SELECTIVE MONOSACCHARIDE TRANSPORT THROUGH LIPID BILAYERS USING BORONIC ACID CARRIERS

Twenty-one boronic acids were studied for their ability to transport s accharides 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 wer e highly hydrophilic, facilitated monosaccharide transport. The order of transport selectivity was sorbitol > fructose > glucose. The disacc harides maltose and sucrose were not transported to any significant de gree. Facilitated transport was demonstrated with a variety of liposom e types, including multilamellar and unilamellar vesicles with anionic or cationic polar lipid additives. Transport mechanism studies includ ed 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 strong ly in favor of a membrane carrier mechanism. The boronic acid combines reversibly with a diol group on the monosaccharide to produce a tetra hedral, anionic boronate, which is the major complexed structure in bu lk, aqueous solution. At the bilayer surface, the tetrahedral boronate is in equilibrium with its neutral, trigonal form, which is the actua l transported species. At low carrier concentrations, a first-order de pendence on carrier was observed indicating that the transported speci es was a 1:1 sugar-boronate. At higher carrier concentrations the kine tic 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 usi ng appropriate reporter molecules. Adding cholesterol to the liposome membranes reduced translational fluidity by ''packing and ordering'' t he bilayer. Addition of lipophilic arylboronic acids (either free or c omplexed with monosaccharides) induced a similar but smaller effect.