On the functional symmetry of nucleoside transport in mammalian cells.

The transport of uridine and thymidine has been examined in HeLa cells, in Novikoff rat hepatoma cells and in human erythrocytes, with the purpose of comparing influx, efflux and isotopic exchange at chemical equilibrium. The results support the following conclusions: (i) In all three cell types influx and efflux are comparable; (ii) HeLa and Novikoff cells show no trans-effect, while erythrocytes show a 5-fold trans-stimulation; (iii) a single kinetic entity accounts for nucleoside transport in HeLa and Novikoff cells - no parallel routes of permeation with Km less than 40 microM were detected. For the cultured cells, the flux data conform to the kinetic model of a single, carrier-mediated transport system symmetrical with respect to direction, and with equal mobilities of substrate-loaded and empty carrier.