UPTAKE OF NUCLEOSIDES BY CELLS IN CULTURE .I. INHIBITION BY HETEROLOGOUS NUCLEOSIDES

Millimolar levels of nucleosides inhibited the uptake of trace levels of other, labeled nucleosides into the RNA and DNA of chicken, rat and mouse cells in culture. The incorporation of label into cold acid-soluble and insoluble fractions was invariably inhibited to the same extent. Every ribo- and deoxyribonucleoside tested inhibited the uptake of each of the other nucleosides. The effect was immediate and rapidly reversible. The synthesis of RNA was shown not to be inhibited, since the incorporation of 32P into RNA was unaffected by excess nucleosides. In contrast, the incorporation of 32P into DNA was inhibited by thymidine, which is known to block the formation of deoxycytidine triphosphate. The active inhibitory compound probably is the nucleoside itself rather than a phosphorylated derivative. Cells deficient in thymidine kinase activity were as responsive to the inhibitory action of thymidine as were cells containing the kinase. Also, the extracted nucleoside kinases were generally unaffected by the various nucleosides (and their derivatives) found inhibitory in whole cells. The results suggest enzyme-like transport systems for nucleosides susceptible to inhibition by heterologous nucleosides. The use of radioactive nucleosides in the study of the metabolic effect of millimolar levels of other nucleosides may be complicated by the block occurring in their assimilation into the intracellular pool. © 1969.