SINGLE HALF-TURNOVERS OF THE GLUCOSE TRANSPORTER OF THE HUMAN ERYTHROCYTE

Abstract

Much of the kinetic data describing the transport of glucose in human erythrocytes is consistent with the four-state carrier model for (l), which can readily account for both transport asymmetry (with & and V , for glucose zero trans efflux exceeding those for influx) and the fact that the maximum rate of equilibrium exchange of glucose is substantially higher than the maximum rates of net efflux and influx. However, some of the reported kinetic data appear to be inconsistent with four-state carrier model and consequently alternative models, notably the simultaneous site model of Carruthers (2). have been put forward. In the 4-state carrier model binding of glucose can occur at either an outward-facing or an inwardfacing binding site with a change in conformation of the transporter effecting transport of glucose between these sites, whereas in the model of Carruthers the transporter presents external and internal binding sites for glucose simultaneously. This difference provides a means of distinguishing between the models experimentally since if the 4state carrier model is correct it should be possible to recruit glucose-binding sites to the external sites using an impermeant reversible inhibitor, then induce a single half-turnover of the transporter by simultaneously adding glucose and diluting the inhibitor, whereas there should be no single half-turnover under these conditions if the simultaneous site model is correct.