The Uptake of Pyrimidine Nucleosides in Tetrahymena. I. Uridine*

Abstract

Uridine uptake was examined in T. pyriformis GL-7 in defined medium under conditions where food vacuole formation was not a significant factor in solute acquisition by the cell. Results indicated the presence of a saturable mechanism which followed Michaelis-Menten kinetics. When corrected for diffusion the apparent Km for the carrier was 2.3 .+-. 0.6 .mu.M and the Vmax was 7.3 .+-. 0.2 .times. 10-7 nmol/cell/min. Nucleotide pool analysis showed that most of the radioactivity of externally supplied [3H]uridine appeared in UMP with the remainder in UTP. Uridine was apparently phosphorylated immediately upon entry into the cell and neither uridine-cytidine kinase activity nor RNA synthesis were rate-limiting in the uptake process. Uridine transport was competitively inhibited by a variety of ribo- and deoxyribonucleosides as well as several nucleoside analogs. Neither uracil nor ribose or deoxyribose were effective inhibitors of uridine transport indicating the carrier was specific for the nucleoside. There was little difference between the Ki values for ribo- as opposed to deoxyribonucleosides except in the case of deoxyguanosine which was much less effective as an inhibitor under these conditions than all the other nucleosides including guanosine.