Vol. 62, Issue 5, 1036-1042, November 2002

Modulation of the Hydrophobic Domain of Polymyxin B Nonapeptide: Effect on Outer-Membrane Permeabilization and Lipopolysaccharide Neutralization

Haim Tsubery, Itzhak Ofek, Sofia Cohen, Miriam Eisenstein, and Mati Fridkin

Department of Organic Chemistry (H.T., M.F.) and Chemical Services Unit (M.E.), the Weizmann Institute of Science, Rehovot, Israel; and Department of Human Microbiology, Sackler Faculty of Medicine, Tel Aviv University, Tel-Aviv Israel (H.T., I.O., S.C.)

Polymyxin B nonapeptide (PMBN), a cationic cyclic peptide derived from the antibacterial peptide polymyxin B, is capable of specifically increasing the permeability of the outer membrane (OM) of Gram-negative bacteria toward hydrophobic antibiotics. In this study, we evaluated the contribution of the hydrophobic segment of PMBN (i.e., D-Phe⁵-Leu⁶) to this activity. Accordingly, we synthesized four analogs of PMBN by replacing D-Phe⁵ with either with D-Trp or D-Tyr and Leu⁶ with Phe or Ala and evaluated their ability to bind cell-free lipopolysaccharide (LPS) and increase bacterial OM permeability. Compared with PMBN, [D-Tyr⁵]PMBN and [Ala⁶]PMBN possessed reduced LPS affinity (IC₅₀ = 2.5, 25, and 12 µM, respectively) and significantly reduced OM permeability and LPS neutralization activity. [Phe⁶]PMBN exhibited rather similar affinity to cell-free LPS (IC₅₀ = 5 µM) and the same OM permeability capacity as PMBN. However, [D-Trp⁵]PMBN, despite its similar affinity to cell-free LPS (IC₅₀ = 4 µM), had moderately reduced OM permeability capacity. These results demonstrate the significant role of the PMBN hydrophobic segment in promoting biological activity.