PHYSICOCHEMICAL CHARACTERIZATION OF THE NISIN-MEMBRANE INTERACTION WITH LIPOSOMES DERIVED FROM LISTERIA-MONOCYTOGENES

Mechanistic information about the bacteriocin nisin was obtained by ex amining the efflux of 5(6)-carboxy-fluorescein from Listeria monocytog enes-derived liposomes, The initial leakage rate (percentage of efflux per minute) of the entrapped dye was dependent on both nisin and lipi d concentrations, At all nisin concentrations tested, 5(6)-carboxyfluo rescein efflux plateaued before all of the 5(6) -carboxyfluorescein wa s released (suggesting that pore formation was transient), but efflux resumed when more nisin was added, Isotherms for the binding of nisin to liposomes constructed on the basis of the Langmuir isotherm gave an apparent binding constant of 6.2 X 10(5) M(-1) at pH 6.0. The critica l number of nisin molecules required to induce efflux from liposomes a t pH 6.0 was approximate to 7,000 molecules per liposome. The pH affec ted the 5(6)-carboxyfluorescein leakage rates, with higher pH values r esulting in higher leakage rates, The increased leakage rate observed at higher pH values was not due to an increase in the binding affinity of the nisin molecules towards the liposomal membrane, Rather, the cr itical number of nisin molecules required to induce activity was decre ased (approximate to 1,000 nisin molecules per liposome at pH 7.0). Th ese data are consistent with a poration mechanism in which the ionizat ion state of histidine residues in nisin plays an important role in me mbrane permeabilization.