Freeman M, Moner J G
J Protozool. 1976 Aug;23(3):465-72. doi: 10.1111/j.1550-7408.1976.tb03814.x.
Uridine uptake was examined in Tetrahymena pyriformis GL-7 in defined medium under conditions where food vacuole formation is not a significant factor in solute acquisition by the cell. The results indicate the presence of a saturable mechanism which follows Michaelis-Menten kinetics. When corrected for diffusion the apparent Km for the carrier is 2.3 +/- 0.6 muM and the Vmax is 7.3 +/- 0.2 X 10(-7) nmoles/cell/min. It is evident from nucleotide pool analysis that most of the radioactivity of externally supplied [3H]uridine appears in UMP with the remainder in UTP. Uridine is apparently phosphorylated immediately upon entry into the cell and neither uridine-cytidine kinase activity nor RNA synthesis are rate-limiting in the uptake process. Uridine transport is competitively inhibited by a variety of ribo- and deoxyribonucleosides as well as several nucleoside analogs. Neither uracil nor ribose or deoxyribose are effective inhibitors of uridine transport indicating the carrier is specific for the nucleoside. There is little difference between the Ki values for ribo- as opposed to deoxyribonucleosides except in the case of deoxyguanosine which is much less effective as an inhibitor under the conditions of this study, than all the other nucleosides, including guanosine.
在梨形四膜虫GL - 7处于特定培养基中且食物泡形成并非细胞溶质摄取的重要因素的条件下，对尿苷摄取进行了研究。结果表明存在一种遵循米氏动力学的可饱和机制。经扩散校正后，载体的表观Km为2.3±0.6 μM，Vmax为7.3±0.2×10⁻⁷纳摩尔/细胞/分钟。从核苷酸库分析可以明显看出，外部供应的[³H]尿苷的大部分放射性出现在UMP中，其余出现在UTP中。尿苷进入细胞后显然立即被磷酸化，并且尿苷 - 胞苷激酶活性和RNA合成在摄取过程中均不是限速因素。尿苷转运受到多种核糖核苷和脱氧核糖核苷以及几种核苷类似物的竞争性抑制。尿嘧啶、核糖或脱氧核糖均不是尿苷转运的有效抑制剂，这表明载体对核苷具有特异性。核糖核苷与脱氧核糖核苷的Ki值之间几乎没有差异，除了脱氧鸟苷在本研究条件下作为抑制剂的效果比所有其他核苷（包括鸟苷）差得多。
