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

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