title: Transport and countertransport of thymidine in ATP depleted and thymidine kinase-deficient Novikoff rat hepatoma and mouse L cells: evidence of a high Km facilitated diffusion system with wide nucleoside specificity.
abstract: Incubation of cultured Novikoff rat hepatoma and mouse L cells in a glucose-free basal medium containing 5 mM KCN and 5 mM iodoacetate for about 10 minutes resulted in a complete depletion of the cells of ATP. ATP-depleted wild type cells or thymidine kinase-deficient sublines of Novikoff or L cells took up thymidine rapidly from the medium without concentrating it intracellularly, and exhibited countertransport of thymidine. Thus uptake was by facilitated diffusion. This transport system differs from the substrate-specific, low-Km (0.5 muM] thymidine transport system previously described for various types of cultured cells in that it exhibits an at least 100-fold higher Km and transports equally well various ribo- and deoxyribonucleosides. The results suggest that the rate-limiting step in thymidine incorporation into the nucleotide pool by wild type cells is phosphorylation rather than transport, or that the cells possess two transport systems, a facilitated diffusion system with low substrate specificity and a second system which involves substrate phosphorylation by thymidine kinase.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D001693:Biological Transport, Active; D002460:Cell Line; D003486:Cyanides / Q000494:pharmacology; D003841:Deoxycytidine / Q000378:metabolism; D004058:Diffusion; D007461:Iodoacetates / Q000494:pharmacology; D007700:Kinetics; D007739:L Cells; D009154:Mutation; D009705:Nucleosides / Q000494:pharmacology; D011188:Potassium; D013936:Thymidine / Q000378:metabolism*; D013937:Thymidine Kinase / Q000378:metabolism*
chemical_list: D003486:Cyanides; D007461:Iodoacetates; D009705:Nucleosides; D003841:Deoxycytidine; D000255:Adenosine Triphosphate; D013937:Thymidine Kinase; D011188:Potassium; D013936:Thymidine