Hypoxanthine and thymidine compete for transport in Chinese hamster fibroblasts |
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Authors: | Robert S Slaughter Raymond G Fenwick Eugene M Barnes |
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Institution: | Departments of Biochemistry, Medicine, and Cell Biology, Baylor College of Medicine, Houston, Texas 77030 USA |
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Abstract: | The transport of thymidine and hypoxanthine was investigated in mutant Chinese hamster lung fibroblasts deficient in both thymidine kinase and hypoxanthine-guanine phosphoribosyltransferase. Kinetic data from rapid uptake experiments (0.5–4.5 s) indicate that thymidine is transported by a monophasic saturable system (Km = 0.29 mM, V = 6.7 nmol/min · mg) which is competitively inhibited by hypoxanthine (Ki = 3.3 mM). The cells displayed a single transport system for hypoxanthine (Km = 2.0 mM, V = 8.9 nmol/min · mg) that is inhibited competitively by thymidine (Ki = 0.43 mM). Both hypoxanthine and thymidine entry were noncompetively inhibited by nitrobenzylthioinosine, but thymidine transport was more sensitive. A kinetic model in which hypoxanthine and thymidine share a common transporter can account for the competitive inhibition and the observation that the inhibition constants are similar to the Michaelis constants. |
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