Evidence from temperature studies that the human erythrocyte hexose transporter has a transient memory of its dissociated ligands

The inhibition constant of L-sorbose efflux (K-i(sorbose)) from human erythrocytes for inhibition by D-glucose increases from 5.15 +/- 0.89 to 12.24 +/- 1.9 mM on cooling from 50 degrees C to 30 degrees C; the K-i(sorbase) of D-mannose increases similarly on cooling. The activation energy E-a(sorbose) of net L-sorbose exit from human erythrocytes is 62.9 +/- 3.1 kJ mol(-1); but in the co-presence of 5 mM D-glucose E-a(sorbose) is reduced to 41.7 +/- 1.6 kJ mol(-1) (P < 0.005). These data are consistent with the view that when D-glucose binds to the hexose transporter it leads to an activated transporter state which remains transiently activated after glucose dissociates; if L-sorbose binds to this excited state it is more mobile than otherwise and consequently the apparent K-i(sorbose) of D-glucose is raised. Cooling prolongs the decay time of the activated state; hence the K-i(sorbose) of D-glucose rises as temperature is reduced.