Interaction of 2'-halogeno-2'-deoxyuridines with the human erythrocyte nucleoside transport mechanism.

The efflux of radioactive thymidine from human erythrocytes at 25 degrees was accelerated in the presence of extracellular 2'-fluoro-2'-deoxyuridine to a maximal velocity 120% of that observed in the presence of extracellular nonradioactive thymidine. Efflux in the presence of 2'-chloro-2'-deoxyuridine and 2'-bromo-2'-deoxyuridine did not exceed 56% and 49%, respectively. 2'-Iodo-2'-deoxyuridine did not accelerate thymidine efflux. In comparison, 2'-fluoro-2'-deoxycytidine and 2'-deoxyuridine accelerated thymidine efflux to maximal velocities of 170% and 91%, respectively. The half-saturation constant for acceleration of thymidine efflux by 2'-fluoro-2'-deoxycytidine was higher (0.90 mM) than those estimated for the other substances (0.22 mM or lower). Influx competition experiments at 25 degrees showed that all of the above nucleosides competitively inhibited influx of thymidine into human erythrocytes. The Km for the zero-trans influx of thymidine was 0.051 +/- 0.008 mM, while the Ki values for 2'-deoxyuridine and the 2'-halogeno-2'-deoxyuridines were similar, ranging from 0.04 to 0.09 mM. The Ki for 2'-fluoro-2'-deoxycytidine was 0.18 mM. These results suggest that, although all nucleosides tested appeared to bind to the same transport site on the external membrane surface, their ease of transport through the membrane was determined by the properties of the halogen substituent at position 2'.