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Antimicrobial Agents and Chemotherapy, April 2002, p. 1073-1079, Vol. 46, No. 4
0066-4804/02/$04.00+0     DOI: 10.1128/AAC.46.4.1073-1079.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Interaction between Polyamines and Bacterial Outer Membranes as Investigated with Ion-Selective Electrodes

Faculty of Pharmaceutical Sciences, Okayama University, Tsushima, Okayama 700-8530, Japan

Received 19 October 2001/ Returned for modification 20 December 2001/ Accepted 14 January 2002

We analyzed the interaction between polyamines and the outer membrane of Escherichia coli cells using potentiometric measurements with Ca²⁺, tetraphenylphosphonium (TPP⁺), and K⁺ electrodes. The Ca²⁺ electrode was used to examine the ability of the polyamines to release Ca²⁺ from the outer membrane. The TPP⁺ electrode was used to examine the ability to permeabilize the outer membrane, since the uptake of TPP⁺ was enhanced when the permeability barrier of the outer membrane was disrupted. The K⁺ electrode was used to examine permeabilization in the cytoplasmic membrane by monitoring the efflux of K⁺ in cytosol. Although Ca²⁺ release was remarkably enhanced by increasing the number of amino groups in polyamines, no TPP⁺ uptake was observed with polyamines of a simple structure, such as ethylenediamine, spermidine, and spermine. TPP⁺ uptake was observed when appropriate lipophilic moieties were further attached to the polyamines with three or four amino groups, indicating that the existence of bulky moieties as well as the number of amino groups is important to induce outer membrane permeabilization. Thus, 1-naphthylacetylspermine and N,N'-bis[6-[[(2-methoxyphenyl)methyl]amino]hexyl]-1,8-octanediamine (methoctramine) were especially effective in increasing the permeability of the outer membrane of E. coli cells, being comparable to polymyxin B nonapeptide, a well-known cationic peptide showing such action.

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