The asymmetry of the facilitated transfer system for hexoses in human red cells and the simple kinetics of a two component model

4, 6-O-Ethylidene-alpha-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.2. 50% inhibition of glucose exchange at 20 mM and 16 degrees C required 200 mM ethylidene glucose when on the inside in contrast to 25-30 mM when 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.