Asymmetry of the hexose transfer system in human erythrocytes. Experiments with non-transportable inhibitors

Abstract

1. The asymmetrical nature of sugar affinity for the hexose transfer system in human red cells has been demonstrated using purified 4,6-O-ethylidene-alpha-D-glucopyranose (ethylidene glucose) to inhibit the exchange of glucose, 3-O-methyl glucose and galactose. 2. The half-saturation concentration for ethylidene glucose inside the cell is estimated at ca. 110 mM whereas on the outside the value for exchange inhibition is ca 11mM. 3. The asymmetrics of affinities of two related non-transportable inhibitors 1,2-O-isopropylidene-D-glucofuranose and methyl-2,3-di-O-methyl-alpha-D-glucopyranoside have also been studied. 4. From experiments at varying concentrations and on theoretical grounds the half-saturation concentration for non-transportable inhibitors on the outside surface is shown to be over-estimated by measuring inhibition of exchange. In consequence the actual asymmetry of affinities may be greater than observed. 5. Experiments with ethylidene glucose also suggest that conformational changes redistributing components of the hexose transfer system between inward and outward facing modes may occur.