Nucleoside transport in human erythrocytes. A simple carrier with directional symmetry and differential mobility of loaded and empty carrier.

Rapid kinetic techniques were employed to measure the transport of uridine and thymidine in human erythrocytes in zero-trans entry and exist and equilibrium exchange procedures. The kinetic parameters of transport were computed by fitting appropriate integrated rate equations to time courses of transmembrane equilibration of radiolabeled substrate. Transport of uridine and thymidine conformed to the simple carrier model with directional symmetry, but differential mobility of loaded and empty carrier. As was apparent from comparison of zero-trans influx and equilibrium exchange flux, the loaded carrier moved 3 to 18 times faster than the empty carrier in batches of erythrocytes obtained from different individuals. The maximum equilibrium exchange velocities also differed for different batches of erythrocytes. Storage of the cells at 4 degrees C for 4 days or treatment of cells with oxidizing or reducing agents or suspension in hypotonic solutions had no effect on the kinetic properties of the nucleoside transporter. All natural ribo- and deoxyribonucleosides tested when present on the trans side at concentrations well above the Michaelis-Menten constant for transport accelerated the influx and efflux of uridine and inhibited uridine influx when present at these concentrations on the cis side, but nucleobases had no significant effect.