Selective Monosaccharide Transport through Lipid Bilayers Using Boronic Acid Carriers

Twenty-one boronic acids were studied for their ability to transport saccharides in and out of liposomes. The rates of liposome efflux were determined using an enzymatic assay, whereas the influx studies used a radiotracer method. All boronic acids examined, except those that were highly hydrophilic, facilitated monosaccharide transport. The order of transport selectivity was sorbitol > fructose > glucose. The disaccharides maltose and sucrose were not transported to any significant degree. Facilitated transport was demonstrated with a variety of liposome types, including multilamellar and unilamellar vesicles with anionic or cationic polar lipid additives. Transport mechanism studies included the accumulation of structure−activity data, as well as systematic investigations of various environmental changes such as pH, added salt, membrane potential, and temperature. Overall, the evidence is strongly in favor of a membrane carrier mechanism. The boronic acid combines reversibly with a diol group on the monosaccharide to produce a tetrahedral, anionic boronate, which is the major complexed structure in bulk, aqueous solution. At the bilayer surface, the tetrahedral boronate is in equilibrium with its neutral, trigonal form, which is the actual transported species. At low carrier concentrations, a first-order dependence on carrier was observed indicating that the transported species was a 1:1 sugar−boronate. At higher carrier concentrations the kinetic order approached 2, suggesting the increased participation of a 1:2 sugar−bisboronate transport pathway. The effect of boronic acids on liposomal bilayer fluidity was probed by fluorescence spectroscopy using appropriate reporter molecules. Adding cholesterol to the liposome membranes reduced translational fluidity by "packing and ordering" the bilayer. Addition of lipophilic arylboronic acids (either free or complexed with monosaccharides) induced a similar but smaller effect.