Vijayalakshmi D, Belt J A
Department of Biochemical and Clinical Pharmacology, St. Jude Children's Research Hospital, Memphis, Tennessee 38101.
J Biol Chem. 1988 Dec 25;263(36):19419-23.
Nucleoside transport was examined in freshly isolated mouse intestinal epithelial cells. The uptake of formycin B, the C nucleoside analog of inosine, was concentrative and required extracellular sodium. The initial rate of sodium-dependent formycin B transport was saturable with a Km of 45 +/- 3 microM. The purine nucleosides adenosine, inosine, guanosine, and deoxyadenosine were all good inhibitors of sodium-dependent formycin B transport with 50% inhibition (IC50) observed at concentrations less than 30 microM. Of the pyrimidine nucleosides examined, only uridine (IC50, 41 +/- 9 microM) was a good inhibitor. Thymidine and cytidine were poor inhibitors with IC50 values greater than 300 microM. Direct measurements of [3H]thymidine transport revealed, however, that the uptake of this nucleoside was also mediated by a sodium-dependent mechanism. Thymidine transport was inhibited by low concentrations of cytidine, uridine, adenosine, and deoxyadenosine (IC50 values less than 25 microM), but not by formycin B, inosine, or guanosine (IC50 values greater than 600 microM). These data indicate that there are two sodium-dependent mechanisms for nucleoside transport in mouse intestinal epithelial cells, and that formycin B and thymidine may serve as model substrates to distinguish between these transporters. Neither of these sodium-dependent transport mechanisms was inhibited by nitrobenzylmercaptopurine riboside (10 microM), a potent inhibitor of one of the equilibrative (facilitated diffusion) nucleoside transporters found in many cells.
在新鲜分离的小鼠肠上皮细胞中研究了核苷转运。肌苷的C核苷类似物福米韦生的摄取是浓缩性的，并且需要细胞外钠。钠依赖性福米韦生转运的初始速率是可饱和的，Km为45±3微摩尔。嘌呤核苷腺苷、肌苷、鸟苷和脱氧腺苷都是钠依赖性福米韦生转运的良好抑制剂，在浓度低于30微摩尔时观察到50%抑制（IC50）。在所研究的嘧啶核苷中，只有尿苷（IC50，41±9微摩尔）是良好的抑制剂。胸苷和胞苷是较差的抑制剂，IC50值大于300微摩尔。然而，[3H]胸苷转运的直接测量显示，这种核苷的摄取也是由钠依赖性机制介导的。胸苷转运受到低浓度胞苷、尿苷、腺苷和脱氧腺苷（IC50值小于25微摩尔）的抑制，但不受福米韦生、肌苷或鸟苷（IC50值大于600微摩尔）的抑制。这些数据表明，小鼠肠上皮细胞中存在两种钠依赖性核苷转运机制，福米韦生和胸苷可作为区分这些转运体的模型底物。这些钠依赖性转运机制均未被硝基苄基巯基嘌呤核糖苷（10微摩尔）抑制，硝基苄基巯基嘌呤核糖苷是许多细胞中发现的一种平衡型（易化扩散）核苷转运体的有效抑制剂。
