Carboxylic ionophore (lasalocid A and A23187) mediated lanthanide ion transport across phospholipid vesicles

Complexation of ytterbium to human transferrin and its uptake by K562 cells

There is an increasing interest in the use of lanthanides in medicine. However, the mechanism of their accumulation in cells is not well understood. Lanthanide cations are similar to ferric ions with regard to transferrin binding, suggesting transferrin-receptor mediated transport is possible; however, this has not yet been confirmed. In order to clarify this mechanism, we investigated the binding of Yb3+ to apotransferrin by UV-Vis spectroscopy and stopped-flow spectrophotometry, and found that Yb3+ binds to apotransferrin at the specific iron sites in the presence of bicarbonate. The apparent binding constants of these sites showed that the affinity of Yb3+ is lower than that of Fe3+and binding of Yb3+ in the N-lobe is kinetically favored while the C-lobe is thermodynamically favored. The first Yb3+ bound to the C-lobe quantitatively with a Yb/apotransferrin molar ratio of < 1, whereas the binding to the other site is weaker and approaches completeness by a higher molar ratio only. As demonstrated by 1H NMR spectra, Yb3+ binding disturbed the conformation of apotransferrin in a manner similar to Fe3+. Flow cytometric studies on the uptake of fluorescein isothiocyanate labeled Yb3+-bound transferrin species by K562 cells showed that they bind to the cell receptors. Laser scanning confocal microscopic studies with fluorescein isothiocyanate labeled Yb3+-bound transferrin and propidium iodide labeled DNA and RNA in cells indicated that the Yb3+ entered the cells. The Yb3+-transferrin complex inhibited the uptake of the fluorescein labeled ferric-saturated transferrin (Fe2-transferrin) complex into K562 cells. The results demonstrate that the complex of Yb3+-transferrin complex was recognized by the transferrin receptor and that the transferrin-receptor-mediated mechanism is a possible pathway for Yb3+ accumulation in cells.

Mechanism and specificity of lanthanide series cation transport by ionophores A23187, 4-BrA23187, and ionomycin

A23187, 4-BrA23187, and ionomycin transport several lanthanide series trivalent cations at efficiencies similar to Ca2+, when compared at cation concentrations of approximately 10(-5) M, ionophore concentrations of approximately 10(-6) M, and a pH of 7.00. Selectivity sequences and the range of relative rates are as follows: A23187, Nd3+ > La3+ > Eu3+ > Gd3+ > Er3+ > Yb3+ > Lu3+ (approximately 34-fold); 4-BrA23187, Nd3+ > Eu3+ > Gd3+ > La3+ > Er3+ > Yb3+ > Lu3+ (approximately 34-fold); ionomycin, La3+ > Yb3+ > Nd3+ > Lu3+ > Er3+ > Eu3+ > Gd3+ (approximately 4-fold). At concentrations between 9 and 250 microM, La3+ is transported by an electroneutral mechanism, predominately through mixed complexes of the type (ionophore)2La-OH (A23187 and 4-BrA23187) or (ionophore)La-OH (ionomycin), when no membrane potential is present. For all three ionophores, an induced potential of approximately 160 mV accelerates transport by approximately 50-100%. However, measured values of H+/La3+ exchange indicate that only 4-BrA23187 displays a significant electrogenic activity under these conditions. At a La3+ concentration of 17 mM, transport by all three ionophores is electroneutral and apparently occurs through complexes of type (ionophore)3La (A23187 and 4-BrA23187) or (ionophore)La-OH (ionomycin). Analysis of these patterns in a context of comproportionation equilibria involving the transporting species and free La3+ indicates that the species containing three ionophore molecules are formed on the membrane when aqueous phase solution conditions would strongly favor a 1:1 complex, based upon previous studies in solution. The implications of this and other findings are discussed.

Ca2+ transport properties of ionophores A23187, ionomycin, and 4-BrA23187 in a well defined model system

Models for the electroneutral transport of Ca2+ by ionophores A23187, ionomycin, and 4-BrA23187 have been tested in a defined system comprised of 1-palmitoyl-2-oleoyl-sn-glycerophosphatidylcholine vesicles prepared by freeze-thaw extrusion. Quin-2-loaded and CaCl2-loaded vesicles were employed to allow the investigation of transport in both directions. Simultaneous or parallel measurements of H+ transport and membrane potential, respectively, indicate that for any of these ionophores, electrogenic transport events do not exceed 1 in 10,000 when there is no preexisting transmembrane potential. When a potential of approximately 150 mV is imposed across the membrane, transport catalyzed by A23187 remains electroneutral; however, for ionomycin and 4-BrA23187, approximately 10% of transport events may be electrogenic. The defined vesicle system has also been utilized to determine how the rate of Ca2+ transport varies as a function of ionophore and Ca2+ concentration and with the direction of transport. Some aspects of the results are unexpected and should be considered by investigators using ionophores in biological systems. These include the apparent failure of these compounds to fully equilibrate Ca2+ with a high affinity Ca2+ indicator when these species are separated by a membrane, rates of transport that vary markedly with the direction of transport, and extents of transport that are a function of ionophore concentration. At least some of these unexpected behaviors can be explained by a strong influence of delta pH on forward and reverse transport kinetics. In the case of A23187, the data also give some initial insights into the relationship between formation of the transporting species and the entry of this species into the membrane hydrophobic region.

Aggregation of lasalocid A in membranes: a fluorescence study

The aggregation behavior of the carboxylic ionophore, lasalocid A, has been studied in egg phosphatidylcholine vesicles by monitoring the intrinsic fluorescence of lasalocid A. Self quenching of lasalocid A fluorescence in vesicles of egg phosphatidylcholine suggests aggregation of lasalocid A. When aggregated lasalocid A is treated with increasing concentrations of lipid, there is an increase in fluorescence due to gradual reduction of self quenching on lateral dilution. This confirms the presence of loosely held non-covalent aggregates of lasalocid A in the membrane. This result is relevant in elucidating the molecular mechanism of cation transport by lasalocid A across membranes.

Effects of divalent cations on lipid flip-flop in the human erythrocyte membrane

Treatment of human erythrocytes with ionophore A23187 (10 mumol.l-1) and Ca2+ (0.05-0.5 mmol.l-1) or Sr2+ (0.2-1 mmol.l-1) in results in a concentration-dependent acceleration of the transmembrane reorientation (flip) of the lipid probes lysophosphatidylcholine and palmitoylcarnitine to the inner membrane leaflet after their primary insertion into the outer leaflet. Mg2+, Mn2+, Zn2+ and La3+ do not accelerate flip. Ca2(+)-induced flip acceleration depends also on the ionophore concentration. It is reversed by removal of Ca2+ with EDTA. A causal role of Ca2(+)-induced membrane protein degradation and decrease of the polyphosphoinositide level in flip acceleration could be excluded. Likewise, calmodulin-dependent processes are probably not involved since the calmodulin antagonist calmidazolium (2-10 mumol.l-1) does not suppress but even enhances the Ca2(+)-induced flip acceleration. The same is true for the Ca2+ antagonist flunarizine. These drugs do not alter flip rate in the absence of Ca2+. At high Ca2+ (1-5 mmol.l-1) an initial flip acceleration is followed by flip normalization. High concentrations of Mn2+ and Mg2+ slow down flip rates. The selective acceleration of flip by Ca2+ and Sr2+ is discussed to be due to a local detachment of the membrane skeleton from the bilayer, whereas the unselective slow down of flip by divalent cations might be due to a stabilization of the membrane bilayer by the cations. After loading of cells with Ca2+ (but not with Mn2+) the inner membrane leaflet phospholipid phosphatidylserine becomes rapidly exposed to the outer membrane surface, as detectable by its accessibility to phospholipase A2 (5 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic properties of cation/H(+)-exchange: calcimycin (A23187)-mediated Ca2+/2H(+)-exchange on the bilayer lipid membrane

The calcimycin (A23187)-mediated electrically silent flux of hydrogen ions coupled with a counter transport of calcium or magnesium ions was measured by the method of local pH changes recording in the unstirred layers near the planar bilayer lipid membrane (BLM). It was shown that: (1) the pH dependence of calcimycin-mediated Ca2+/2H+ exchange had a maximum at pH 7; (2) the apparent Michaelis constant for the alkali earth cations were higher at acidic pH than the corresponding values at alkaline pH; (3) the apparent Michaelis constant for calcium was similar to that for magnesium ions in agreement with calcimycine cation binding constants; (4) the ratio of calcium and magnesium fluxes was independent of pH in the pH range from 5 to 8. (5) the flux was proportional to the calcimycin concentration at pH greater than 6.3 and proportional to the square of the carrier concentration at pH less than 5; (6) the addition of calcium ion chelator EDTA increased the flux significantly. These data were discussed in terms of the model of cation/H(+)-exchange and it was concluded that the dissociation of the cation-carrier complex at the membrane/water interface played an important role in the process of calcimycine operation. The comparison of the kinetic properties of calcimycin with the previously described kinetics of nigericin (Antonenko and Yaguzhinsky (1988) Biol. Membr. (Russian) 5, 718-728) revealed much similarity. On the other hand, a significant difference was found between the mechanism of the nigericin K/Na selectivity and calcimycin Ca/Mg selectivity.

General features in the stoichiometry and stability of ionophore A23187-cation complexes in homogeneous solution

Existing literature describing the stoichiometry and stability of complexes between A23187 and divalent cations in solution has been extended to include additional transition series cations, the heavy-metal cations Cd2+ and Pb2+, plus seven lanthanide series trivalent cations. Stability constants of 1:1 complexes between the ionophore and the divalent cations vary by 6.2 orders of magnitude between Cu2+ and Ba2+ which are the strongest and weakest complexes, respectively. Considering alkaline-earth and first-series transition cations together, the pattern of stability constants obeys the extended Irving-Williams series as is seen with many nonionophorous liganding agents. Cd2+ and Pb2+ are bound with an affinity similar to those of Mn2+ and Zn2+, whereas the lanthanides are bound with little selectivity and slightly higher stability. Titration of the ionophore in the 10(-5) M concentration range with di- and trivalent cations gives rise first to complexes of stoichiometry MA2 and subsequently to MA as the metal concentration is increased. The second stepwise stability constants for formation of the MA2 species exceeds the first constant by approximately 10-fold. With lanthanides, heavy metals, and transition-metal cations, OH-, at near physiological concentrations, competes significantly with free ionophore for binding to the 1:1 complexes. This competition is not apparent when Ca2+ or Mg2+ are the central cations. Possible implications of the 1:1 complex selectivity pattern, the ionophore-hydroxide competitive binding equilibria, and potential ternary complexes involving 1:1 ionophore:cation complexes and other anions present in biological systems are discussed with respect to the ionophore's transport selectivity and biological actions.

Factors affecting solute entrapment in phospholipid vesicles prepared by the freeze-thaw extrusion method: a possible general method for improving the efficiency of entrapment

It is often assumed that the internal solute concentrations of phospholipid vesicles are equal to those in the medium in which they were prepared, particularly when freeze-thaw cycles are employed during the procedure. Conditions are reported here which when used to prepare vesicles by the polycarbonate filter extrusion method, produce approximately 12- and approximately 7-fold higher internal concentrations of Ca2+ and sucrose, respectively, than exist in the external medium. Formation of these large gradients is dependent upon the use of freeze-thaw cycles during preparation, on the presence of tetraethylammonium perchlorate in the medium, and is independent of media pH across the region of pH 5-9. Gradient formation is antagonized by high concentrations of an impermeant solute (NaCl). It is proposed that gradients form because solutes are concentrated by exclusion from ice during freezing but that they are normally dissipated by osmotic lysis during thawing. The presence of a permeant solute such as tetraethylammonium perchlorate provides an alternative mechanism to balance osmotic pressure, thereby preserving the gradients of impermeable species.

Effect of changes in cation concentration near bilayer lipid membrane on the rate of carrier-mediated cation fluxes and on the carrier apparent selectivity

A new approach was applied for the measurements of ion transport through bilayer lipid membranes (BLM) induced by electrically neutral cation/H+ exchangers. This is an improved version of the method of the measurements of the cation/H+ exchange rate based on recording pH shifts in the unstirred layers near the BLM. Using this approach, the pH gradient in the unstirred layers induced by the cation/H+ exchanger was reduced by successive addition of the acetate on one side of the BLM until the pH shift reached zero. The difference in acetate concentration across the membrane is a measure of the cation/H+ exchange rate. In the second part of the work we found that the changes in cation concentration in the unstirred layers under the conditions imposed when measuring cation selectivity (according to Antonenko, Yu.N. and Yaguzhinsky, L.S., Biochim. Biophys. Acta 1988; 938, 125-130) can significantly decrease the apparent value of cation selectivity. It was shown that more accurate results can be obtained if low concentrations of the carrier are used. The values of nigericin cation selectivity for the alkali metals were measured (K+/Rb+ 19 +/- 1, Rb+/Na+ 1.9 +/- 0.2, Na+/Cs+ 8 +/- 0.5, Cs+/Li+ 1.8 +/- 0.3).

Physical characterization of and ionophore-mediated europium(III) transport through unilamellar phosphatidylcholine vesicles. A laser-induced europium(III) luminescence spectroscopy study

A continuation of the study of phospholipid bilayer vesicles as model membrane systems by laser-induced europium(III) luminescence spectroscopy is presented here (B.M. Cader and W. DeW. Horrocks, Jr, Biophys. Chem. 32 (1988) 97). This spectroscopic technique was used to characterize further the physical properties of small and large vesicles composed of dipalmitoylphosphatidylcholine and egg phosphatidylcholine, respectively. Unilamellar preparations were confirmed and internal aqueous volumes were calculated. The calcium-binding carboxylic ionophores, lasalocid A and A23187, were incorporated into the lipid bilayers of these vesicles for the purpose of modeling the mobile carrier mechanism of ion transport across cell membranes. Spectroscopic data implicate the presence of 1:1 and 1:2 europium(III)/lasalocid A complexes within the hydrophobic region, both capable of efficient transport and containing no water molecules in the inner sphere of europium(III). First-order rate constants for lasalocid A-mediated europium(III) transport were determined at 37 and 62 degrees C (0.018 and 0.11 min-1, respectively) using EGTA as a 'flag' to bind and detect the post-transported metal ion.

Ca2+ binding cyclic octapeptides having an alternating Sar and a hydrophobic amino acid in the sequence

Cyclic octapeptides having alternating Sar and hydrophobic amino acid sequences, such as cyclo[Lys(Z)-Sar-Leu-Sar-Leu-Sar-Leu-Sar] (C8KL), cyclo[Glu(OMe)-Sar-Lys(Z)-Sar-Leu-Sar-Leu-Sar] (C8KE, and cyclo[Lys(Suc)-Sar-Leu-Sar-Leu-Sar-Leu-Sar] [C8K(Suc)L, Suc represents succinic acid], were synthesized. These cyclic octapeptides formed a complex selectively with Ca2+. Upon complexation, trans peptide bonds of Sar residues were isomerized to cis peptide bonds. C8KL and C8KE showed very similar characteristics of Ca2+ binding, extraction of Ca2+ from an aqueous solution to a chloroform solution, and Ca2+ transport through a liquid chloroform membrane. C8KL transported Ca2+ across the lipid bilayer membrane above the phase-transition temperature, while C8KE and C8K(Suc)L did not. Therefore, the transport of Ca2+ through the lipid bilayer membrane is very sensitive to the hydrophobicity of the carrier molecule.

1H-NMR of phosphatidylcholine liposomes at low p2H in the presence of a paramagnetic shift reagent

1H-NMR spectra of egg phosphatidylcholine liposomes in 2H2O were obtained at several p2H values in the presence of 10 mM PrCl3 added after sonication of the phospholipid. It has been found that as the p2H is lowered below 2, the two distinct signals corresponding to the outer and inner phospholipid trimethylammonium groups which arise by the shifting effect of the paramagnetic cation on the external surface of vesicles, tend to coalesce into a single, high-field peak, at the position corresponding to the internal, non-shifted -N+ (CH3)3 protons. These results can be interpreted to mean that the shifting effect of Pr3+ on phosphatidylcholine NMR spectra, is due to electrostatic interaction between the lanthanide and the ionized group of the lipid. At low p2H, as the phosphodiester becomes protonated, the paramagnetic cation is no longer attracted by the liposome surface and its shifting effect on the phospholipid NMR signals disappears. The plot of the p2H dependence of the chemical shift of the outer trimethylammonium resonance of phosphatidylcholine liposomes with praseodymium ions present only on the outside of vesicles, results in a sigmoidal titration curve with its midpoint at p2H 1.5. In contrast, the inner signal is not affected by p2H. If coalescence of signals is considered as indicative of complete protonation of the phosphate moiety, the value of 1.5 can be taken as the apparent pK for the ionization of that group under the experimental conditions employed, i.e., 10 mM PrCl3 in 2H2O. That the low p2H-induced merging of the signals is reversible, is shown by the reappearance of the two peaks when the p2H of the phospholipid dispersion is raised from 1 to 5.7. Since the recovery of the trimethylammonium signal splitting indicates that Pr3+ has remained excluded from the liposome inner compartment, these experiments also demonstrate that the vesicles have not been disrupted by exposure to such an extreme acidic condition as p2H 1.