gramicidin-a and fluorexon

The effects of hydrophobic thickness and the molar phosphatidylglycerol (PG) content of lipid bilayers on the structure and membrane interaction of three cationic antimicrobial peptides were examined: aurein 2.2, aurein 2.3 (almost identical to aurein 2.2, except for a point mutation at residue 13), and a carboxy C-terminal analog of aurein 2.3. Circular dichroism results indicated that all three peptides adopt an alpha-helical structure in the presence of a 3:1 molar mixture of 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPC/DMPG), and 1:1 and 3:1 molar mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPC/POPG). Oriented circular dichroism data for three different lipid compositions showed that all three peptides were surface-adsorbed at low peptide concentrations, but were inserted into the membrane at higher peptide concentrations. The (31)P solid-state NMR data of the three peptides in the DMPC/DMPG and POPC/POPG bilayers showed that all three peptides significantly perturbed lipid headgroups, in a peptide or lipid composition-dependent manner. Differential scanning calorimetry results demonstrated that both amidated aurein peptides perturbed the overall phase structure of DMPC/DMPG bilayers, but perturbed the POPC/POPG chains less. The nature of the perturbation of DMPC/DMPG bilayers was most likely micellization, and for the POPC/POPG bilayers, distorted toroidal pores or localized membrane aggregate formation. Calcein release assay results showed that aurein peptide-induced membrane leakage was more severe in DMPC/DMPG liposomes than in POPC/POPG liposomes, and that aurein 2.2 induced higher calcein release than aurein 2.3 and aurein 2.3-COOH from 1:1 and 3:1 POPC/POPG liposomes. Finally, DiSC(3)5 assay data further delineated aurein 2.2 from the others by showing that it perturbed the lipid membranes of intact S. aureus C622 most efficiently, whereas aurein 2.3 had the same efficiency as gramicidin S, and aurein 2.3-COOH was the least efficient. Taken together, these data show that the membrane interactions of aurein peptides are affected by the hydrophobic thickness of the lipid bilayers and the PG content.

Topics: Animals; Anti-Infective Agents; Antimicrobial Cationic Peptides; Anura; Benzothiazoles; Carbocyanines; Cell Membrane; Cell Membrane Permeability; Dimyristoylphosphatidylcholine; Fluoresceins; Gramicidin; Lipid Bilayers; Membrane Potentials; Phosphatidylcholines; Phosphatidylglycerols; Protein Structure, Secondary; Staphylococcus aureus

The changes induced by biologically active substances in the permeability to K+ and calcein of liposomes composed of egg phosphatidylcholine and cholesterol were measured simultaneously in order to rapidly screen the sizes of pores formed in a membrane, using different sized markers. The substances examined in the present study were classified into three types based on differences in the rates at which K+ and calcein were released. The first type released only K+, and included gramicidin A. The second type predominantly released K+, preceding the release of calcein, and included amphotericin B and nystatin. The third type, including antimicrobial peptides, such as gramicidin S, alamethicin, and melittin, and several membrane-active drugs, like celecoxib (non-steroidal anti-inflammatory drug), 1-dodecylazacycloheptan-2-one (named azone; skin permeation enhancer), and chlorpromazine (tranquilizer), caused the release of K+ and calcein simultaneously. Thus, the sizes of pores formed in a liposomal membrane increased in the following order: types one, two, and three. We determined the size more precisely by conducting an osmotic protection experiment, measuring the release of calcein in the presence of osmotic protectants of different sizes. The radii of pores formed by the second type, amphotericin B and nystatin, were 0.36 - 0.46 nm, while the radii of pores formed by the third type were much larger, 0.63 - 0.67 nm or more. The permeability changes induced by substances of the third type are discussed in connection with a transient pore formed in a lipid packing mismatch taking place during the phase transition of dipalmitoylphosphatidylcholine liposomes.

Topics: Alamethicin; Amphotericin B; Azepines; Celecoxib; Chlorpromazine; Fluoresceins; Gramicidin; Liposomes; Melitten; Membranes, Artificial; Nystatin; Permeability; Potassium; Pyrazoles; Sulfonamides

The cytolytic peptides melittin and gramicidin S are naturally occurring agents that provide a comparative model for studies of complement, immunotoxin and cell-mediated membrane permeability. Most attempts to characterize cytolytic peptides have used model membrane systems including phospholipid vesicles or erythrocytes. Membrane vesicles permit the use of self-quenching concentrations of fluorescent permeability markers, while erythrocytes release measurable hemoglobin. Attempts at measuring early membrane permeability changes in nucleated mammalian cells have been limited. To measure the kinetics of mammalian cell membrane permeability changes induced by cytolytic peptides, we developed a 96-well fluorescence cytolysis assay using the cytoplasmic fluorescent dye calcein as the membrane permeability marker. To facilitate rapid assessment of membrane permeability, trypan blue was added to the assay solution to quench (a) released fluorescence and (b) retained intracellular fluorescence. Trypan blue also provided a complementary visual assessment of cell viability. Using this assay, a detailed kinetic analysis demonstrated permeability of the cell membranes within seconds of exposure to the cytolytic peptides. The rapid permeabilization of the cell membranes was confirmed by flow cytometry using the calcium indicator dye fluo-3. The assay also demonstrated a second slower phase of marker release over the next several hours. The fluorescence cytolysis assay was able to reliably detect the biphasic permeability changes associated with the melittin and gramicidin S peptides suggesting the potential utility of this assay in the assessment of other cytolytic agents.

Topics: Animals; Cell Line; Cell Membrane Permeability; Cytotoxins; Fluoresceins; Fluorescent Dyes; Gramicidin; Humans; Mammals; Melitten; Peptides; Sheep; Time Factors; Trypan Blue