Glucose transport kinetics in human red blood cells

D-[14C]Glucose self exchange and unidirectional efflux from human red blood cells were studied at 20 degrees C (pH 7.2) by means of the Millipore-Swinnex filtering technique whose time resolution is greater than 1 s and the continuous flow-tube method with a time resolution of greater than 2 ms. The unidirectional efflux data were analyzed using both the method of initial rates and the integrated rate equation. Simple Michaelis-Menten kinetics apply to the results obtained under both experimental conditions. In self-exchange mode, the half-saturation constant, K1/2ex, was 10 (S.E. +/- 1) mM. In unidirectional efflux mode K1/2ue was 6.6 (S.E. +/- 0.5) mM (initial rates) or by the method of integrated rates 7.7 mM, with a range of 2.7-12.1 mM, K1/2ue increasing with an increased initial intracellular glucose concentration. Our results of K1/2ex oppose previous published values of 32 mM for self exchange (Eilam and Stein (1972) Biochim. Biophys. Acta 266, 161-173) and 25 mM for unidirectional efflux (Karlish et al. (1972) Biochim. Biophys. Acta 255, 126-132) that have been used extensively in kinetic considerations of glucose transport models. Under self-exchange conditions Jmaxex was 1.8 x 10(-10) mol cm-2s-1, and in unidirectional efflux mode Jmaxue was 8.3 x 10(-11) mol cm-2s-1 (initial rates) and 8.6 x 10(-11) mol cm-2s-1 (integrated rates). We suggest that the previous high values of Jmax and in particular K1/2 are due to the use of methods with insufficient time resolution. Our results indicate that the transport system is less asymmetric than was generally accepted, and that complicated transport models developed to account for the great difference between the determined K1/2 and J max values are redundant.