Evidence of multiple operational affinities for D-glucose inside the human erythrocyte membrane.

1. The Michaelis-Menten parameters of labelled D-glucose exit from human erythrocytes at 2 degrees C into external solution containing 50 mM D-galactose were obtained. The Km is 3.4 +/0 0.4 mM, V 17.3 +/- 1.4 MMOL . 1(-1) cell water . min-1 for this infinite-trans exit procedure. 2. The kinetic parameters of equilibrium exchange of D-glucose at 2 degrees C are Km = 25 +/- 3.4 mM, V 30 +/- 4.1 mmol . 1(-1) cell water . min-1. 3. The Km for net exit of D-glucose into solutions containing zero sugar is 15.8 +/- 1.7 mM, V 9.3 +/- 3.3 mmol . 1(-1) cell water . min-1. 4. This experimental evidence corroborates the previous finding of Hankin, B.L., Lieb, W.R. and Stein, W.D. [(1972) Biochim. Biophys. Acta 255, 126--132] that there are sites with both high and low operational affinities for D-glucose at the inner surface of the human erythrocyte membrane. This result is inconsistent with current asymmetric carrier models of sugar transport.