Monensin-induced cation movements in bovine erythrocytes

Identification of new leishmanicidal peptide lead structures by automated real-time monitoring of changes in intracellular ATP

Leishmanicidal drugs interacting stoichiometrically with parasite plasma membrane lipids, thus promoting permeability, have raised significant expectations for Leishmania chemotherapy due to their nil or very low induction of resistance. Inherent in this process is a decrease in intracellular ATP, either wasted by ionic pumps to restore membrane potential or directly leaked through larger membrane lesions caused by the drug. We have adapted a luminescence method for fast automated real-time monitoring of this process, using Leishmania donovani promastigotes transfected with a cytoplasmic luciferase form, previously tested for anti-mitochondrial drugs. The system was first assayed against a set of well-known membrane-active drugs [amphotericin B, nystatin, cecropin A-melittin peptide CA(1-8)M(1-18)], plus two ionophoric polyethers (narasin and salinomycin) not previously tested on Leishmania, then used to screen seven new cecropin A-melittin hybrid peptides. All membrane-active compounds showed a good correlation between inhibition of luminescence and leishmanicidal activity. Induction of membrane permeability was demonstrated by dissipation of membrane potential, SYTOX trade mark Green influx and membrane damage assessed by electron microscopy, except for the polyethers, where ATP decrease was due to inhibition of its mitochondrial synthesis. Five of the test peptides showed an ED50 around 1 microM on promastigotes. These peptides, with equal or better activity than 26-residue-long CA(1-8)M(1-18), are the shortest leishmanicidal peptides described so far, and validate our luminescence assay as a fast and cheap screening tool for membrane-active compounds.

Cationomycin and monensin partition between serum proteins and erythrocyte membrane: consequences for Na+ and K+ transport and antimalarial activities

The ionophore properties of cationomycin and monensin were studied on human erythrocytes by measuring Na+ influx by 23Na NMR and concomitant K+ efflux by potentiometry in the presence of increasing amounts of serum. Both ion currents (Na+ or K+) decreased linearly with the reciprocal of serum amount. The serum effects on ion currents were stronger with cationomycin than with monensin. Assuming this decreased transport activity was due to drug binding to serum proteins, a partition coefficient between the protein and the membrane phase was determined for each ionophore by using a novel model. This partition coefficient is about 30 times higher for cationomycin than for monensin; the same result was obtained with purified human serum albumin, indicating that albumin may be the major ionophore binding protein of serum. In parallel, we also measured IC50 for 50% in vitro growth inhibition of Plasmodium falciparum, the agent of malaria. In the presence of increasing serum concentrations, the antimalarial activity was decreased for both ionophores. Serum effect was less severe for monensin than for cationomycin, in agreement with the weaker interaction of monensin with proteins as shown from the partition coefficient values. A correlation was established between the ion transport currents (sodium and potassium) and the IC50 measured on P. falciparum in the presence of the various concentrations of serum. The relative value of the ion transport currents (expressed as percentage of control in absence of serum) can be indicative of the ionophore unbound fraction in the medium.

Heterogeneity of tachykinin receptors in the rabbit lung

The tachykinins, substance P (SP) and neurokinin A (NKA), are agonists for the NK(1) and NK(2) receptors, respectively. Tachykinins have various respiratory effects, including bronchoconstriction. This study characterizes tachykinin binding sites in the rabbit lung. We hypothesize that (2-[(125)I]iodohistidyl(1))Neurokinin A ([(125)I]NKA) interacts with NK1 and NK2 binding sites in the rabbit lung. The K d determined from saturation isotherms was 0.69 times/divided by 1.14 nM (geometric mean times/divided by SEM) and the B max was 4.15 + or - 0.22 femtomole/mg protein (arithmetic mean + or - SEM). Competitive inhibition studies with NKA, SP and various selective tachykinin agonists showed the rank order of potency; [beta-Ala(8)]-Neurokinin A 4-10 = SP >> NKA >> [Sar(9),Met(02)11]-Substance P. [beta-Ala(8)]-Neurokinin A 4-10, a selective NK(2) agonist, and SP inhibition of [(125)I]NKA binding were best described using a two-site model. Competitive inhibition studies using the selective nonpeptide NK(2) antagonist (SR 48968) and the selective nonpeptide NK(1) antagonist (CP 96,345) revealed Ki's of 5.5 nM and 8.1 nM, respectively. Our data therefore suggest that [(125)I]NKA binds to both the NK(1) and NK(2) receptors in the lung.

Ionophore properties of monensin derivatives studied on human erythrocytes by 23Na NMR and K+ and H+ potentiometry: relationship with antimicrobial and antimalarial activities

Eight derivatives of monensin with a modified C25-C26 moiety were synthesized. Their ionophore properties were studied on human erythrocytes by measuring Na+ influx with 23Na NMR and concomitant K+ and H+ efflux by potentiometry. Modification of OH-26 led to inversion of selectivity of transport in favor of K+/Na+ in comparison with monensin. This selectivity disappeared by suppression of the C26-OH moiety. Finally the ionophore ability was lost if the head-to-tail chelation of the monensin skeleton was prevented by blocking the terminal OH-25 and -26 functions. All the compounds were inactive on Gram-negative bacteria and fungi. MIC measured on Bacillus cereus showed that derivatives with increased K+/Na+ selectivity were clearly the most active against Bacillus growth. Most of the compounds showed potential antimalarial properties in the nanomolar range when tested in vitro against Plasmodium falciparum. The IC50S measured were correlated with the whole Na+ and K+ transport efficiency rather than with the ionic selectivity. In both cases determination of initial fluxes of transport for both cations (Na+ and K+) was necessary to investigate the relationship between biological and ionophore properties.

Na+ and K+ transport by 4-chlorophenylurethane-monensin in Enterococcus hirae de-energized and energized cells studied by 23Na-NMR and K+ atomic absorption

Na+ and K+ movements induced by 4-chlorophenylurethane-monensin, which presents an inverted ion selectivity (K+ > Na+) in model systems compared with monensin, were followed on Enterococcus hirae cells by 23Na-NMR and K+ atomic absorption. For de-energized cells, the urethane derivative is much more selective for K+ than monensin, but only at low concentrations (10(-3)-10(-4) mM). For higher concentrations, as previously shown for monensin, the sodium and potassium movements are driven by the ion gradients present. On energized cells, both K+ and Na+ gradients were highly perturbed, and this can be related to the higher toxicity in mice and bacteria for this derivative.

Conditions modulating the ionic selectivity of transport by monensin examined on Enterococcus hirae (Streptococcus faecalis) by 23Na-NMR and K+ atomic absorption

Factors likely to modulate the ionic selectivity of monensin were examined on Enterococcus hirae (Streptococcus faecalis) in two states previously characterized: the resting (de-energized) cell and the active (energized) cell. Internal and external Na+ were followed by corresponding 23Na-NMR resonances K+ concentrations were measured by atomic absorption. For a given cellular population of de-energized cells, the apparent transport rates and the final cationic concentrations reached at the steady state were decreasing with the ionophore dose. Monensin was selective for sodium only at low concentrations, in the range 1 mM-10(-4) mM the transport was depending on the effective cationic gradients. Comparison of the activity curves for two cell populations (7.10(9) and 7.10(10) cells/ml) showed the importance of the ratios of monensin/mg phospholipid and also of the ratios of external/internal volumes. On energized cells, except for low monensin concentrations, the main effect was a K(+)-induced efflux and not a Na+ influx. Two factors were modulating the resulting selectivity of this ionophore: the response of the intrinsic bacterial carriers and the generation of the gradients (mainly the external pH) which were favourable to a K+/Na+ transport. Once again the results obtained for two cell populations could be compared, the determining factors were the ratio external/internal volume and the generation of the pH gradient.