Purine transport in cultured-normal and mouse sarcoma virus-transformed rat kidney cells

Abstract

The characteristics of adenine, hypoxanthine, and guanine uptake in normal rat kidney (NRK) cells and NRK cells nonproductively transformed (TRK) by Kirsten mouse sarcoma virus have been studied. The data show that adenine and hypoxanthine are transported by saturable and nonsaturable processes in both cell types. The nonsaturable process for adenine uptake (about 2.5 pmoles/mg of protein/min/μ m external concentration) and hypoxanthine uptake (about 1.3 pmoles/mg of protein/min/μ m external concentration) is similar in both cell lines. The apparent K m and V max values for the facilitated transport process of both purines are more than twofold higher for TRK cells than the corresponding values for NRK cells. Uptake by NRK cells is greater than that by TRK cells at low extracellular concentration of adenine or hypoxanthine, whereas the reverse is true at high concentration of adenine or hypoxanthine. In contrast to the characteristics of adenine and hypoxanthine uptake by two cell types, guanine uptake is slightly greater in NRK cells than in TRK cells at all concentrations tested. The structural requirements for the adenine transport system are very specific in both cell types, as indicated by the inability of uric acid, hypoxanthine, and several purine analogs to inhibit adenine uptake. 6-Thioguanine and 6-mercaptopurine inhibit uptake of guanine and hypoxanthine in NRK and TRK cells. Inhibition is competitive with the apparent K i values lower than the corresponding K m values in each case. Hypoxanthine inhibits guanine uptake competitively with apparent K i values of 0.19 and 6 μ m for NRK and TRK cells, respectively. Guanine, however, does not inhibit hypoxanthine uptake in either cell line. Therefore, hypoxanthine and guanine have separate transport systems in both cell lines which do not involve hypoxanthine-guanine phosphoribosyltransferase. Purine inhibits guanine and hypoxanthine uptake competitively in NRK cells but is without effect in TRK cells, and adenine inhibits the uptake of guanine and hypoxanthine to a greater extent in NRK cells than in TRK cells. These results show that differences exist in the transport systems of NRK and TRK cells for these purines.