Drastic reduction in antimicrobial activity by replacement of Orn residues with Lys in cyclized amphiphilic beta-structural model peptides

Recent investigation have indicated that cyclic dodeca- and tetradecapeptides, cyclo(-Leu-Orn-Leu-Orn-D-Phe-Pro)2 (Orn-DLL-12) and cyclo(-Leu-Orn-Leu-Orn-Leu-D-Phe-Pro)2 (Orn-DLL-14), which are designed on the basis of a cyclic beta-structural antibiotic, gramicidin S (GS), inhibit the growth of Gram-positive and -negative bacteria with high potency [Ando, S., Nishikawa, H., Takiguchi, H., Lee, S. & Sugihara, G. (1993) Biochim. Biophys. Acta 1147, 42-49]. In this study we designed and synthesized two analogs, Lys-DLL-12 and Lys-DLL-14, in which four Orn residues in Orn-DLL-12 and Orn-DLL-14 were replaced by Lys residues, respectively, and investigated their interactions with model membranes in terms of CD and dye-leakage experiments, antimicrobial activity and lytic activity for human erythrocytes. Both peptides newly designed showed no antimicrobial activity and no lytic activity of erythrocytes. The present CD study showed that the presence of neutral liposomes and of acidic liposomes of natural or synthetic phospholipids results in no remarkable conformational difference between Orn-DLL-12/-14. The leakage experiment showed a clear relation between the antimicrobial activity and the leakage ability in acidic synthetic phospholipid liposomes but no correlation in acidic natural ones. The difference in hydrophobic and hydrophilic balance between Orn-DLL-12/14 and Lys-DLL-12/14 (derived from the increasing hydrophobicity due to an increase of four methylene units by the substitution of Lys for Orn) may be one of the important factors in the drastic decrease in activity.

Antimicrobial specificity and hemolytic activity of cyclized basic amphiphilic beta-structural model peptides and their interactions with phospholipid bilayers

We synthesized a series of cyclic antiparallel beta-sheet model peptides with various ring sizes, which were designed on the basis of a cyclic beta-structural antibiotic, gramicidin S (GS); cyclo(Val-Orn-Leu-D-Phe-Pro)2, and investigated in terms of their antimicrobial activity and specificity against Gram-positive and Gram-negative bacteria and lytic activity for human erythrocytes. In our planning, in order to compare the peptides with GS, D-Phe-Pro sequence forming beta-turn in GS molecule remained unaltered and repeating sequences of alternately hydrophobic (Leu)-hydrophilic (Orn) residue were introduced into the beta-structural parts. CD study in acidic liposomes as well as leakage study of carboxyfluorescein encapsulated in phospholipid vesicles indicated that the peptides strongly interacted with lipid bilayers by taking an amphiphilic beta-structure. Antimicrobial study showed that although GS is active only against Gram-positive bacteria, the antimicrobial spectra of the model peptides transformed gradually to be active against Gram-negative ones and finally only against Gram-negative bacteria whose repeating sequences increased. It should be noted that the designed cyclic model peptides show antibacterial activity but accompany no hemolysis. This indicates that an appropriate hydrophobicity together with a proper orientation of hydrophilic (cationic) and hydrophobic groups in cyclic beta-structural molecules can hold antimicrobial activity against both types of bacteria without damaging eukaryotic cells.