tritrpticin and Hemolysis

Antimicrobial peptides (AMPs) constitute a diverse family of peptides with the ability to protect their host against microbial infections. In addition to their ability to kill microorganisms, several AMPs also exhibit selective cytotoxicity towards cancer cells and are collectively referred to as anticancer peptides (ACPs). Here a large library of AMPs, mainly derived from the porcine cathelicidin peptide, tritrpticin (VRRFPWWWPFLRR), were assessed for their anticancer activity against the Jurkat T cell leukemia line. These anticancer potencies were compared to the cytotoxicity of the peptides towards normal cells isolated from healthy donors, namely peripheral blood mononuclear cells (PBMCs) and red blood cells (RBCs; where hemolytic activity was assessed). Among the active tritrpticin derivatives, substitution of Arg by Lys enhanced the selectivity of the peptides towards Jurkat cells when compared to PBMCs. Additionally, the side chain length of the Lys residues was also optimized to further enhance the tritrpticin ACP selectivity at low concentrations. The mechanism of action of the peptides with high selectivity involved the permeabilization of the cytoplasmic membrane of Jurkat cells, without formation of apoptotic bodies. The incorporation of non-natural Lys-based cationic amino acids could provide a new strategy to improve the selectivity of other synthetic ACPs to enhance their potential for therapeutic use against leukemia cells.

Topics: Animals; Anti-Bacterial Agents; Antimicrobial Cationic Peptides; Antineoplastic Agents; Cathelicidins; Circular Dichroism; Erythrocytes; Escherichia coli; Hemolysis; Humans; Leukocytes, Mononuclear; Microbial Sensitivity Tests; Oligopeptides; Peptides; Swine

The positively charged side chains of cationic antimicrobial peptides are generally thought to provide the initial long-range electrostatic attractive forces that guide them towards the negatively charged bacterial membranes. Peptide analogs were designed to examine the role of the four Arg side chains in the cathelicidin peptide tritrpticin (VRRFPWWWPFLRR). The analogs include several noncoded Arg and Lys derivatives that offer small variations in side chain length and methylation state. The peptides were tested for bactericidal and hemolytic activities, and their membrane insertion and permeabilization properties were characterized by leakage assays and fluorescence spectroscopy. A net charge of +5 for most of the analogs maintains their high antimicrobial activity and directs them towards preferential insertion into model bacterial membrane systems with a similar extent of burial of the Trp side chains. However the peptides exhibit significant functional differences. Analogs with methylated cationic side chains cause lower levels of membrane leakage and are associated with lower hemolytic activities, making them potentially attractive pharmaceutical candidates. Analogs containing the Arg guanidinium groups cause more membrane disruption than those containing the Lys amino groups. Peptides in the latter group with shorter side chains have increased membrane activity and conversely, elongating the Arg residue causes slightly higher membrane activity. Altogether, the potential for strong hydrogen bonding between the four positive Arg side chains with the phospholipid head groups seems to be a determinant for the membrane disruptive properties of tritrpticin and many related cationic antimicrobial peptides.

Topics: Anti-Bacterial Agents; Antimicrobial Cationic Peptides; Arginine; Bacillus subtilis; Cations; Erythrocytes; Escherichia coli; Guanidine; Hemolysis; Humans; Hydrogen Bonding; Lysine; Membranes; Methylation; Oligopeptides; Peptides; Spectrometry, Fluorescence; Staphylococcus aureus

Tritrpticin, a member of the cathelicidin family, is a Trp-rich or Pro/Arg-rich peptide. Since the Trp, Pro and Arg residues are important in membrane disruption and/or cell entry, tritrpticin is a particularly attractive template around which to design novel antimicrobial peptides. Although tritrpticin is effective against a broad spectrum of microorganisms, it also has relatively strong haemolytic activity, which may compromise its therapeutic effects. To identify antimicrobial analogues of tritrpticin that lack cytotoxicity, we have designed and synthesised several molecules based on the amphipathic turn structure of tritrpticin. C-terminal amidation of tritrpticin enhanced its antimicrobial activity, comparable with indolicidin, another Trp-rich peptide. In contrast, the additional insertion of positively-charged amino acids resulted in only small variations in antibiotic activity, suggesting that a total of five positive charges is sufficient for high antimicrobial activity. We found that perfectly symmetric analogues of tritrpticin with C-terminal amidation showed two- to eight-fold improved antimicrobial activity compared with tritrpticin, as well as significantly reduced haemolytic activity. This reduction in cytotoxicity was correlated with decreased permeabilization of the zwitterionic phosphatidylcholine membrane, the major component of the outer leaflet of red blood cells. In addition, we designed a symmetric indolicidin analogue that possessed antimicrobial potency and selectivity. Moreover, we found that these analogues of tritrpticin and indolicidin were effective against several antibiotic-resistant clinical bacterial isolates. Circular dichroism spectroscopy suggested that the structure of these symmetric analogues resembled that of tritrpticin or indolicidin in a membrane mimetic environment. Overall, our findings suggest that these symmetric peptides with an amphipathic turn structure may serve as useful templates for pharmaceutical compounds that may be effective against increasingly antibiotic-resistant microbes.

Topics: Anti-Bacterial Agents; Antimicrobial Cationic Peptides; Bacteria; Bacterial Infections; Cell Membrane Permeability; Circular Dichroism; Drug Design; Drug Resistance, Bacterial; Erythrocytes; Fluorescent Dyes; Hemolysis; Liposomes; Microbial Sensitivity Tests; Oligopeptides

Tritrpticin is a member of the cathelicidin family of antimicrobial peptides. Starting from its native sequence (VRRFPWWWPFLRR), eight synthetic peptide analogs were studied to investigate the roles of specific residues in its biological and structural properties. This included amidation of the C-terminus paired with substitutions of its cationic and Phe residues, as well as the Pro residues that are important for its two-turn micelle-bound structure. These analogs were determined to have a significant antimicrobial potency. In contrast, two other peptide analogs, those with the three Trp residues substituted with either Phe or Tyr residues are not highly membrane perturbing, as determined by leakage and flip-flop assays using fluorescence spectroscopy. Nevertheless the Phe analog has a high activity; this suggests an intracellular mechanism for antimicrobial activity that may be part of the overall mechanism of action of native tritrpticin as a complement to membrane perturbation. NMR experiments of these two Trp-substituted peptides showed the presence of multiple conformers. The structures of the six remaining Trp-containing analogs bound to dodecylphosphocholine micelles showed major, well-defined conformations. These peptides are membrane disruptive and show a wide range in hemolytic activity. Their micelle-bound structures either retain the typical turn-turn structure of native tritrpticin or have an extended alpha-helix. This work demonstrates that closely related antimicrobial peptides can often have remarkably altered properties with complex influences on their biological activities.

Topics: Amino Acid Substitution; Anti-Infective Agents; Escherichia coli; Fluoresceins; Hemolysis; Humans; In Vitro Techniques; Liposomes; Magnetic Resonance Spectroscopy; Micelles; Microbial Sensitivity Tests; Models, Molecular; Oligopeptides; Phosphorylcholine; Staphylococcus aureus; Structure-Activity Relationship

To investigate the effect of Pro --> peptoid residue substitution on cell selectivity and the mechanism of antibacterial action of Pro-containing beta-turn antimicrobial peptides, we synthesized tritrpticin-amide (TP, VRRFPWWWPFLRR-NH(2)) and its peptoid residue-substituted peptides in which two Pro residues at positions 5 and 9 are replaced with Nleu (Leu peptoid residue), Nphe (Phe peptoid residue), or Nlys (Lys peptoid residue). Peptides with Pro --> Nphe (TPf) or Pro --> Nleu substitution (TPl) retained antibacterial activity but had significantly higher toxicity to mammalian cells. In contrast, Pro --> Nlys substitution (TPk) increased the antibacterial activity but decreased the toxicity to mammalian cells. Tryptophan fluorescence studies indicated that the bacterial cell selectivity of TPk is closely correlated with a preferential interaction with negatively charged phospholipids. Interestingly, TPk was much less effective at depolarizing of the membrane potential of Staphylococus aureus and Escherichia coli spheroplasts and causing the leakage of a fluorescent dye entrapped within negatively charged vesicles. Furthermore, confocal laser-scanning microscopy showed that TPk effectively penetrated the membrane of both E. coli and S. aureus and accumulated in the cytoplasm, whereas TP and TPf did not penetrate the cell membrane but remained outside or on the cell membrane. These results suggest that the bactericidal action of TPk is due to inhibition of the intracellular components after penetration of the bacterial cell membrane. In addition, TPK with Lys substitution effectively depolarized the membrane potential of S. aureus and E. coli spheroplasts. TPK induced rapid and effective dye leakage from bacterial membrane-mimicking liposomes and did not penetrate the bacterial cell membranes. These results suggested that the ability of TPk to penetrate the bacterial cell membranes appears to involve the dual effects that are related to the increase in the positive charge and the peptide's backbone change by peptoid residue substitution. Collectively, our results showed that Pro --> Nlys substitution in Pro-containing beta-turn antimicrobial peptides is a promising strategy for the design of new short bacterial cell-selective antimicrobial peptides with intracellular mechanisms of action.

Topics: Amino Acid Sequence; Amino Acid Substitution; Animals; Anti-Bacterial Agents; Bacteria; Cell Membrane; Cells, Cultured; Circular Dichroism; Dose-Response Relationship, Drug; Erythrocytes; Fluoresceins; HeLa Cells; Hemolysis; Humans; Mice; Microbial Sensitivity Tests; Microscopy, Confocal; NIH 3T3 Cells; Oligopeptides; Peptides; Peptoids; Plasmids; Proline; Protein Binding

In antimicrobial peptides, the cationic property due to basic amino acids has been widely recognized as an important factor to promote electrostatic interaction with negatively charged phospholipids. However, little is known about the differences between two basic residues, Arg and Lys, in membrane binding affinity. Tritrpticin is an Arg- or Trp-rich antimicrobial peptide with a broad spectrum of antibacterial and antifungal activity. To investigate the structural and functional differences between Arg and Lys residues, here we designed and synthesized Arg-containing peptides, tritrpticin and SYM11, and their counterpart Lys-substituted peptides, TRK and SYM11KK, respectively. Although there were no remarkable conformational differences between Arg-containing and Lys-substituted peptides, TRK and SYM11KK exhibited almost two-fold enhanced antibacterial activity but significantly reduced hemolytic activity as compared to tritrpticin and SYM11, respectively. Furthermore, Arg-containing peptides showed strong binding affinity to both zwitterionic and anionic liposomes, whereas Lys-substituted peptides interacted weakly with zwitterionic liposomes but strongly with anionic liposomes. These results suggest that the primary amine of Lys interacts less electrostatically with zwitterionic phospholipids than the guanidinium group of Arg. Our results obtained in this study may be helpful in the design of drugs that target negatively charged phospholipids.

Topics: Amino Acid Substitution; Anti-Bacterial Agents; Arginine; Circular Dichroism; Hemolysis; Liposomes; Lysine; Microbial Sensitivity Tests; Oligopeptides; Protein Conformation