A simple resolution of the kinetic anomaly in the exchange of different sugars across the membrane of the human red blood cell

1. 1. Using human red blood cells, we measured the time courses of efflux of labelled mannose, glucose or galactose (all at 130 mM) into equimolar concentrations of different sugars. 2. 2. We confirmed that the rates of exchange of mannose with mannose and galactose with galactose are somewhat slower than that of glucose with glucose at the particular concentration studied. The rates of exchange for glucose into mannose and for glucose into galactose are (we confirm) faster, but those for mannose into glucose and galactose into glucose are (we show) slower than for the exchange of either glucose into glucose, mannose into mannose or galactose into galactose. 3. 3. We determined directly, using the single sugars alone, the kinetic parameters K_m and V for exchange transport. The different sugars demonstrated different values for V as well as for K_m. 4. 4. Based on the values obtained, the theoretical time course of the exchange between pairs of sugars at 130 mM was computed according to the conventional carrier model and the recently proposed tetramer model. The experimental results fitted well with the predictions of the tetramer model and to a lesser extent with the carrier model. Thus the previously reported anomaly in the exchange experiments was merely an apparent one due, first, to the fact that different sugars possess different maximum velocities and, second, to the accumulation of the unlabelled sugars within the cell.