Expression of a novel high-affinity purine nucleobase transport function in mutant mammalian T lymphoblasts.

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Abstract
The single nucleoside transport function of mouse S49 lymphoblasts also transports purine bases (B. Aronow and B. Ullman, J. Biol. Chem. 261:2014-2019, 1986). This transport of purine bases by S49 cells is sensitive to inhibition by dipyridamole (DPA) and 4-nitrobenzylthioinosine, two potent inhibitors of nucleoside transport. Therefore, wild-type S49 cells cannot salvage low hypoxanthine concentrations in the presence of 10 microM DPA and 11 microM azaserine; the latter is a potent inhibitor of purine biosynthesis. Among a mutagenized wild-type population, a cell line, JPA2, was isolated which could proliferate in 50 microM hypoxanthine-11 microM azaserine-10 microM DPA. The basis for the survival of JPA2 cells under these selective conditions was expression of a unique, high-affinity purine nucleobase transport function not present in wild-type cells. JPA2 cells could transport 5 microM concentrations of hypoxanthine, guanine, and adenine 15- to 30-fold more efficiently than parental cells did. Kinetic analyses revealed that the affinity of the JPA2 transporter for all three purine bases was much greater than that of the wild-type nucleobase transport system. Moreover, nucleobase transport in JPA2 cells, unlike that in parental cells, was insensitive to inhibition by DPA, 4-nitrobenzylthioinosine, sulfhydryl reagents, and nucleosides. No alterations in nucleoside transport capability, phosphoribosylpyrophosphate levels, or purine phosphoribosyltransferase enzymes were detected in JPA2 cells. Thus, JPA2 cells express a novel nucleobase transport capability which can be distinguished from the nucleoside transport function by multiple biochemical parameters.