Ionophoric activity of the antibiotic peptaibol trichorzin PA VI: a 23Na- and 35Cl-NMR study

Trichorzin PA VI (Ac Aib1 Ser Ala Aib Iva Gln Aib Val Aib Gly10 Leu Aib Pro Leu Aib Aib Gln Pheol18) is one of the seven main peptaibols forming the natural antibiotic 18-residue peptide mixture biosynthesised by a Trichoderma harzianum strain. Trichorzins exhibit antimycoplasmic activity resulting from membrane permeability perturbations. The membrane permeabilisation process by trichorzin PA VI has been examined in egg yolk phosphatidylcholine large unilamellar vesicles (LUV) and under conditions of ionic equilibrium by 23Na- and 35Cl-NMR experiments conducted in the presence of a chemical shift reagent and a relaxation agent, respectively. In such conditions, trichorzin PA VI exchanges both cations and anions across the vesicle bilayers, indicating the absence of ion- and charge-selectivity, in contrast to antibiotic ionophores, such as monensin or nigericin; the Na+ exchange is not influenced by the ionic strength. The kinetics of the Na+ exchange have been found to be third to fourth order with respect to the peptide concentration. The permeabilisation process of liposomes has been shown to be due to the formation of aggregates of three to four helical peptide monomers arranged into a supramolecular complex including presumably lipid molecules and forming a badly-defined pore in the bilayer. The major mechanism by which ions may exchange through the bilayer involves a long-lasting opening of the pores allowing complete exchange of the internal and external media in an 'all or nothing mode'.