[Determination of the distribution coefficient of rifabutin by the fluorescence study in the system liposome-water]

The distribution coefficient (Kd) of tuberculostatic rifabutin in a liposome/water system at pH 6.4 and 7.4 has been determined by the fluorescence quenching method. Large unilamellar vesicles composed of phosphatidylcholine alone or its combination with cholesterol or cardiolipin were used. The fluorescent probe anthrylphosphatidylcholine, which contains the anthryl moiety in the hydrophobic part, was incorporated into large unilamellar vesicles. The Kd values calculated with the use of dynamic quenching theory (Stern-Volmer model), were comparable for phosphatidylcholine and its combination with cholesterol at both pH. The value of lgKd (24-2.6) demonstrates the hydrophobicity of the rifabutin molecule. After the introduction of negatively charged cholesterol, Kd increases more than tenfold at pH 6.4. At pH 7.4, a second phosphate group of cholesterol undergoes ionization, and Kd of rifabutin gains an additional increase. The results obtained demonstrate a strong influence of electrostatic forces on rifabutin-model membranes interaction.