Abstract

Initial rates of adenosine uptake by HeLa cell monolayers were measured by a replicate culture technique. Under the assay conditions, the cellular content of adenosine was almost entirely in the form of metabolites, primarily ATP, ADP, and AMP, and thus the time course of adenosine uptake was essentially that of the formation of the adenosine phosphates; both time courses were linear and could be extrapolated through zero time. Adenosine uptake was mediated by a mechanism that appeared to be distinct from the thymidine, uridine, and guanosine uptake mechanisms. In the presence of nitrobenzylthioinosine (NBMPR), mediated entry of adenosine was eliminated and a nonsaturable component of uptake, evidently simple diffusion, was apparent. In the presence of partially inhibitory concentrations of NBMPR, the apparent K_m values of the uptake process were increased and V_max values were unchanged. In a structure-activity study, comparisons were made of inhibition of adenosine uptake by graded concentrations of various nucleoside derivatives related to NBMPR. Inhibitors more potent than NBMPR were identified, and it was apparent that the 2'-hydroxyl group of NBMPR and related compounds was not involved in the cell-inhibitor interaction. NBMPR and its 5'-monophosphate were comparable inhibitors of adenosine uptake.