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

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Abstract
Under equilibrium-exchange (ee) conditions, the respective values of Vmax and Km at 25.degree. C are 7.54 .+-. 0.45 mM/min and 1.29 .+-. 0.11 mM. Under zero-trans (zt) conditions the kinetic properties of efflux (io) differ significantly from those of influx (oi), revealing a 4:1 asymmetry in the system: .**GRAPHIC**. = 1.98 .+-. 0.31 mM/min and .**GRAPHIC**. = 0.40 .+-. 0.12 mM; .**GRAPHIC**. = 0.53 .+-. 0.038 mM/min and .**GRAPHIC**. = 0.073 .+-. 0.069 mM. These data are analyzed in terms of the simple carrier model as formulated by Lieb and Stein. Using this model and the data of equilibrium-exchange and zero-trans the half-saturation constants were predicted in infinite-cis conditions (ic) and compared with the experimental values (given in parentheses): .**GRAPHIC**. = 0.231 (0.252) mM and .**GRAPHIC**. = 1.08 (0.937) mM. This indicates the internal consistency of the simple carrier model for uridine transport. 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, it was inferred 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, the turnover rate for uridine transport was calculated to be 7,600 molecules/min for influx and 35,000 molecules/min for efflux (both at 25.degree. 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.