Characterisation of the kinetic parameters of facilitated hexose transfer in the human red cell by inhibitor studies

Abstract

The transfer of glucose across the human erythrocyte membrane occurs by a process of facilitated diffusion, the mechanism of which is not yet fully understood.In an attempt to provide information about the structure of the binding site, the apparent half-saturation constants of various glucose derivatives were determined by the glucose exit technique.The variation of these half-saturation constants with temperature showed that with the exception of cellobiose all the other glucose derivatives examined possessed the same DeltaE (approximately 42 kJ mole-1) whether they were transported by the carrier or not.

The examination of a previously untried glucose derivative, 4,6-0-ethylidene-a-D-glucopyranose showed that this substance penetrated the ceil readily hut did not do so on the glucose system although reacting strongly with it. Instead it was found to penetrate the membrane by a process of diffusion.

Using the properties of this substance it was found possible to inhibit glucose exchange independently from each side of the membrane. This disclosed a marked asymmetry of the system for its substrates, the half-saturation constant for glucose at the inner surface being ten times that for the outer surface at 16° C, whilst ethylidene glucose showed a forty-fold asymmetry under the same conditions.

Studies of the inhibition of sorbose transfer by various sugars showed that the half-saturation constants determined by this process were about 2.5 times larger than those determined by the glucose exit procedure for transported sugars and 1.5 times larger for non-transported sugars.

The action of Chlorpromazine on glucose exchange and sorbose transferwas studied and it was found that whilst the former suffered negligible inhibition the latter was inhibited up to three times at the concentrate used.