Transport of Monosaccharides I. ASYMMETRY IN THE HUMAN ERYTHROCYTE MECHANISM

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Transport of D-glucose across human erythrocyte membranes occurs via a facilitated diffusion process which demonstrates influx-efflux asymmetry. The mechanism of the asymmetry has been studied by estimating unidirectional fluxes in the presence or absence of trans equilibrium hexose. In the absence of transhexose, the half-saturation constant for efflux at 15°C was approximately 10 mM as compared with 27 mM for influx; the corresponding values for maximal transfer rates (μmol/min per ml cell H2O) were approximately 51 vs. 18. The estimation of kinetic parameters, including the constant Fs, which is the ratio of maximal transfer rate/half-saturation constant, indicates a unique effect of intracellular hexose on the transfer system. Further evidence to support this conclusion was obtained by studying the effects of noncompetitive inhibitors on efflux vs. influx. N-ethylmaleimide, p-chloromercuribenzenesulfonate, and dichloroallyldiethylstilbestrol all inhibited efflux much more than influx. Glucose rendered the transport system more reactive to N-ethylmaleimide as assayed by efflux, whereas influx was much less affected. The results support the hypothesis that the transport system exists in two states. Transition from one state to the other is dependent on the presence of intracellular hexose.

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