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SUMMIARY1. 4,6-O-Ethylidene-a-D-glucopyranose (ethylidene glucose) has been used to study the competitive inhibition of glucose exchange fluxes when the reagent was (i) inside the cells and (ii) on the outside. 3. The inhibitions at different inhibitor/glucose concentration ratios were measured and analysis of the data suggested that the half-saturation constant for ethylidene glucose was 6 times that for glucose inside the cell as against 1-5 outside. The analysis, however, suggested an asymmetry in respect to the affinities for glucose of approximately ten-fold and this would make the asymmetry towards ethylidene glucose forty-fold.4. Such asymmetries make it necessary to consider a transfer mechanism for sugars with different components on the outer and inner membrane interfaces and simple kinetics for a two component system have been developed and used for analysing the experimental data quantitatively.5. The kinetic similarities to and difference from the kinetics of a simple mobile carrier and those of some more recent models are briefly discussed.

SUMMARY1. The asymmetrical nature of sugar affinity for the hexose transfer system in human red cells has been demonstrated using purified 4,6-O-ethylidene-oz-D-glucopyranose (ethylidene glucose) to inhibit the exchange of glucose, 3-0-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 11 mM.3. The asymmetries of affinities of two related non-transportable inhibitors 1,2-0-isopropylidene-D-glucofuranose and methyl-2,3-di-0-methyl-a-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.

SUMMARY1. The glucose derivative 4,6-0-ethylidene-ax-D-glucopyranose (ethylidene glucose) was found to inhibit glucose exit competitively but its penetration into human red cells was unaffected by glucose in the medium.2. In penetrating red cells ethylidene glucose followed diffusion type kinetics without any evidence of saturation up to 360 mm. Besides its penetration being unaffected by glucose, 105 M phloretin, which powerfully inhibits the facilitated transfer of hexoses, did not inhibit penetration.3. Red cells incubated with 1-fluoro-2,4-dinitrobenzene (FDNB) until glucose exit was reduced by 95 % showed no slowing of penetration by ethylidene glucose. 4. The potentiation of the development of FDNB inhibition by sugars in the incubating medium was absent when ethylidene glucose was used and there was a slight protective action. Cells pre-incubated with 76 mm ethylidene glucose did not show an uphill transfer from 4 mM-[14C]glucose in the outside medium in contrast to cells pre-incubated in 76 mm glucose or in 76 mm 3-0-methyl glucose.5. The possibility that ethylidene glucose penetrated human red cells by simple diffusion was supported by its penetration of guinea-pig red cells at similar rates, by the occurrence of osmotic haemolysis in isosmotic solutions which was unaffected by copper ions and by the relatively high ether/water partition of the compound.