrimination. Chirality 28:420-428, 2016. © 2016 Wiley Periodicals, Inc.

The effects of lipid composition on the binding of lasalocid A to small unilamellar vesicles

The binding of the carboxylic ionophore lasalocid A (X537A) to small unilamellar phospholipid vesicles of varying composition was examined in an effort to determine what structural features of the phospholipid membrane influence the ionophore-membrane interaction. Apparent dissociation constants (Kapp) were calculated for both the acidic and anionic forms of the ionophore using the change in fluorescence intensity observed for lasalocid A upon addition of phospholipid vesicles. The Kapp for binding to fluid phase dimyristoylphosphatidylcholine (DMPC) vesicles is 46 microM for the anion and 14 microM for the acid. While the phase transition of DMPC had no effect on the Kapp of the anion, an increase was observed in the Kapp of the acid below the phase transition temperature. The Kapp of the anion was not affected by the incorporation of 10% dimyristoylphosphatidylethanolamine (DMPE), but increased slightly upon incorporation of cholesterol. The pKa values of the ionophore were the same in DMPC and DMPC/DMPE membranes. Incorporation of the negative lipids phosphatidylglycerol, phosphatidic acid, or phosphatidylethanolamine (at pH 9.4 where PE carries a negative charge) decreases binding of the anion in accord with the increase in surface potential estimated from Gouy-Chapman theory. The CD spectrum of membrane-bound lasalocid A anion indicated the ionophore to be in an extended acyclic conformation on the membrane surface with the C-1 carboxylate rotated out of the plane of the salicylate ring. The out-of-plane rotation of the carboxylate may be the result of facial binding by the amphiphilic ionophore on the membrane surface or of weak ion pairing to the polar lipid head groups. These results suggest that the primary determinants of binding of the anionic ionophore on the membrane surface are packing density of the polar head groups and membrane surface potential. There is no evidence of strong hydrogen bond formation between the lipid polar head groups and the ionophore as has previously been suggested.

The transport of Na+ and K+ ions through phospholipid bilayers mediated by the antibiotics salinomycin and narasin studied by 23Na- and 39K-NMR spectroscopy

Addition of the ionophoric antibiotics salinomycin or narasin to preparations of large unilamellar vesicles made from egg yolk phosphatidylcholine in sodium or potassium chloride solutions gives rise to dynamic effects in the 23Na- and 39K-NMR spectra. The dynamic spectra arise from the ionophore-mediated transport of the metal ions through the membrane. The kinetics of the transport are followed as a function of the concentrations of ionophore and the metal ion and are compatible in all cases with a model in which one ionophore molecule transports one metal ion. For both ionophores the transport of potassium ions is appreciably faster than that of sodium and in both cases the rate-limiting step for sodium transport is dissociation of the ionophore-metal complex. Assuming dissociation to be rate limiting in all four cases it is shown that the transport rate differences between the pairs of complexes of each metal arise solely from differences in the rates of formation. The stability constants for ionophore-metal complex formation in the membrane/water interface are evaluated.

The role of molecular conformation in ion capture by carboxylic ionophores: a circular dichroism study of narasin A in single-phase solvents and liposomes

Conformational and thermodynamic aspects of cation binding by the carboxylic ionophore narasin A were studied by circular dichroism (CD). In single-phase solvents, dramatic increases in the maximum differential absorption (delta epsilon) of the C-11 carbonyl were observed upon the binding of K+, Na+ and protons to the free anionic form. These changes were associated with major shifts in the conformation equilibrium between extended and pseudocyclic conformers of narasin. Similar CD changes observed upon the binding of K+ to narasin A in dimyristoylphosphatidylcholine vesicles provided evidence that in the membrane environment, comparable conformation changes were associated with ion binding. Variation of the polar and protic properties of single-phase solvents was also found to influence the delta epsilon of the cation bound species of narasin A, supporting previous evidence for polarity-mediated modulation of conformation. Comparison of cation binding affinities indicated that in both single-phase solvents and liposomes, narasin had a marked equilibrium selectivity for K+ over Na+.

The solvent polarity dependent conformational equilibrium of the carboxylic ionophore narasin: a proton NMR study

Two dimensional homonuclear (1H-1H) chemical shift correlation, double resonance and nuclear Overhauser effect difference spectroscopy were used to determine spectral parameters of narasin acid in different solvents approximating the range of polarities encountered within a biological membrane. The observed chemical shift and coupling constant changes were consistent with a polarity mediated shift between two conformational states, with the major conformational adjustments occurring in two specific backbone regions of the molecule previously described as "hinges" (1,2). Evidence suggests that the conformational equilibrium is not only mediated by solvent polarity but may in part be determined by the intrinsic propensity of narasin to form inclusion complexes with H+.