Transport of uridine in human red blood cells: Demonstration of a simple carrier-mediated process

Abstract

The kinetic properties of the mediated transport of uridine in human erythrocytes are investigated. Different methodological procedures are used to acquire a complete kinetic description of the system. Under equilibrium- exchange (ee) conditions the respective values of maximum velocity and Michaelis constant at 25°C are 7.54 ± 0.45 mM/min and 1.29 ± 0.11 mM. Under zero-frans (zt) conditions the kinetic properties of efflux (to) differ significandy from those of influx (oi), revealing a 4:1 asymmetry in the system: Vztio = 1.98 ± 0.31 mM/min and Kztio = 0.40 ± 0.12 mM; Vztio = 0.53 ± 0.038 mM/min and Kztio = 0.073 ± 0.069 mM. These data are analyzed in terms of the simple carrier model as formulated by Lieb and Stein (1974. Biochim. Biophys. Acta. 373:178). Using this model and the data of equilibrium-exchange and zero-trans we have predicted the half-saturation constants in infinite-eis conditions (ic) and compared them with the experimental values (given in parentheses): Kztio, = 0.231 (0.252) mM and Kztio = 1.08 (0.937) mM. This indicates the internal consistency of the simple carrier model for uridine transport. Furthermore, application of several rejection criteria developed for the simple carrier failed to indicate lack of fitness of the model in the present case. From the analysis of kinetic data we infer that the movement of the unloaded carrier is the rate-limiting step in transport of uridine. From the value of 104 uridine carrier molecules per cell we calculate the turnover rate for uridine transport to be 7,600 molecules/min for influx and 35,000 molecules/min for efflux (both at 25°C). The present work provides a unique example of an asymmetric transport mechanism which is fully consistent with the predictions of a simple carrier model. The mechanism is discussed in terms of current concepts of membrane structure. © 1977, Rockefeller University Press., All rights reserved.