Summary: The uptake of pyrimidines and their derivatives into and was measured using a novel technique in which the cells were rapidly separated from their suspending medium by centrifugation through a layer of an inert oil. The uptake of [C]cytosine was linear for 30 s for all concentrations of pyrimidine tested. In but not cytosine transport was mediated by both a high affinity ( 0·8±0·1 μ), low capacity [ 40±4 pmol(μl cell water) s] and a low affinity [ 240±35 μ], high capacity system [ 770±170 pmol (μl cell water) s. The cytosine permease in was specific for cytosine and 5-fluorocytosine. In there was only one cytosine transport system [ 2·4±0·3 μ; 50±4 pmol (μl cell water) s]; this system also transported adenine, guanine and hypoxanthine. Differences in nucleoside transport were also observed for and , with the uridine permease in transporting only uridine and 5-fluorouridine whereas cytidine and adenosine were also transported by the uridine permease in . Studies on the effect of nucleoside analogues on uridine transport in demonstrated the importance of the sugar moiety in determining the specificity of transport, with a hydroxyl residue on C-2 being apparently essential for transport.


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