Pre-steady-state uptake of -glucose by the human erythrocyte is inconsistent with a circulating carrier mechanism

Simulation shows that the four-state mobile carrier model for sugar transport in which the asymmetry arises from unequal rate constants of inward and outward translation of the free-carrier and carrier-sugar complex, does not fit with the observed data for pre-steady-state uptake recently obtained by A.G. Lowe and A.R. Walmsley ((1987) Biochim. Biophys. Acta 903, 547–550). The main reason for this discrepancy is that pre-steady-state fluxes are determined mainly by the dissociation constants K_s of glucose and maltose for the external sites, rather than the K_m (zero-trans_oi) of glucose and the K_i of maltose. The data are also inconsistent with other forms of asymmetric carrier but are fairly consistent with a symmetrical carrier with high-affinity sites for -glucose or with a fixed site carrier model.