The kinetic parameters of the monosaccharide transfer system of the human erythrocyte

1. 1. The kinetics of sugar transfer across the human erythrocyte membrane are complex and require for their description the measurement of at least three independent parameters: K_S (the carrier-substrate dissociation constant), r (the ratio of the rates of transfer of loaded and unloaded carrier) and V_{max., equil.} (the limiting transfer rate when both sides of the membrane are saturated with permeant). 2. 2. Methods are described for determining K_S and r is analysed. half-saturation constant K_m in terms of K_S and r is analysed. 3. 3. For glucose, K_S is 11 mM at 25°, 20mM at 13° and 44mM at 5°. V_{max., equil.} (in μmoles per ml cell water per min) decreases from 400 at 25% to 66 at 5°. 4. 4. For galactose, K_S is 95 mM at 25°. 5. 5. At 25°. values for K_m are 3-5 mM for glucose entry and egress and 40-50 mM for galactose entry. These data require that r=3 or 4 if accord with the K_S data is to be achieved. 6. 6. For glucose egress at 8δ, K_m decreases to approx. 1 mM while V_{max., zero} (the limiting rate of transfer when only one side of the membrane is saturated and the other side sugar-free) is 20 μmoles per ml cell water per min.