Abstract
Mutant cell lines have been isolated from wild type S49 cells that possess a novel high affinity purine base transport system that is not found in parental cells or any other mammalian cell line. These cells are capable of transporting purine bases at a rate 10 to 30-fold greater than wild type cells. The Km value of this novel nucleobase transporter for purine bases is exceptionally low, approximately 10-100 micromolar. The high affinity transporter does not recognize nucleosides or pyrimidine nucleobases and can be distinguished from other previously described nucleobase transporter in mammalian cells on the basis of its high affinity for ligands and its sensitivity to inhibition by DPA and NBMPR. That the expression of this high affinity purine base transport system is independent of the NBMPR- and DPA-sensitive nucleoside transport system has been demonstrated by the insertion of the high affinity nucleobase transport mutation into nucleoside transport-deficient S49 cells. Two classes of high affinity purine base transport mutants were generated that can be differenfiated on the basis of their substrate specificities, affinities for purine bases, and sensitivities to inhibitors of nucleoside transport. The data suggest that the expression of the high affinity transport probably requires the unmasking or alteration of a specific genetic locus that is silent or different in wild type cells.
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