FLUCTUATIONS IN NUCLEOSIDE UPTAKE AND BINDING OF THE INHIBITOR OF NUCLEOSIDE TRANSPORT, NITROBENZYLTHIOINOSINE, DURING THE REPLICATION CYCLE OF HELA-CELLS
Binding of the potent nucleoside transport inhibitor 6-[(4-nitrobenzyl)thio]-9-.beta.-D-ribofuranosylpurine (NBMPR) and rates of uptake of several nucleosides were examined at 4 intervals during the replication cycle of HeLa S3 cells. Monolayer cultures of synchronous cells, obtained by mitotic detachment, were assayed for high-affinity binding of NBMPR and for rates of uptake of thymidine, uridine, cytidine, adenosine, inosine and guanosine. The number of NBMPR binding sites per cell doubled between 4 and 16 h after detachment (late G1 and S phase); during this interval, Vmax S for uptake of cytidine and adenosine doubled, and for uridine and thymidine uptake increased about 4- and 8-fold, respectively. Rates of inosine and guanosine uptake at extracellular concentrations below saturation increased 2-fold between G1 and S phase of the cell cycle. Km for cellular uptake of thymidine, uridine, cytidine and adenosine did not change with progress through the cycle. Changes in nucleoside uptake during the HeLa cell cycle probably were due, in part, to changes in the activity of NBMPR-sensitive transport elements in the membrane.